Acheron

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 is more important than morality.

Acheron is where those who followed orders blindly go, or 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.

The plane is also either the source or the destination of the River Styx, depending on which direction you figure the river flows: Condensing from the winds of Pandemonium and making 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.

Layers

Acheron is an infinite void filled with floating geometric shapes, at the top most 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 down the layers the shapes become smaller and more irregular, until you get to the bottom layer 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 connecting facing unless they had the presence of mind to look up and escape before that happened. Some layers also have other dangerous environmental effects.

Avalas

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

Avalas is also the home of both the Goblin and Orc pantheons where they are in constant warfare with one another.

The Goblins inhabit Clangor which is almost entirely given over fighting with the Orcs, with its 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 Orc. The Hobgoblin deity Huggrek 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. 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 and has the mildly-positive trait, meaning dead warriors do get up at the end of the day ready to fight again. Also, 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 the 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.

Thuldanin

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 it 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 spontanesouly 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.

Tintibulus

As one descends down the layers into the plane, the floating objects get smaller and more irregular. Within Tintibulus other odd shapes outnumber cubes by a huge margin and are also far more numerous. 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.

Not much else goes on in Tintibulus.

Ocanthus

Unlike the rest of the plane, Ocanthus is not a void filled with floating shapes, but is rather a single immense frozen wall. 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 inhabitants of this layer are the 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 will cut to pieces any idiot still out in the open.