Chaotic Evil Plane of Existence and home of demons
It is an infinity of clutching horror. It is the home of the tanar'ri. It is where morality crumbles and ethics perish. The Abyss is all that is ugly, evil and chaotic reflected in its infinite varieties through layers beyond count. Its virtually endless layers spiral downward into ever more atrocious forms. Conventional wisdom places the number of layers in the Abyss at six hundred and sixty-six, though no one has ever explored all of them. The Abyss is far more terrible than conventional wisdopm can encompass.
Each layer has its own unique and horrible environment. No theme unites the multifarious layers other than their harsh and inhospitable natures. Lakes of caustic acid, clouds of noxious fumes, caverns of razor-sharp spikes and landscapes of magma all exist. So are parched deserts that suck the moisture fron your body, subtly poisonous winds and plains of biting insects.
The Abyss is home to tanar'ri, creatures devoted to death and destruction. A tanar'ri in the Abyss sees visitors as food or amusement. Others see especially powerful visitors as potential recruits (willing or not) in their never-ending war against the baatezu known as the Blood War.
Tanar'ri lords and deities inhabit the Abyss, including Demogorgon, Graz'zt, Pazuzu, Blibdoolpoolp, Diirinka, the Great Mother, Gruumsh, Hruggek, Lolth and many, many more. While the Abyss has layers beyond count, the top layer is well-known: the Plain of Infinite Portals.