The Abyss, also known as The Infinite Layers of the Abyss, or Demonholme, is one of the planes of existence in the cosmos of Alfaysia. It is chaotic evil in alignment, and best known as the home of demons, including the numerous varieties of tanar'ri.
The Abyss is an ever spreading realm and cancer of evil and chaos that long ago somehow infected the plane of Limbo.
The Abyss is unique in that it is divided into an uncountable number of independent planes. Each has its own unique and terrible terrain. Controlling at least one level of the Abyss usually qualifies a demon to claim the rank of demon lord.
The total number of Abyssal layers are not fixed at any given time, due to the ever-expanding and corrupting nature of the plane . If left unchecked, the continued presence of demons in a world in the Million Spheres could erode the barriers between that place and the Abyss, leading to the opening of one or more portals and an ever-growing demonic presence. Over time, the Abyssal influence and the long-term presence of one or more demon lords could permanently corrupt that entire world, effectively converting it into a new ream or even layer of the Abyss. Even "spatially" separate abyssal realms are somehow connected.
The first layer of the Abyss (named thus because it is the most common layer normal encountered in travel, and has the most known connections to other realms) was called Pazunia, the Palace of 1,001 Closets, the Plain of a Thousand Portals, and the Plain of Yawning Pits. The layer was a barren wasteland of scorching winds under a relentless red sun. The surface was marked by massive dark pits: portals that plunged into many, but not all, of the deeper layers. The surface of the layer, cut through by the river Styx, connected to other dark realms in the cosmos. Its air space was ruled by the obyrith Pazuzu, Prince of the Lower Aerial Kingdoms. The landscape was marked with scattered iron fortresses claimed as fiefs by other demon lords, such as Aldinach, the Lady of Change, and Baltazo, a retired general of the Blood War, who claimed smaller domains on the layer as their own.