"I was just following orders."
- – The Nuremburg Defense
Acheron (later expanded into The Infernal Battlefield of Acheron) is a plane which exists on the Evil side of Lawful Neutral on the cosmological wheel of the multiverse, though not actually evil because it represents a plane of existence where imposing order and conformity is more important than morality or reasoning, to the point that you are just one more faceless pawn who gets played in a pointless stuggle that never ends.
Acheron is the afterlife for those who followed dubious orders blindly or without question, or for those who fought for causes they never believed in. They come here to fight to all eternity in futile wars that have no conclusion and offer no glory... and unlike Ysgard there is (usually) no miraculous resurrection at the end of the day: if you die on Acheron, even if you are a petitioner you're still dead for good.
It is the plane where things that are lost inevitably end up; armies that fought lost causes continue to fight for all eternity never to win anything substantial, lost items gravitate here and end up being petrified in time and lie forgotten under scrapheaps, and even lost memories end up frozen in ice, never to be reclaimed.
The plane is also either the source or the destination of the River Styx, depending on which direction you figure the river flows: it condenses from the winds of Pandemonium and makes its way across the lower planes to the bottom of Acheron where it freezes into an infinite sheet of ice containing all of the memories the Styx has stolen over the cosmos.
Acheron is an infinite void filled with floating geometric shapes; at the topmost layer of the plane these are almost uniformly shaped as cubes each with their own directional gravity and are large enough that entire nations can be housed on them. As you descend the layers the shapes become smaller and more irregular, until you get to the bottom layer of Ocanthus where only things flying around are razor sharp shards of ice.
Travel is always dangerous, not least because of all of the roving armies that would be happy to conscript a wandering traveller. The cubes/shapes are in constant motion and have a tendency to crash into each other every now and then, squashing any unfortunates who happen to be on the colliding faces unless they had the presence of mind to look up and escape before that happened. Some layers also have other dangerous environmental effects.
The topmost layer of Acheron is the battlefield. It contains the largest cubes where armies can claim their territory and space enough for military actions to be fought. Here the sound of battle persists endlessly
The Goblins inhabit Clangor which is almost entirely given over to fighting with the Orcs, with caverns filled with Goblinoids of various types as well as Wolves for riding. Depending on your perspective, the Kobold deity Kurtulmak also makes his home either on Clangor or in Avernus, though quite possibly the two locations are linked and the Goblins spend as much time fighting the Kobolds as the Orcs. The Hobgoblin deity Hruggek supposedly resides here but maintains links to the Abyss. Frequently the Blood War spills its way into Clangor where the Goblins and Kobolds are happy enough to let it resolve itself while acting in an "advisory" capacity for both sides.
It might seem odd that the typically Chaotic Orcs make their home on a lawful plane, but it's a place of constant battle so it would probably be natural that they would end up there (more likely is that the orcs in 2e, when Planescape was created, were Lawful Evil. It was only in 3e that they became Chaotic.). Also the presence of the deities cancels out lawful-aligned trait of the plane, meaning they suffer no penalties. Also (at least according to the Forgotten Realms rules, where the cosmological arrangement is different, although it need not be entirely incompatible with core D&D, since cosmological perspectives are often subjective depending on where you start from) the Orc realm of Nishrek is also mildly evil aligned instead of being lawful and also has the mildly-positive trait, meaning dead warriors get up at the end of the day ready to fight again. What's more, inside Gruumsh's Iron Fortress any Orc gets damage reduction 25/- which means it is quite possibly one of the most well-defended locations in the entire cosmos outside of Malsheem in the Nine Hells.
Hextor also made his realm here, called Scourgehold (which presumably became Banehold when Bane seemed to usurp/replace Hextor's position as a god of strife and tyranny during the transition to 4e) which is a place of constant battle replete with siege engines and constantly rebuilt fortifications. If the Bane connection is to be followed, it probably has more in common with Hades and is less intense with battle, but filled with far more foreboding.
