"During the time men live without a common power to keep them all in awe, they are in that conditions called warre; and such a warre, as if of every man, against every man."
- – Thomas Hobbes
The Abyss (later expanded into The Infinite Layers of the Abyss) is the Chaotic Evil plane in Dungeons & Dragons's Great Wheel cosmology. It's home to an infinite number of demons, the Chaotic Evil fiends. Expect METAL.
The plane is such a mess of chaos and upheaval that it doesn't even stay the same in basic state from time to time, which is another way of saying the writers can't keep it straight and change it whenever it's convenient. It's an infinite plane, like the others, but actually has an infinite number of different layers as well, meaning that the aforementioned changes might just be an examination of a different subset of layers. According to one faction in the Planescape setting it also specifically has 666 layers, because that's totally the same as infinite. Then again, it's one of the chaotic planes, so not making sense is probably the whole idea (ignoring the fact that Arborea is just as consistent as any of the Lawful planes). Oddly enough though, none of the other outer planes even have ten layers. A clear in-universe reason for the abyss alone getting so many layers has never been provided, but a narrative-based reason might be to leave room for DMs to write their own BBEGs into the Cosmology.
It's also a plane in the World Axis cosmology of D&D too, but here is a more modestly sized "universe-swallowing" vortex of corruption and evil that lies in the Elemental Chaos, created when Tharizdun planted a crystallized shard of pure evil from the dying universe of the obyriths in the Elemental Chaos.
In the World of Darkness games, the Abyss is a realm of cold blackness from which shades and demons and the god of Oblivion hail, and from which vampires wielding Obtenebration summon either a black ectoplasmic substance for various effects, such as the creation of tentacles, or a raw Oblivionic energy for disintegrating targets. Thematically it is closer to D&D's Negative Energy Plane or Shadowfell plane.
The political structure of the Abyss actually greatly resembles that of the (not)Holy (not)Roman (not)Empire in its latter years: In Theory there's one Demon Prince, Demogorgon, who rules over the others, but in practice it's a complete Hot Mess where each Demon Prince operates entirely independently, warring with eachother or even Demogorgon himself just as often as they do entities from other planes. At least planar layers have neatly-arranged borders.
Most of the layers are often described as containing wildly different inhospitable biomes, sometimes on the same layer side-by-side. All of them are full of the kind of Chaoticy-Evilly fuckery you'd expect, so none of them are great places for vacation homes. The named layers are also given numbers, but given that they are in no way consistently arranged in space, those numbers are meaningless and changed often (except for the first one). Officially, though, they are numbered in order of their discovery by mortals.
The descriptions are, naturally, weighted to the lower numbers.
A major source for the below is the Planescape box, Planes of Chaos.
Pazunia: 1st layer
Plane Plain of Infinite Portals and The Wasteland of Gates‚ the way you get to any of the others. It's covered in giant sinkholes, each one a portal to another layer of the Abyss. This is also the layer that the River Styx flows through on its path from Pandemonium to Acheron. Iron fortresses on this layer are also where Demon Princes leave their bodies when they're busy roaming around the Astral Plane. Dozens of minor demon princes co-rule this layer, though, officially, Pazuzu is in charge. The Grand Abyss is sometimes the 4th layer and Pazunia is the Styx shorelines.
This is one of the great battlefields of the Blood War. Mostly, this is a staging ground for the various demon hordes being flung at the Nine Hells, but that creates a whole set of problems by itself. As some of you may already know, beings that are inherent Chaotic and Evil are just as likely to start axe-murdering one another as anything else. So put a million such beings of pure Chaotic Evil side by side in one place, and count the seconds until they just start killing each other instead of doing what they were told. Now, complicate that problem with the fact that some demon princes may actually want such an outburst to occur to weaken the forces of a rival. Now throw in yugoloth mercenaries turning stag on a demon employer (devils, after all, are much better at keeping their words on contracts and payments and the like). Toss in a force of devils being sent for scouting and/or raiding duties to better prepare for said horde before it even gets to Avernus. And finally, sprinkle in the dozens of other planars - celestials, adventurers, etc. - who are visiting and know that the best way to deal with an ornery demon is basically to fucking destroy it.
Now, it's not all murderhobos and axes to the face. There's a few fiends that run some odd little fortified establishments out here. Some are mercenaries who can press-gang enough demons to make it worth someone's effort to pay for it. There's even a few mercantile or entertainment venues around. But mostly, if you run into a fiend out here, you murder the shit out of it and hope it didn't have any friends who really gave a shit about it. This is the first layer of an infinite amount of Chaotic Evil, by the way; it only gets more fun the further down the spiral you go.
It should be noted that oddly, this is the most stable layer of the Abyss. No, really: it hasn't actually changed all that much, fundamentally, in all of existence. The demons that inhabit it, and who's running what, that does change, and yes, the land itself has been ripped up, rebuilt, wash, rinse, repeat. But this is basically where the teeming, seething primordial chaos of the Abyss touches reality, and thus this connection is by necessity stable. (Since if the Abyss disconnected, even temporarily, it would probably be adrift in the Astral, or possibly it would fuck off from reality altogether for the Far Realm; it's hard to say what the metaphysics of that scenario would look like.)
