Baator: aka Inferno; the Halls of the Damned; the Fire Everlasting. Few names evoke as many dramatic and terrifying images in the minds of mortals as The Nine Hells, and with good reason. Baator personifies the epitome of the Lawful Evil ethos as a place of punishment, oppression, and endless, soul-crushing conformity. It is the eternal destination for those who force their will through unjust violence, tyrants, cruel overseers, malicious judges, unscrupulous barristers, and (most) politicians. Basically, anyone who dominates others through lawful force or manipulates the law at the expense of others will probably find him-or-herself on a one way trip to the big H-E-Double Hockey Stick when they get their ticket punched.
True to its lawful nature, each layer of Baator is ruled by a despotic head of state known collectively as the Lords of Nine. The Lords of Nine are the princes and archdukes of Hell, wielding absolute tyrannical authority over their respective layers. Despite their immense power, they concern themselves with little else aside from internal (infernal) politics and the orderly administration of their dominion. They form the pinnacle of a vast, rigid caste system populated primarily by Baator's most numerous inhabitants, the baatezu (also known as Devils). Greater Baatezu serve the Lords of Nine directly as generals and officers in Hell's massive armies as well its barons and overseers. The least and lesser Devils serve as rank and file soldiers as well as the clerks and petty officials who maintain the labyrinthian bureaucracy that keeps Baator running. Baatezu of all descriptions fill in the middle of the hierarchy and serve as everything from lieutenants to emissaries, courtiers, spies, and administrators. The very bottom caste of infernal society is made entirely of petitioners, the souls of dead mortals whose deeds in life earned them an extended stay in Baator as their eternal reward. Petitioners are treated more like cattle than slaves by the baatezu, who see it as their duty and privilege to torture, humiliate, and degrade these unfortunate souls in a manner appropriate to their individual sadistic whims.
Baatezu are almost without exception arrogant, cruel, opportunistic, paranoid, self-serving, and megalomaniacal. As the very embodiment of malign lawfulness, however, baatezu frame all of their actions within a twisted, nearly incomprehensible code of honor. A Devil can always be counted on to keep its word and will never violate the terms of an agreement or act in such a way as to breach its duty to its leaders. That being said, however, baatezu are true masters of exploiting loopholes and manipulating words. They will only ever honor the exact wording of a deal or command but never the spirit (unless it is to the baatezu's benefit). Though they must honor their duty to their rulers and execute their orders, they have no concept of loyalty to them. Baatezu are constantly looking for the means to undermine their superiors and sabotage their peers while promoting their own self-interest. All the while they must plan and execute their machinations with exceeding craft and subtlety as outright insubordination is considered among the gravest of transgression by baatezu. Thus, Baatorian society is an endless game of political maneuvering up a particularly vicious corporate ladder where the penalty for failure is limited only by the patience and imagination of the Devil meting out punishment.
Baator is made up of nine descending layers and does not resemble a certain Italian poet's vision of Hell at all. These nine descending layers form (conceptually, at least) an inverted cone with the "largest" layer (Avernus) at the top and the "smallest" (Nessus) at the bottom (although some layers do have specific areas that are effectively infinite in size). Baator's layers each embody a unique vision of torment, misery, exile, and suffering. Trying to physically leave the boundary of any Hellish layer by magical or mundane means will get you torn to pieces by the "gnawing vacuum" beyond the layers' edges in short order.
4th Edition D&D changed the layout of the Nine Hells, so that instead of resembling a stack of floating pancakes, the Nine Hells can be seen from the Astral Sea like a single planet, with the visible "surface" of the "planet" being the first layer, Avernus, and the other layers now arranged as smaller spheres nested inside each other, with the final layer Nessus situated on the underside of the Eighth layer Cania, facing the "planet's" fiery core.
Regardless of the configuration you choose to play with, remember that the Nine Hells is not a nice place. The Devils may be somewhat more predictable than the Demons or Yugoloths, but they are still incarnations of evil and entering the Nine Hells means you're on their turf. Abandon hope, all ye who enter here!
Avernus, the first layer of Baator, serves as the proverbial "Gates of Hell" and is thus the layer most readily accessible from other planes of existence. Avernus is an endless, rocky, blasted wasteland where lightning streaks across the blood-red sky and balls of molten fire periodically streak overhead before crashing into the dust-choked hellscape in a pyroclastic explosion. Lesser Devils herd throngs of newly arrived petitioners through the gates like cattle, prodding with spears and lashing with whips, pausing at their fancy to torment and humiliate the terrified, shrieking wretches. Avernus also serves as the martial fields where the infernal legions muster and drill endlessly in preparation for the next battle in the eternal Blood War. The Pit Fiend Bel (AKA the Pretender) oversees Avernus as both military governor and commander of its forces. Other notable inhabitants of Avernus include Tiamat, the evil mother goddess of all chromatic dragons and Kurtulmak, the spiteful god of Kobolds.
