Abyss

The Abyss, sometimes more completely called the Infinite Layers of the Abyss features an endless variety of evil. Shaped as an endless number of layers, one atop the other, the Abyss stretches into infinity, though some Faerûnian scholars have theorized that the plane has no more than 666 layers. But this is unknown, as no adventurers have been known to return from anywhere below around the two hundredth layer.

The only constant in this chaotic realm is the presence of the adjacent Blood Rift where the demons fight their mortal enemies, the devils. The proximity of the Blood Rift to both the Abyss and the Nine Hells allows for the demons and devils to wage an unceasing war upon each other. But unlike the Nine Hells, which is always connected to the Blood Rift by its top most layer (called Avernus), the ever shifting nature of the Abyss brings different layers into proximity with the Blood Rift at different times, allowing the demons to constantly bring fresh forces to bear in their constant struggle known as the Blood War.

Layers
Each layer is different from all the others, the only apparent similarity being that the plane seems to try and inflict as much harm on travelers with each layer as possible. It might even be said that the plane is as varied and evil as the demons who inhabit it. Even deities dwell in the Abyss, like Umberlee and Merrshaulk, as well as Demon Lords that are mostly worshiped by one kind of species, such as Sess'Inik, the lord of Lizardfolk, who makes his home on the Seventh layer, Phantom Plane and Vaprak, patron of Ogres, Ogre mages and Trolls. Vaprak is located on the layer of Shatterstone.

The 66th layer used to be home to the Demonweb Pits and the drow pantheon (excluding Eilistraee who makes her home in Arvandor) until they were torn out of the Abyss and became a separate outer plane, following the Silence of Lolth.

Abyss Links
Various layers of the Abyss include portals to the Demonweb Pits, Beshaba's realm in the Barrens of Doom and Despair, Hruggek's realm in Clangor, the realm of Urdlen in Hammergrim and the Fugue Plane. Portals also exist to other planes like the Deep Caverns and Fated Depths, and indeed the River of Blood flows through the Abyss and into most of the other Fiendish Planes.

The first layer of the Abyss is called the Plain of a Thousand Portals, a pock-marked plane containing Pazuzu, demon lord of the skies. The plane is covered in holes that when climbed down, lead to various layers of the Abyss.

Abyss Inhabitants
The infinite layers of the Abyss are home to the tanar'ri, like the succubi and other demons such as the obyrith and loumara, as well as normal demons that fall into no such classification. The most noteworthy inhabitants of the Abyss however are the Demon Lords that make their home there. Such beings are demons who have almost ascended to god-hood, and rule over other demons with an iron fist. Kostchtchie, Demogorgon, Pale Night, Dagon, Pazuzu, Graz'zt, Fraz-Urb'luu, Baphomet, Yeenoghu Juiblex, and Zuggtmoy are noteworthy powers in the Abyss, especially Demogorgon, the "Prince of Demons," but the Demon Lord closest to ascending to godhood currently is Orcus, the demon prince of the Undead.

Abyssal Petitioners
When demons steal souls from the Fugue Plane and bring them to the Abyss, those souls become petitioners called manes. A mane has pale white skin, cruel claws, sharp teeth, sparse hair and white eyes. The few manes that survive for many years are sometimes "promoted" to lesser demon types like quasits, though they retain no memory of their former lives.

Manes have the following qualities:
 * Additional Immunities: Electricity and poison.
 * Resistances: Acid and fire.
 * Other Special Qualities: Acidic vapor, no planar commitment
 * Acidic Vapor: When a mane is slain, it discorporates into a cloud of noxious vapor.
 * No Planar Commitment: Unlike most other petitioners, manes can leave the plane they call home.

The Abyss has no divine realms, and no true deities make their home there. However, a great many demon princes vie for power and territory on this plane. A few prominent beings are Demogorgon, Eltab, Yeenoghu, Orcus, Graz'zt, Baphomet and Kostchtchie.