A map of Pazunia's World Axis counterpart.
Driller's Hives: 2nd layer
This horrible place is one of the most unpleasantly inhospitable layers in all of the Abyss. It's not so much because of the indigenous fiendish insects who try to kill anything on sight, but because of how all visitors who enter risk contracting a horrible disease that causes the sufferer to break out in hatching insect eggs until the afflicted dies and decomposes into a mass of rot and insect eggs over the course of minutes. The addition of the giant, intelligent fiendish insect thralls of the ruler, Tharzax the Chattering Prince, help make the whole place a big, sentient shit-sprinkle-covered crapcake of NOPE.
The Forgotten Land: 3rd layer
This is an almost endless landscape of empty ruins that suggest a once bustling city. No demons, other than the layer's Lord, Zzyczesiya The Ungrasped, master of malevolent ignorance and confusion, come here. The place was abandoned due to a catastrophe, for which Zzyczesiya has been traditionally blamed, and now, entering the layer causes any sentient being's mind, even those of demons and sapient undead, to fritter away like moist cotton candy, leaving visitors as catatonic and braindead. Constructs are immune to the effects, and persistent visitors often use golem stand-ins to explore this godawful place. Allegedly, Zzyczesiya destroyed all the original inhabitants by accident, 1st through 3rd edition claiming by changing the course of the River Styx, and was punished by imprisonment in the Wells of Darkness. Zzyczesiya would escape, and return to the Forgotten Land in order to use the power of this layer to unremember something really, really, really horrible (possibly a first draft of the script for Gigli).
The Grand Abyss: 4th layer
What starts as a gaping chasm on the first layer is an infinitely deep pit where the walls are dotted with portals which connect to all of the other layers in the plane. Navigation is tricky because the only dimensions of relevance are vertical, so you need to either fly or be a patient climber. This place was created by the now-dead Lord, Asima The Unanticipated, who apparently died because she(?) didn't anticipate pissing off the Queen of Chaos enough to assassinate her. Now, Asima's corpse lies in her tomb, the Tumulus of Abhorrence, which is filled with treasure and secrets, and is guarded by her "seraph," a super-demon named "Guardian of the Gates" who pretty much super-kills anyone unlucky enough to encounter him. Guardian of the Gates periodically leaves his master's tomb to harass/super-kill schmucks in the Grand Abyss, as per his master's original orders: adventurers stupid enough to think that there are no other defenders of the Tumulus tend to die screaming.
Wormblood: 5th layer
Realm of a Million Eyes: 6th layer
Home of the Great Mother, insane princess of the Beholders. They're a Lawful race so have no idea that their heaven is Abyssal - but then, tanar'ri don't visit much, so to nonAbyssals it's an easy mistake to make. Her realm is an infinite cavern of tunnels and chambers, populated by every kind of Beholder-kin many, perhaps, birthed right here. The walls, ceilings and floors of the realm are dotted with eyes, making attempts at concealment nearly impossible considering that you're being watched all of the time. The eyes are believed to be those of the Great Mother herself, meaning that she practically is the layer. If you're not supposed to be here, you're in trouble.
The realm was apparently first discovered by Illithids, who declared that it must be the source of all corruption and foulness in the multiverse and then attempted to destroy it. It says a lot when even Mind Flayers think a realm is too corrupt to let live.
The Phantom Plane: 7th layer
Ruled by Sess'lnnek the other god of the lizardfolk. He grew bored with the Tanar'ri's Blood War, withdrew and officially closed the layer off so that no one can enter without his explicit permission.
Skin-shedder: 8th layer
Ruled by Volisupula the Flensed Marquesse, lord of ostentatious finery. The layer itself makes you more evil the longer you stay there. Tieflings are immune.
Burningwater: 9th layer
Acid ocean, so the crustaceans and fish have shells of silica.
That Hellhole: 10th layer
It's the body of an actual fiend. The dwarf who found this place got chewed up (through adamantite!) before beaming out of there.
Molrat: 11th layer
Dark tunnels, where the tanar'ri hunt by smell. Besides the demon horde the plane is more friendly to life than most.
Twelvetrees: 12th Layer
Definitely a place of horror, and a living example of why you can never, ever trust a demon.
Eons ago, some demons tricked some astral devas into having a meeting for the sake of peace. Basically, the powers from the Upper Planes saw the tanar'ri overthrow the obyriths, and thought maybe that the new fiends were "better" than the old ones. Big fucking mistake there: the demons turned on the angels, bound them to twelve of the biggest trees in the layer, and then performed a blasphemous arcane ritual that sacrificed them to the layer, allowing their eternal suffering to become fuel for all kinds of evil shit.
