Abyss, the

The Abyss is the plane where Chaos and Evil mix in equal amounts. It has an unknown, possibly infinite number of layers, each showcasing depravity and atrocity in a unique way.

The Abyss is the native plane of the tanar'ri, who rule it, although there are lesser demonic species as well, such as the varrangoin or abyss bats, quasits, gremlins, and shadow fiends.

Contrary to popular representations, the layers of the Abyss are not stacked neatly on top of one another like a pack of cards. Instead they grow from the first and central layer, the Plain of Infinite Portals, like obscene, branching tentacles.

The Plain of Infinite Portals is a barren, infinite wasteland peppered with mountains and black, gaping pits. Each of these pits leads to a "deeper" layer of the Abyss.

Each layer is dimly sentient, and bonds with a demon lord powerful enough to master it. Even so, being an Abyssal Lord is a perpetual struggle, and the Abyss has been known to devour lords whose wills are less than that of their layers. On the other hand, an Abyssal layer that goes without a lord to define it for too many centuries eventually is absorbed back into the layer it originally budded from, so the relationship between lord and layer is one born of mutual necessity.

Sources:
Planes of Chaos
Faces of Evil: The Fiends
Fiendish Codex I: Hordes of the Abyss (3rd ed)

Additional Resources:
The Concordance of the Abyss - A listing of known abyssal layers
Fiendish Codex I: The Lost Entries