Acherion

Acheron is an outer plane on the border of lawful neutrality and evil.
Acheron is also known as the Infinite Battlefield of Acheron. It is what happens, in the cosmic sense, when you take the concentrated order of Arcadia and have it clash with the powerful evil of the nine hells. Acheron is comprised of 4 layers, each sharing a similarity of geometric shapes. Each layer is made up of geometric block like pieces of land that float effortlessly through a void of air. Each face of a block has its own gravity and walking from one face to the other can go by unnoticed if you weren't paying attention. These blocks would float through the void, colliding with each other and parting again allowing for the souls on each block to transfer from one to another. Although the floating of the blocks seems random, an individual of high intelligence could discern a pattern among them and use it to their advantage. The denizens of this plane are mostly comprised of souls from those on the material plane that had been tormented by war and unable to let go.
Avalas
Avalas, known simply as the Battleplains, was the first layer of Acheron. Amongst the four layers, this one is the most populated and has the most blocks. These blocks could range in size from small cities to large countries and were mostly cubes. These cubes were a hard metalic black and when the tired armies walked across them, the reverberations could be felt on the opposite face. Because of the large number of these blocks, the rate of collision was constant happening at least once a week giving those that lived on any given particular block very little rest. When the blocks collided, it was always an edge into a face, causing deep ridges, mountains, and crevices to form on each. The scar that was left was always perpendicular and almost always a piece of the cube would break off forming new cubes. Along the cubes surfaces were fortresses that the cursed armies would mindlessly build so they could fortify and fight amongst other cubes for causes that they aren't aware of. When a cube gets too small and collides it shatters and a new, large, and featureless cube would form from the void. The armies would often avoid these until new souls arrived to populate them as well. The river Styx flowed through this top layer as well. It did not touch every cube, but it does change its course so that it comes out of one crevice, around a mountain to another cube to disappear into another crevice and so on. It is also possible for a new unaware army to build one of their cities in the path of the Styx which ends in a flooding. Other times, the river may push its ways through and out of one cube only to push its way into another creating deadly waterfalls. Also on this top layer, were portals to the neighboring planes of Arcadia, the Nine Hells, and the Material Plane. These portals are in fixed locations in the void often being enveloped by a cube and therefore shut. When a visitor was able to come through however, they would immediately begin falling towards the closest cube, the unprepared or unaware would surely die from the fall.
Thuldinan
The second layer, Thuldinan, is known simply as, the Graveyard. The blocks on this layer are hollow and dotted with pits that are miles wide and several miles deep. On the inside lays the countless numbers of armor, weapons, contraptions, miscellaneous items, failed inventions, carriages, ships of both sea and air. The junk, refuge, and unwanted items throughout the eras appear within the cubes and turn to stone within days. There are none that dwell on this layer other than the occasional visitor hoping to find something magical before it too turns to stone.
Tintibulus
Tintibulus is the third layer. This layer is slightly different than the two above it as it blocks are made of hexagonal shapes with anywhere from 5 to 12 sided faces although any number of sides were possible. This layer is plagued by a constant ringing sound all throughout because of the near constant colliding of these blocks. When one crashes into another, it creates these large breaks in the face along natural fault lines that allow the debris to create more perfectly shaped blocks to float around the void. The only sign of life on this layer is known as the Hopping Tower. Somewhere amidst the maze of different sized blocks, exists a wizards' tower that teleports from place to place with no pattern known. It is believed to be the creation of a mad mage who either fled to the layer or was banished there, it is unknown of this mage is still alive or not.
Ocanthus
The last and most bottom layer is Ocanthus. The blocks on this layer are square like the first two. Unlike the first two layers, these blocks are so thin that they may appear two dimensional. These blocks are made of a black ice like material, paired with them being razor thin, and that they can be as small as a few inches makes them near impossible to see in the black air-filled void. The size varies between a few inches and some fragments are large enough to support structures resting on top of them, some are even so large that they generate their own gravity and pull in objects to them. Unless you had some magical or alternative means, you would not be able to see all of the fragments around you creating the dangerous situation of running into them. These ice fragments are so thin that you can float into one and it would go clean through you and most other materials as these fragments have the same quality as a Vorpal blade. At the bottom of this layer is also the actual surface of Acheron which is also covered in an almost metalic like black ice. The nature and purpose of this endless ice surface is unkown and speculated on. It is however the source for all of the razor ice fragments that float through this layer. As the shards that have already broken off collide with the surface, both the surface and fragment shatter creating even more fragments. Other than the occasional mining operation from Mechanus, this layer is uninhabitable.