The angels can still be heard screaming, and a big altar now commemorates the site of the sacrifice, surrounded by twelve sickly pines that ooze some kind magically-infused slime. Evil-descriptor spells are Maximized and Extended automatically for free as a result of the power of the place. Demons make pilgrimages to the layer, to honor the screw job their kind put to the forces of good, and the overwhelming evil of the place is like a drug to the fiends, as they bask in a haze of quasi-religious ecstasy and basically don't give a shit what's happening around them, making it scarily easy for non-demons to wander around pretty freely (you only gotta dodge some chasmes that patrol the skies, maybe some kelvezus that look for intruders to murder).
But that's not all. See, someone figured out how to harness all that deliciously evil energy to create a new weapon of war: the Ship of Chaos. This horrible thing is basically made of flesh, bone, and spirit; it's hull seems to be comprised of some kind of twisted, thick, ropelike fibers, woven into an organic mass of mottled white and yellow, with translucent blue blisters along the flanks. They are powered by the annihilation of soul larvae... meaning that the ship itself absolutely destroys souls beyond recovery in order to operate. It can generate an "entropy field" almost the length of 5 football fields that either drives beings insane or simply repulses them, and shoots waves of force out of the mouths at the front and back. It can warp itself to travel through small portals, but this causes magic stuff onboard to possibly break unless it is made from the Abyss, and tanar'ri and their kin aren't particularly messed up by the warping.
The scaffolds for these ships dot the area around Twelvetrees. Six have been completed for sure. There was a seventh ship in the 2nd edition Planescape adventure In The Abyss, but it's highly likely that it was destroyed. The 3rd edition supplement FC2: Hordes of the Abyss notes another seventh ship is being built, which seems likely. In theory, these ships will not only be awesome weapons to use against the devils, but will mean the demons don't need the yugoloths to help them travel to Baator anymore.
And all this time, the angels keep screaming.
Blood Tor: 13th Layer
Steaming Fen: 14th Layer
The Courts of Mustering 15th layer
The Endless Graveyard 16th layer
Pleroma/Death's Reward: 17th layer
Ruled by demon prince Abraxas the Unfathomable. He has a roosters head and a bunch of snakes for a lower body. He's big on magic and the occult. Diovengia is his city which houses some of the best libraries in the entire Abyss. Overlapping with the Whispering Lights.
Carnaxius Sekhmeth's realm.
The Sixth Pyre: 21st layer
Ruled by Prince Kardum, the lord of Balors.
The Iron Wastes: 23rd layer
Also commonly know as the Ice Wastes, Kostchtchie is its resident Demon Prince. An infinite expanse of jagged mountains, glaciers and frozen tundras. Kostchtchie commands a big army of frost giants which he used to further "conquer" his layer, when he isn't siccing them on other demon lords for laughing at him, or conquering other tribes of frost giants on the Prime Material Plane.
Malignebula: 27th layer
The acid-strewn clouds of this layer are ruled by the gaseous Lady Lissa’aere, a swirling funnel of malevolence and poison mist. Alu-Fiends, Nabassu, and Vrocks make up her army, sallying forth only when Baatezu troops cross through Carceri and threaten the borders of The Abyss or when one of Lissa’aere's rivals comes too close to the layer’s boundary. No solid ground exists in the layer being completely composed of acidic clouds, sort of like Venus minus the ground. Clouds harden into floating ice on occasion, but they quickly melt when too many Tanar'ri cluster there to rest their weary wings. What happens to those who fall too far isn't known, some say that they're consumed by the Lissa’aere.
Sholo-Tovoth/ Fields of Consumption: 32nd layer
Ruled by Demon Prince Turaglas, The Ebon Maw, who was spawned directly from the Abyss. A 100% pure demon and therefore envied by Orcus and Demogorgon for this (Demogorgon was a creation of the Obyriths, while Orcus was a former mortal) who joined against him and imprisoned him for a while. His goal is to devour and destroy, he has spawned his own breed of demons, the turagathshee. The layer itself, like That Hellhole #10, tries to eat anyone that enters it that's not either Turaglas or one of his minions, with mouths and tongues popping up everywhere. Those minions number millions of insatiably hungry demons.
Azzagrat: 45th, 46th, and 47th Layers
Ruled by Graz'zt, demon lord of depending on your source, A) tyrants, manipulation, and, unofficially, blatant nepotism, or B) lust, hedonism, and incubi. There's a salt river that connects all three layers. Zelatar is a big, shiny, evil pastiche of the Emerald City of Oz that stretches across the three layers. This city is populated by relatively well-behaved demons, not that that's saying much, as all of Graz'zt's minions know that Graz'zt torturing them to death for a minor screw up is the least he can do to them. Not a terrible place for a party, assuming it's the right kind of party.
Shaddonon: 49th Layer
Ruled by Rhyxali.
Vorganund: 52nd Layer
Has no ruler.
