Abyss

The Infinite Layers of the Abyss is the Chaotic Evil plane in Dungeons & Dragons great wheel cosmology. It's home to an infinite number of Demons, the Chaotic Evil fiends.

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.

Notable Layers

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.

Pazunia: The Plane of Infinite Portals, and 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.

The Grand Abyss: The largest sinkhole in Pazunia leads to The Grand Abyss, which is a botomless canyon full of portals to other layers.

Realm of a Million Eyes: Home of the Great Mother, princess of the Beholders. Presumably not a great place to be for anyone else, because no one's ever bothered to describe it in detail.

The Demonweb Pits: The place where Lolth hangs out with her spiders. In 3e this was changed to be an entirely separate plane, though it's still connected to the Abyss.

The Gates of Heaven: 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.

Thanatos: Orcus's crib. He spends most of his time chilling here not doing things.

The Gaping Maw: The realm of Demogorgon. Pretty much a tropical nightmare, with demon infested jungles and a roiling ocean. Dagon lives in his basement.

Inhabitants

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, and are somehow the most numerous in spite of the fact that, again, there are an infinite number of demons. Most of the demon princes are Tanar'ri, though there are some Obyriths.

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.