If Avernus had a theme, it might be "the wages of betrayal" or "the wages of sin." Not only is it a horrible place where Lawful Evil damned souls end up to be painfully stripped of their identities and turned into fodder for the Blood War or fuel for Hell's Archdukes, but Bel (as detailed on his own section of the Archdevil page), who won his position via betrayal of his liege and became one of the nine Archdukes of Hell, is for all his strength and schemes effectively stuck where he is in Hell's hierarchy and can do little other than fight the demons who continually invade his layer. For all its awfulness, Avernus is just a taste of the horrors to come. What? You say you need to go deeper? Don't say the sign didn't warn you . . .
Dis is both the name of the second layer of Baator as well as its principal city. The two are used interchangeably, as the "City of Pain" sprawls across a blackened iron wasteland to the point where it is difficult to determine exactly where one ends and the other begins. Black mountain peaks thrust into the ash-green skies all along the horizon, although no one can tell if they are a geographic feature or simply a mirage brought about by planar metaphysics.
Within the city itself, the iron walls smoke with intense heat, causing painful burns to any exposed flesh. Once you are inside, the city is effectively infinite in size due to the way space is warped there, and short of magic you can't leave the city without devilish permission. The air is filled with the acrid stench of hot iron and the screams of the miserable souls who are condemned to spend eternity building, tearing down, and rebuilding the city with their bare hands. The archduke Dispater, Lord of the Second, rules the City of Pain from an enormous citadel of iron and lead known as the Iron Tower. From here, he oversees the ceaseless toil of petitioners while lording over his vassals with an iron fist. Mirroring Dispater's omnipresent paranoia, the Iron Tower always seems to be one block away due to spatial warping unless you have a Devil guide or magical means to truly approach the structure. A similar effect can be seen in how the City of Dis is visible on the horizon no matter where you look. The City itself also has a location named "God Street" where any Lawful Evil deities (including homebrew ones) not powerful enough to have their own godly realms can be found.
The theme of Dis is likely how it embodies the nature of Hell as a torturous prison for Lawful Evil souls, from the newest damned soul to the highest Archduke. Funnily enough, Dispater is both Dis' chief ruler and its chief prisoner, his paranoid nature leaving him holed up in his Iron Tower where he is invulnerable but still a slave to his fears, leaving the structure only at Asmodeus' summons, and then only when just sending an avatar won't do.
The Third Hell is a largely a cold, fetid swamp. The air is thick with fog and the stench of rot and decay. Stinking, oily rain and sleet fall from the leaden skies, which are periodically illuminated by streaks of green and purple lightning. Sharp ridges of obsidian thrust out irregularly from the muck along with small, muddy islands containing the twisted boles and skeletal limbs of lifeless trees. At the center of the great bog looms the layer's namesake -- Minauros, the Sinking City. The world-sized metropolis is built upon great stone plinths that thrust into the fathomless depths of the swamp below. Here petitioners are lashed to the columns, screaming and wailing with increasing panic as they sink helplessly, inch by inch, to drown in the brackish, frigid waters. The walls and avenues of the city proper remain in a constant state of decay as the city slowly, inexorably sinks into the ooze upon which it was built. Work details of petitioners constantly extract stone from the surrounding mire, using it to reinforce the city's support pillars in a futile attempt to delay its inevitable plunge into the muck. It is said that the current city is but the tip of an impossibly vast structure, as each layer is built upon the ruins of layers that have been reclaimed by the endlessly hungering mire.
Somewhere far from the crumbling and reeking avenues of the city of Minauros lies Jangling Hiter, called the City of Chains. The city looms like a ghost out of the fog and quagmire. Turrets, spires, and towers thrust skyward along with massive links of corroded metal, disappearing into the dense lower atmosphere. These mammoth chains prevent The Jangling City from suffering the same sinking fate as Minauros, as it is anchored to the underside of Dis. As its name implies, the entire city of Jangling Hiter is fashioned of chains. That's right -- every dwelling, shop, tower, warehouse, abattoir, avenue, and alleyway is fashioned from links of metal which continually rust in the frequent rain, giving the entire city an acrid, metallic stench that barely overpowers the noisome stench of the swamp beneath.