Phage Breeding Ground: 53rd Layer
Ruled by Urae-Naas, a "female" slaad Demon Princess so bloated with eggs she can't walk anymore and has to drag herself along on her long arms and enormous tongue. Comprised from the interiors of the ever-rotting organs of her former boyfriend, the slain Primordial Ramenos. Iconic local feature: screaming, insane humanoids sewn into the walls begging ceaselessly for help before she gets around to implanting her offspring in their bodies. Created in 4th edition, and probably lost with the change to 5th edition seeing as how Slaad are Chaotic Neutral in all non-4th editions.
Torturous Truth: 57th layer
Ruled by Alvarez the Purging Duke, the head of the demonic Inquisition. His forces investigate the tanar'ri for any signs of lawful behavior. His torture is notorious throughout the Abyss.
The Demonweb Pits: 66th layer usually
The place where Lolth, the deity of the drow, and the rest of the drow pantheon, hangs out with her spiders. In Jeff Grubb's entry, the Demonweb was 65th and her actual abode on the 66th. In 3e this was changed to be an entirely separate plane, though it's still connected to the Abyss.
The Heaving Hills: 67th layer
Swamps and hills, with Varrangoin ruins. Those swamps and hills swap places after frequent earthquakes. The Varrangoin survive because they can fly; how their ruins are still recognisable after all this tumult, we dunno.
Morgenta's Layer: 68th layer
Barren, "abandoned", and airless. No-one's named it but Planes of Chaos does have a name for the planewalker who survived to enter it into the list: Morgenta, the mage. At least that was the case until Fiendish Codex named it The Swallowed Void.
The Crushing Plain: 69th layer
A surface made up of chasme, manes, and lemures all "crawling over one another". Planes of Chaos claims they are avoiding suffocation but it's more likely they end up squished like bugs further down. Later on it is instead claimed that the 69th layer is called Gibbering Hollow and ruled by Ollomegh.
The Ice Floe: 70th layer
That ice would be thin, over black running water. The cold acts as energy-drain. As to whence / whither the water flows, we're not told but we can guess probably the Styx.
Spirac: 71st layer
Tree-ferns and spiky mountains. A happy hunting ground like Molrat / #11. The demons might be friendlier, or maybe that's just because they got along better with the cutter who penned this entry.
Darklight: 72st layer
Visitors get a halo of darkness (if evil) or light (good). Either is a beacon for the demons here, who enslave the former and kill the latter. Is ruled over by Nocticula the Undeniable, a Demon Prince of Night. Her name was later used by Paizo for their Pathfinder version of Malcanthet, Nocticula.
Wells of Darkness: 73rd layer
A spa-cum-prison/graveyard for some of the worst individuals the plane could play host to. Jacuzzi tubs of black ink keep unspeakably powerful beings trapped beneath the surface, unable to get out, but perfectly conscious and able to communicate. The demons don't visit. It's amusing because the idea of an unbreakable prison on a chaotic plane constantly attracts break-in attempts by those who just want to prove otherwise. Notable prisoners include Shami-Amourae and the layer's own ruler Ahazu. Ahazu in fact imprisoned himself and there's various reasons given as to why he did this from simple madness to a ritual to obtain true godhood. Never the less he's still in full control of the plane and is the only reliable way to release a prisoner.
Smaragd: 74th layer
Merrshaulk rules here, he's the deity of the Yuan-ti snake men. Basically a nightmarish, evil, rainforest full of poison and acid(one of several in the Abyss). Despite that it is relatively safe compared to the the other layers of The Abyss, the only real danger is the acid rain and poisonous animals. Also for some weird reason Ramenos was also lying around here and was considered a co-ruler as well, which makes no sense since they were usually asleep til they died. Still while they were alive his followers would chuck offerings and any unlucky adventures into his perpetually open mouth.
The Gates of Heaven: 77th layer
Actually not named ironically, it's apparently a gate to Celestia in the middle of a Chaotic Evil hell. Must be a really sturdy gate. Ruled by demon lords and brothers, Munkir and Nekir who keep anyone from entering the layer's eponymous gate.
The Emessu Tunnels: 79th layer
Abysm/The Gaping Maw: 88th layer
The realm of Demogorgon. Also known as "The Upside Down" thanks to Stranger Things. Pretty much a tropical nightmare, with demonic-disease-infested jungles and a roiling ocean full of rocky islands that host flying demons and krakens. Dagon, his special friend and mentor, lives in his basement.
Shadowsea: 89th layer
Ruled by Dagon one of the few obyrith lords still part of the establishment. It's an underwater realm without a surface although it connects to the 88th. Filled with myriads of aquatic demons.
Demonwing: layer unknown
An entire layer of the Abyss transformed into a crazy demonic pirate ship by Demogorgon that can move freely about the planes.
The Guttering Grove: 90th layer
Ruled by Ilsidahur, the Howling King, who's a bootlicker to Demogorgon. He leads the barlgura, these orangutan looking demons, as well as these winged gorilla looking guys. The Guttering Grove was recently subsumed into Abysm by Demogorgon as punishment for Ilsidahur screwing up an invasion of Arcadia, though Ilsidahur is apparently either too unaware, too uncaring or too stupid to notice.
