The lower plane, sectioned into 666 layers, inhabited by the demons of chaotic evil and destination of chaotic evil souls
It is an infinity of clutching horror. It is home to demons. It us where morality crumbles and ethics perish.
The Abyss is all that is ugly, all that is evil, and all that is chaotic reflected in infinite variety through layers beyond count. Its virtually endless layers spiral downward into ever more atrocious forms. Conventional wisdom places the number of layers of the Abyss at 666, though there may be far more. The whole point of the Abyss, after all, is that it's far more terrible than conventional wisdom could ever encompass.
Each layer of the Abyss has its' own unique, horrific environment. No theme unifies the multifarious layers other than their harsh, inhospitable nature. Lakes of caustic acid, clouds of noxious fumes, caverns of razor-sharp spikes, and landscapes of magma are all possibilities. So are less immediately deadly terrains such as parched salt deserts, subtly poisonous winds, and plains of biting insects.
The Abyss is home to demons, creatures devoted to death and destruction. A demon in the Abyss looks upon visitors as food or a source of amusement. Some see powerful visitors as potential recruits (willing or not) in the Blood War.