The 666 Layers of the Abyss
The Abyss is the Chaotic Evil home to Tanar'ri the demonic rulers of the plane. As well a number of demon lords and deities call this place home. To describe the Abyss would be to describe chaos, the layers range from extreme climates to an endless void of infinite space.
The Chief ruling class of demon the Tanar'ri are in a constant war called the "blood war" with the demons of the Nine Hells.