Ulgurshek: 92nd layer
The Realms: 99th layer
Plotted out by Rick Swan in the Tales of the Outer Planes anthology. The Abyssal chaos sometimes diverts visitors here when they're aiming for elsewhere, Kali's plane in this one. It's a network of sub-layers, attached to each other through portals:
- Gray Realm. Desert of black sand, infested with dretch.
- Lightning Realm. "Blank white" plain with pillars. Disputed between Orcus and Demogorgon. Yeah, this is oldschool.
- Swirling Realm. Connects to Juiblex's crib. It's a waterfall and ocean... made of puddings and slimes. Yecch!
- Yellow Realm. Primordial tropical forest. 50' blades of grass.
- Fog Realm. "Jellyfish" surface and it stinks. It's a colossal demon uterus.
- Silver Realm. A silvery ice glacier, where sound doesn't travel.
- Star Realm. Another black sand desert, but cratered. Kali's kitchen, preparing souls for her breakfast.
At a guess, only the Gray Realm is truly the 99th. The others look to be other Abyssal layers entirely, like #222 or, indeed, #500.
The Mind of Evil: 111st layer
Ruled by Prince Sch'theraqpasstt, the Serpent Reborn.
Thanatos: 113th layer
Also called "The Belly of Death", it is as full of undead as demons. Cold, black, nearly airless, it's very much not a pleasant place to spend a vaction. Orcus's crib where he spends most of his time chilling here not doing things. When Orcus kicked the bucket in 2e, the minor drow goddess Kiaransalee took over the realm and kept his seat warm until he got back.
Slugbed: 128th layer
Lupercio the Baron of Sloth rules here. It's a tropical, mostly overcast realm filled with lush forests and coral reefs filled with all manner of demon gastropods. Because Lupercio is too busy spending most of his time sleeping, where his perpetually darkness-shrouded form is perpetually parasitized by shadow demons, and too busy spending what precious little waking hours he has maniacally doing something, the demon gastropods of Slugbed do not recognize his authority, and, instead, do their own thing, i.e., eating the scenery, each other, and intruders too stupid to understand the threat posed by demon gastropods until they get eaten. An army of angels has secretly constructed fortresses in the clouds above Slugbed, and eventually intend to use their fortresses here to stage attacks on the demons of the Abyss.
Outcast's End: 137th layer
Ruled by the exiled and outcast Azazel, Prince of Scapegoats.
Lifebane: 142nd layer
Torrent: 148th layer
Has no ruler.
Hollow's Heart: 176th layer
Ruled by Fraz-Urb'luu. Fraz-Urb'luu pissed off virtually all of the other demon lords by learning how to summon them in order to humiliate them in front of his sniveling minions. This led to the demon lord being forced to take a sabbatical in the dungeons of Oerth while lesser Lords moved in, turning the place into a demonic continent of crazy shit, including a jungle of skin, where trees are hairs sprouting from the ground; a desert made of grains of clotted blood; and a city shrouded in powerful illusions to make petitioners think everything is happy and safe, so the demons can take it away piece by piece.
The Writhing Realm: 177th layer
A lightless underground layer ruled by the Obyrith lord Ugudenk and also the place where Golothomas are most commonly found. Ugudenk is a massive worm so long that nobody has ever seen the rear end of his body, and some suspect that he is either infinitely long, or the part of him that has been seen is only a feeler appendage of an even larger creature that exists in (or even is) an unknown layer of the abyss. He is constantly digging through the layer creating new tunnels and sometimes poking his head out into other layers or even the material plane. Fraz-Urb'luu constantly tries and fails to bar Ugudenk from entering Hollow's Heart, but because the Writhing Realm is literally underneath, Fraz-Urb'luu's efforts to exterminate the Squirming King amount to nothing more than an infinitely protracted game of whackamole.
The Rotting Plain: 181st layer
Ruled by Laogzed the god of the troglodyte. It's a massive expanse of baked savannah and shallow swamps, and much like it's ruler it's seriously smelly. Here they archive everyone's browsing history, purchases, and other personal information forever.
Vulgarea: 193th layer
Shedaklah, the Slime Pits: 222nd layer
Offalmound: 223rd layer
No ruler like many of the random unimportant layers.
The Dreaming Gulf: 230th layer
A phantasmal dark and wind-swept realm with no ruler. This is where the dreams of evil gods go when they die, slowly being given form by the Abyss, becoming the incorporeal demons known as Loumaras.
Palpitatia: 241st layer
The Scalding Sea: 245th layer
Another ocean of acid but this one at least has a surface; it might even be the route out for Burningwater. It also has islands... of black glass. Probably deserted, the one island town is empty, with its former occupants encrusting the town's walls while the town's fountain screams eternally and sprays yet more acid.
The Hidden Layer: 248th layer
A huge thunderstorm-ravaged wasteland. Was ruled by Demon Prince Eltab, until he was summoned into the Prime Material and bound there, also sucking small portions of his realm with him to Faerun in the form of demoncysts and leaving his subordinates to fight amongst themselves over control of the rest.