The next layer is made up of slightly smaller cubes than Avalas, and it is the scrapheap of the multiverse. Lost or forgotten objects find their way here and scavengers can make a living for themselves by collecting hidden treasures or technological devices which end up here, but mostly this is a place for the refuse of every war that was ever fought, so the place is entirely littered with broken siege engines or weapons whose owners perished and would never need them again.
There are very few inhabitants on Thuldanin and even among those there are likely to be virtually no permanent residents at all. This is entirely because the layer has a quality which petrifies objects and creatures which remain on the plane for too long; for every thirty days spent in Thuldanin, there is a 1% chance of spontaneously turning to stone. There is no proof or protection against this, and there is no reversal of this petrification short of Wish or Miracle spells either, but travelers can Rules Lawyer themselves around this by spending 29 days in the layer, then leaving for a day and coming back afterwards.
It's also part of the reason that simply searching through the scrap piles is not as lucrative as you might think, because all of the older objects will have been turned to stone themselves, rendering them useless. But that's not to say a scavenger won't get lucky, or find something that can't be copied.
One of the few native inhabitants are rust dragons, which scrabble cockroach-style over the debris. They can corrode and consume metal, which makes any scavenging trip even more hazardous.
The Duergar God Laduguer makes his home here in a fortress made entirely of leftover debris from war; even the walls are made up of shields stapled together. Obviously he can negate the petrification effect because he is a deity.
As one descends down the layers into the plane, the floating objects get smaller and more irregular. Within Tintibulus other odd shapes outnumber cubes in contrast to Avalas. Travel here is particularly perilous because collisions between objects occurs with far more frequency, to the point that the sounds of battle typically audible throughout Acheron is entirely drowned out by the banging and crashing of the floating shapes floating into each other. A layer of thick dust covers many objects, at least partly from other objects that shattered on impact.
What passes for inhabitants are various mages who stay to either stash their libraries behind abjuration spells or research in another's - the constant battering of celestial objects is apparently seen as the ultimate robbery deterrent. Tintibulus is considered a good place for magical research among casters who can survive there.
Unlike the rest of the plane, Ocanthus is not a void filled with floating shapes but rather a single immense frozen floor. As mentioned earlier, some believe the ice to be the destination of the river Styx. It is not known if the ice is simply the "end" of the plane, or if there is anything buried underneath it-- or even if the ice block really infinite at all or whether it is just another floating cube that nobody has bothered to circumvent to find out what's on the other side. Regardless of what might truly be the case, lets just say the ice block is inconceivably huge and leave it at that.
Travelling the surface is dangerous because the only other objects that float around here are tiny shards of broken ice which whip up in storms of blades, each striking with the force of a greatsword which ignores any form of damage reduction.
The only permanent inhabitants of this layer are the Bladelings in their city of Zoronor which is protected by from the flying razors by a forest of magical trees, but they are entirely xenophobic and won't take kindly to unexpected visitors. Anyone else living here will be servants of whichever deities who have chosen to make their realms here, because deities can control their surroundings and impose whichever effects they wish: one such deity is Wee Jas who maintains the Cabal Macabre, a castle of black ice. Wee Jas is said to walk the sheet of Ocanthus searching for lost memories of magical power or dying moments, but occasionally she brings back mortals kidnapped from other planes who inevitably die for failing to pass her tests. Simply put, Ocanthus is inhospitable; if the inhabitants don't kill you, the ice storms simply destroy any mortal attempt at erecting shelter, then cut to pieces any idiot still out in the open.
Since Acheron is not really a place for settling down and making a family, the inhabitants you'll find in Acheron generally depend on which layer you're travelling on. In the upper Avalas there will be numberless Orcs and Goblins, more than you could possibly find anywhere else if only because Acheron is their final destination in the afterlife.
As mentioned there are the Bladelings and their cousin species, the Spikers; both races are similar to Tieflings and according to their history may have migrated to Acheron from elsewhere long in the past, but who don't really come out to play. Their primary export is Assassins, so make of that what you will.
Some might be surprised that it seems more of the major lawful evil gods live here than in Hell, but it makes a lot more sense if you remember that a lot of these big name lawful evil gods would really rather not have to take marching orders from Asmodeus and his Archdevils.