Minauros' theme is almost certainly "the burden of greed." Never mind that Mammon, the Lord of the Third layer, is officially described as a patron of greed, or the way his abode continuously sinks, slowly consigning many of his servants, plundered riches, and booty to a bottomless swamp requiring them to be continuously moved or replaced with MOAR. Don't pay any attention to the fact that Mammon's avaricious and treacherous nature has alienated everyone else in the Nine Hells who might ally with him either; Minauros is not a layer that provides poetic justice to show what happens when greed overrides all other concerns, no sir.
The fourth circle is the Hell that most resembles the stereotype of a fiery world of eternal damnation, filled with active volcanoes, rivers of liquid fire, molten rock, ash hills, smoking pits, unbearable heat, all wracked by tremors and earthquakes. Even the air seems aflame and thus Phlegethos is considered to be fire-dominant. In the World Axis view, Phlegethos is a cavern several miles/kilometers below Minauros, where burning lava pours out of fissures in the ceiling. The city of Abriymoch is the seat of power in this realm, built of hardened magma, obsidian, and crystal in the caldera of an extinct volcano which provides visitors some protection from the elemental environment found throughout the rest of the plane.
Stygia is frickin' cold. Most of the layer is an icy, murky ocean fed by the River Styx. Tantlin, the City of Ice, is built upon an ice floe and is ruled over by Levistus, insomuch as much as he can rule over anything while popsicled in a huge iceberg.
Originally, Malbolge was an endless rocky slope made of craggy black stone and pits of fire. The air was hot and full of choking ash and vapors. There was no flat ground, so avalanches were common. Nobles dwelt in copper fortresses, the metal plating of which helped defend against the frequent rockslides. That, however, was before the gruesome fate of Malagard, the Hag Countess, reshaped the layer forever. Now, her exploded and grotesquely enlarged body is the layer, for all intents and purposes.
Malbolge has had the most known rulers of any layer of Baator. The first was Behrit, who was destroyed by Asmodeus for violating rules regarding devilish promotion. After him was Moloch, until his hag advisor Malagard tricked him into rebelling against Asmodeus. Moloch was deposed and Malagard was made ruler in his place. She didn't rule for long, though- Asmodeus made her body grow until it burst open in order to appoint his daughter Glasya as the new ruler, using Malagard's skull as her palace.
The seventh layer of Baator is made of ruins. Nothing exists here that hasn't been damaged, defaced, destroyed, or otherwise corrupted. Major tourist attractions, if you're crazy enough to try and tour the place, are Grenpoli, the city of diplomacy and treachery run by Erinyes Mysdemn Wordtwister, and Malagard, the city of black spires from which Baalzebul rules over the plane. The Lord of the Flies is never satisfied with his domain, and always demands his cities torn down and reconstructed based on some tiny flaw. He wants nothing less than absolute perfection- even if he has to destroy his layer or all of Baator to achieve it.
Maladomini's theme seems to be how sin ruins good. Its ruler Baalzebul is a fallen Archon whose pride drives him to continuously destroy his own realm because it isn't perfect enough, making the layer one gigantic ruin that will never be truly rebuilt.
Much of Caina is a land of ice-covered boulders and mountains, ruins of stone and sprawling glaciers. The jagged mountains war with frigid glaciers, each grinding fiercely away at the other. Avalanches holding thousands of tons of snow rush down from the mountains regularly, crushing anyone slow or unlucky. A visitor will find Caina numbingly cold (-50°C). Without heat, most warm-blooded creatures can only survive for a few hours before suffering chills, later frostbite, and then death. Certainly any such creature that goes to sleep, falls unconscious, or is rendered immobile in the open without heat will die shortly.
Cania is among the most sparsely populated of the Hells, with a wary traveller standing a fair chance of avoiding notice. Were it not for the cold and the lack of food (some tales speak of remorhaz or glacier worms in Caina, but if any exist they must be very rare), Caina would offer intruders many inviting places of concealment there are many hidden valleys in the mountains, and countless ice caves. Beware though, the denizens of the plane are attracted to any fire, so in the unlikely even that any travellers are able to light one, it will likely be the last thing they ever do. The only parts of Cania that can be called even remotely hospitable are its fringes. These regions are actually uncomfortably hot, and criss-crossed with near-freezing rivers, the slimy waters of which give way to marshes of icy ooze are they creep closer to the center of the plane. The only exception to this are the palaces. The Lords of the Hells all reside in one each, and many lesser (in a relative sense) devils keep palaces that are reportedly kept at temperatures and pressures considered comfortable for most planar mortals
The final layer of Baator is a land of extremes. In its blasted landscape one can find regions as icy as Cania, as hot as Phlegethos, and as unpleasant as both combined. Nessus is cut through by massive canyons, deeper than the greatest ocean trench. Asmodeus rules here from the city of Malsheem, which lies directly underneath the portal from Cania.