Durao: 274th layer
A ruler-less layer dotted by military encampments and demon barracks interconnected by iron roads that snake through the otherwise swampy environs along the River Styx. It was from here that most of the demon armies staged before departing to wage the Blood War in Gehenna.
The Sighing Cliffs: 297th layer
"Ruled" by the self defeating succubus Lynkhab, who probably only controls this layer because no one wants to hang around her depressed self.
Feng-Tu: 300th layer
The Sulfanorum: 303rd layer
As the Coenobites have taught us, what are demons to some are angels to others; so what's a hell to many could be a heaven to some. Here is the tobacconist's heaven, as Planes of Chaos wittily illustrated. Full of flames designed to light pipes, which the Abyssals smoke here.
Gorrion’s Grasp: 313rd layer
No ruler, heck there isn't any info on this layer at all.
The Broken Scale: 333rd layer
The Black Blizzard: 340th layer
A layer with no ruler, or mention outside the Fiendish Codex.
Fortress of Indifference: 348th layer
Rough terrain with fiery skies. An evil 200 foot tower is the main attraction, and is why we don't let Monte Cook listen to Nine Inch Nails anymore. Its the home of outcasts, half-fiends, tieflings, etc. that turned their backs on the Blood War. Its particularly anarchic. Tapheon, a barely held together demon, tries to rule here but mostly goes around torturing random beings.
Arch of Eternity: 359th layer
Ruled by Eldanoth, the Prince of Crime. He once served under Orcus but left after Orcus was overthrown for a time, now his servants sabotage Orcus's plans. He's worshipped by criminals. The realm is, ironically, a huge, but finite plane of fire and snakes with a copper fortress at its center.
The Plains of Gallenshu: 377th layer
A layer constantly suffering from dirt-storms, and "bat-winged creatures" forever fighting the skull helmed tanar'ri slavers here, they may possiblly be called Armanites.
Hungry Tarns: 380th layer
A bleak and poisonous realm of fens and black bottomless lakes that once was ruled by the Obyrith Sertrous but is now abandoned since Sertrous's death. As Sertrous returns to life, the layer becomes more infested with Golothomas and other serpentine monsters.
The Worm Realm: 399th layer
Urdlen's home, another plane of tunnels, specifically a series of constanlly collasping tunnels through dirt, clay, and stone. Everything that stays here for too long gets either cancer or a fungal infection - possibly from a clash with Zuggtmoy.
The Woeful Escarand: 400th layer
Here, under a great mountain, is the court of judgement for chaotic-evils, ruled and staffed by the Nalfeshnees. If you want a legally binding agreement with a specific demon, ask a favor of the Abyss, or even make bail for an inmate of the Wells of Darkness, go to the Woeful Escarand and plead your case. If you can convince the Nalfeshnee judges, especially if you take the time to request a specific official, you can accomplish a great many wondrous and terrifying things. If you fail to successfully plead your case, or worse yet, convince everyone that you've wasted their time, getting eaten alive by the courtroom attendees will be the very least of your problems.
The Rainless Waste: 403rd layer
This layer has no ruler.
White Kingdom: 421st layer
Ruled by the King of Ghouls, who, depending on the author, is either a toady of Orcus, a hostage-turned-minion of Yeenoghu, or an undead god who's free to do his own shit.
Death Dells: 422nd layer
Death Dells: 423rd layer
One of the many rulerless, featureless layers.
Ahriman-abad: 452nd layer
Ruled by Ahrimanes, the Chief of the Cacodaemons.
Androlynne: 471st layer
Owned by Pale Night. This layer has become a battlefield where the forces of good fight to protect a group of Eladrin children who were trapped there by a deal with Pale Night. This may have backfired on Pale Night because the amount of people fighting for good on this layer is slowly causing the layer to change its appearance, and may eventually shift it out of the abyss to a higher plane.
Guttlevetch: 480th layer
A sea eternally swept by perpetual typhoons and hurricanes, its beaches of blood are littered with innumerable shipwrecks. Was ruled by Zuregurex, Lord of the Drowned, but he seriously fucked up when he tried to invade Demogorgon's layer. Zuregurex fled to the 245th layer to escape retribution.
Lair of the Beast and Mansion of the Rake: 487th layer
An entire layer stylized as a perpetually shifting baroque-style palace. Ruled by Kanchelsis the god of vampires, born of the blood of the elven gods. His primary servants are vampires, drow, and the occasional crazy birds in his aviaries. His live-in boarders-turned-sort-of-minions are a trio of crazed blood fiends, Memnul, Dagrobard, and Vonce, who are so unpleasant, they apparently scare even Kanchelsis shitless.
Noisome Vale: 489th layer
Tarnhem is the official ruler here, though he's left for a while to go and hike Mt. Everest to find himself or something (if playing Return to the Tomb of Horrors, he's now his son Acererak's guest). The layer itself is mostly volcanic, creating a poisoned and acidic atmosphere. However, a deep canyon carves its way through the layer which is infested with worms. These worms thrive in the acidic air, breathing it in and expelling breathable air as a waste byproduct. Tarnhem's estate is built along the ravine, and his servants still occupy it awaiting his return.
