The Infinite Layers of the Abyss are the physical manifestation of chaos and evil, the murderous, unpredictable underbelly of the multiverse. Each layer is a unique microcosm of terror. On one, geysers of scalding steam flow from tumorous, fleshy plains to roiling skies of turbid clouds heavy with spoiled pus. On another, what appears to be an idyllic forest dappled with a fresh sunrise instead crawls with chameleon killers and cunning, razor-branched creepers. Each layer presents a different face, but one truth remains constant. The Abyss itself works to destroy its inhabitants with a passion and tenacity that might best be described as sentient.
— The Fiendish Codex I: Hordes of the Abyss