The Steeping Isle: 493rd layer
Siragle, The Ineffible rules over this layer.
Carroristo: 499th layer
Kali: 500th layer
Kali makes her home in a big red India-themed jungle under a cloudless red sky, on the shore of the Screaming Sea; all described in Rick Swan's adventure. Or not. Planes of Chaos has her in layer #643 instead. But she's Kali; she's probably got several layers.
Torremor: 503rd layer
Also called "The Layer of Flies" it's an aerie, built out of bridges, buildings and junk, floating in a void of filthy air, and inhabited by vrock, chasme and other flying demons. Officially ruled by Pazuzu, though, he has, more or less, abandoned it to his ex-girlfriend, Lamashtu. For a while they were a disgustingly close couple, then Lamashtu betrayed him, and then they broke up when he ate out her eyes and impaled her on top of a tower at the very top of Torremor, where ever that is. He's more or less forgiven her, but still can not be assed to un-impale her.
Occipitus: 507th layer
Adimarchus, Prince of Madness, founded this plane and was its first prince. He is a fallen Celestial and thus this layer is actually a fragmented piece of Celestia (the Seven Heavens) that fell with him. Demons avoid this layer all together, it's filled with ruins and basins. Adimarchus fought a big fight against Graz'zt and lost; whereupon Graz'zt had him evicted. The whole ordeal caused him to develop a split personality, one angelic, one demonic, which is why he's called "Prince of Madness." Now he's imprisoned in Skullrot in Pandemonium, the Great Wheel's funny farm.
Shatterstone: 524th layer
Molor, The Stinking Realm: 528th Layer
Juiblex's layer in 4th Edition. Filled with slime, filth and disease. The only settlement is a tiny collection of shacks built from mud and garbage called Thullgrime.
Vudra: 531st layer
Ruled by Shaktari, the queen of Mariliths. An archipelago of tropical, snake-infested, snake-infected islands in a sea of poison, ruled by Shaktari's favored marilith generals and daughters. Anyone who comes to Vudra without Shaktari's permission quickly discovers that literally everything about Vudra, including the snakes, the food, the demons, the foliage, the poison, the air, is all deadly poisonous. Did I mention that everything in Vudra is poisonous?
Garavond: 548th layer
Ruled by Demon Prince Haagenti, Lord of Alchemy. "Garavond" can either refer to Haagenti's orb-shaped, metallic fortress, or the lightless, airless void in which the fortress sits. The Lord of Alchemy is generally described as being a clockwork being shaped either like a winged bull or a winged minotaur, though he can also change his shape as his whim or situation demands. His fortress is filled with laboratories where he conducts his experiments, and is inhabited by his golems and demonic constructs. He often sends his minions to Shedaklah to gather more reagents and components for his many alchemical projects, and has a very comfy relationship with Zuggmoty as a result; the two often collaborate together to create horrible diseases. As a part of his experiments, and to further discourage uninvited guests, Haagenti can fill the void around his fortress with dangerous elemental energies, like lava, electricity, or Fresca.
Fleshforges: 558th layer
Ruled by Demon Prince Dwiergus, the Chrysalis Prince. He has control over the shaping of demonic races. Probably older than Pale Night. The Father of Demons in some ways. A mentor to Baphomet who taught him how to create new demons. Stats in Dragon #359. Dwiergus lives in a vast fortress called the "Chitin Palace," which sits in the center of a lake of molten flesh. Visitors to the Chitin Palace note that it is an extension of Dwiergus' body.
Soulfreeze: 566th layer
Soulfreeze is a vast snow covered glacier 200 miles wide, chocked by a cold so bitter that at the start of a each turn creatures take 1d6 cold damage and attacks with the Cold subtype are enhanced. Is ruled by Aseroth The Winter Warlock.
Shendilavri: 570th layer
Islamic Paradise! ...for Malcanthet, Queen of the Succubi. Not so much for her visitors and guests.
Prison of the Mad God: 586th layer
Goranthis: 597th layer
Shubgottia, The Endless Maze: 600th layer
Ruled by Baphomet, Prince of Beasts. He lets Pale Night, a ghost looking entity, the mother of demons allegedly, have a castle here as well though she rarely does anything. Lyktion is his palace and at his Tower of Fucking Science he experiments and creates new demon breeds and such. He has vast armies of beastmen looking demons and is revered by the minotaurs. He is smart, savage, one of the top tier princes, and is not to be fucked with. He began as a mortal creature, either as a beast that lived as a man or a man that lived as a beast, and was cursed by the gods, though obviously he made the best of it.
Conflagratum: 601rst layer
Vallashan: 628th layer
No Demon Prince claims this layer
Caverns of the Skull: 643rd layer
Kali again, Planescape edition. The authors decided that reading Sanskrit would be too hard so just watched Temple Of Doom for this one. It's full to the brim with her bloodthirsty followers, weird four-armed Xorn, and a variety of demons including Eyewings, Fetch, and Fireshadows. All of which are constantly killing each other and being reborn to fight again. The caverns constantly shift due to Kali's whim and the only portals out can only be opened by killing Kali's proxies.
Nethuria: 651st layer
Layer of the Demon Prince Vucarik the Consort of Chains.
The Rift of Corrosion: 652nd layer
A contested layer.
Zionyn: 663rd layer
Ruled by Obox-ob, Prince of Vermin. As an obyrith and, perhaps, the last surviving aspect of the old Prince of Demons, he plots here to dethrone Demogorgon and to run all tanar'ri off the Abyss. Most of Zionyn is a blasted wasteland that abuts an ocean resin. The wasteland is teeming with demonic vermin, and on the shores of the ocean of resin is a city of ekolids who capture non-ekolid visitors and either dip them in the quick-hardening resin and then use them as grotesque building materials, or inject them full of eggs in order to entertain other ekolids with "performance hatchings," or both. Planar travelers are constantly warned to avoid Zionyn.
Unnamed Layer: 665th layer
A layer with gravity and atmosphere but nothing to stand on, spells that prevent or slow down falling do not work, and all portals connected to it go in but not out, so if you stumble into it and can't teleport out or have someone rescue you, you will endlessly fall until you die of dehydration or get ripped apart by the air friction.
Bottomless Pit: 666th layer
Ruled by the Demon Lord Abaddon, Prince of Apocalypse. The center of the layer is a literal black hole singularity that is slowly eating away at all of creation and the Abyss. Anything that enters the singularity, known as the Maelstrom, is lost forever. It is theorized by the Fraternity of Order in Sigil that a portion of the energy devoured by the Maelstrom is used to keep Tharizdun imprisoned, which is a bit ironic on a plane devoted to chaos and evil.
Abyssian Ocean: Many layers
The Abyssian Ocean is not really a layer of the abyss, but is a vast ocean that connects to all of the oceans that exist in the abyss and so spans across many layers, making it a useful way of traveling around the abyss. No demon lord rules over the Abyssian Ocean as it is protected by the extremely powerful myrmyxicuses, who rule it in floating citadels made of bone, shells, and coral.
Plains of Rust: Unknown layer
Used to be a swampy layer that no one really paid attention to, until devils discovered three portals linking it to Baator. They secretly built several fortresses on the layer to use as forward bases in the Blood War. When the Demon Lords found out about it, Orcus and Juiblex used their powers to make the layer's atmosphere so toxic that no living being can survive and so corrosive that all metal rusts into dust in a matter of days. Now the only remaining inhabitants are wandering groups of undead and constructs. The portals to Baator are all buried and unusable.
Blood Sea: Unknown layer
A literal ocean of blood. Its most famous landmarks are the floating flesh islands above it. It is theorized that this blood and flesh are the remains of some long dead god or primordial. Its most common inhabitants are Blood Fiends.
The main race in the Abyss are demons. There are four different categories of demons: The Tanar'ri, the Obyriths, the Loumaras, and everything else. The Tanar'ri are the only ones who ever get shit accomplished. They are also the "most numerous". This may owe to Georg Cantor's metamathematic: the number of rational numbers (aleph-null, the smallest possible infinity) is infinite but countable (You can count them using a pattern but can't finish) but the number of all real numbers including irrational numbers (possibly aleph-one) is infinite and uncountable (No pattern of counting them can include all of them) - so some infinities are greater than others. The demon princes are finite: most of them are Tanar'ri, though there are some Obyriths. Loumaras are a relatively recent addition to the abyss. They are ghostly demons spawned from the thoughts of dead gods who can possess objects or living things.
The petitioners in the Abyss are called manes, and become a low caste of Tanar'ri. If they survive for a very long time, they can be promoted up into the ranks of the not-worthless, and even to demon lords if they're cunning enough. This is how Orcus got his start.
Most of the chaotic evil deities live in the Abyss as well, for obvious reasons, including the aforementioned Great Mother and Lolth (depending on edition). Deities here are also called demon lords, because that's generally just a title and not any kind of useful disambiguation. Also because there's plenty of layers to go around so if you've got the power you might as well rule one.
Why You Really REALLY Shouldn't Come Here
Whenever some berk (that's an idiot, in Sigil cant) drops by here, there's a chance she gets killed. It's often the case that said berk wasn't Chaotic Evil herself. In the Outer Planes, undead as such don't exist - yeah, we know, Orcus but here we're really dealing with the damned in the form of undead. So a decent person getting into the dead-book on the Abyss should have her soul end up in a final rest suited for her true alignment. Anywhere but here.
As one final barbed grind up the arse, that depends on how one dies here. Here are some chaotic energies so mighty and blasphemous that it twists a person's body and soul into a special, Abyssal sort of unlife. Such a wretch is a Bodak.