Abyss

The Abyss was the birth place of the demons, a chaotic evil universe unto itself with uncountable layers of infinite variety connected haphazardly. The Abyss's place in the cosmology of the Forgotten Realms shifted over time but the nature of the plane remained fairly constant: a violent, malevolent place where the strong survived on the backs of the weak, the weak conspired to overthrow the strong, alliances only lasted while convenient, and the landscape itself tortured the mind and body of all who dared to pass.

Cosmology
In the Great Wheel cosmology model used in first and second edition D&D, the Abyss was an Outer plane on the Great Wheel connected to the Astral plane, Pandemonium, Tarterus, and Concordant Opposition. It was estimated that the Abyss had 666 possibly infinite layers but no one could be certain. The first layer, named Pazunia after the demon lord Pazuzu, was described as a barren, dusty place baking under a red sun, broken only by great iron fortresses, huge holes in the ground, and the river Styx. Some branches of the Styx flowed down the holes (which were conduits to the various layers) and some of these holes supplied water to the Styx in reverse waterfalls.

When the Great Wheel model was replaced by the World Tree cosmology model, the river Styx was renamed the River of Blood and it flowed through all the fiendish planes (except for the Supreme Throne and the Demonweb Pits) originating in the Abyss, passing through the Blood Rift&mdash;an unusual plane that connected the Abyss with the Nine Hells &mdash;bringing the demons even closer to their arch-enemies the devils, resulting in the Blood War. Cosmologists found evidence of portals between the Abyss and the Barrens of Doom and Despair, Clangor, the Demonweb Pits, and Hammergrim, with intermittent portals to Deep Caverns and Fated Depths. The Astral plane connected all of the fiendish planes to the Prime Material Plane, but not directly to each other.

After the Spellplague, Asmodeus consumed the essence of the fallen Azuth and ended the Blood War by casting the Abyss to the furthest depths of the Elemental Chaos. The World Axis cosmology described the ruined Abyss as a domain with ever-changing features and realms, where the demons waged futile wars over meaningless empires. Once again the river Styx flowed through the Nine Hells and the Abyss, but then emptied its pollution into the Astral Sea. Other than the Styx or braving the Elemental Chaos, the only known passages to the Abyss were found in the Demonweb Pits, the Dismal Caverns, and Nishrek.

Layers


The number of layers in the Abyss is unknown and perhaps unknowable, but the main article attempts to list them regardless of which cosmology model was in favor at the time of discovery. Each layer had its own characteristics which were as varied as the demons themselves. All types of terrain, atmosphere, weather, gravity, and material composition were represented in the myriad layers. Some examples are:
 * Desert&mdash;endless tracts of black, white, or red sand with no water
 * Glacier&mdash;infinite black, white, or orange ice with few rocky peaks breaking the surface
 * Space&mdash;zero gravity, no matter except air
 * Maelstrom&mdash;swirling, bubbling chaos like Limbo
 * Volcanic&mdash;lava flows, earthquakes, volcanoes, and smoke
 * Underground&mdash;solid rock tunneled by the inhabitants
 * Water&mdash;a trackless ocean, with a surface
 * Normal&mdash;similar to a Prime plane with various flora and fauna
 * Delusion&mdash;appears to be normal but the flowers are poisonous or the trees attack

Inhabitants
Too numerous to catalog here, see the main article for information on the myriad types of demons. Non-demonic life could be found in the Abyss but was mostly likely imported from other planes.

Afterlife
In the Great Wheel cosmology model, chaotic evil souls would travel directly to the Abyss and become manes, the lowest form of sub-demon. Manes were treated as slaves, cannon fodder, and food by the demon lords. In the later World Tree cosmology model, all souls would first go to the Fugue Plane and await transport to their final resting place. Greedy demon lords would open a portal to the Fugue Plane and conduct raids to steal souls and bring them back to the Abyss to become manes. After the Spellplague, Shar reshaped the Plane of Shadow and folded in what death energy did not get absorbed into the Elemental Chaos and created the Shadowfell. After that, souls on their way to the afterlife started their journey in the Shadowfell and most made it to the Fugue Plane to await judgment, but a few remained behind or were lost.

Realms
Relatively few greater powers have carved out a realm in the Abyss because dealing with the ubiquitous hordes of demons and ambitious demon lords would require much of their attention. Those deities for whom the chaotic evil nature of the Abyss was attractive at some point in time, along with the demon "royalty" whose names are not spoken openly in civil society, are listed here in alphabetical order:
 * Ahazu the Seizer.
 * Baphomet created an infinite maze that changed form as it passed between layers. It was populated by a variety of minotaurs said to have breath weapons or magical abilities.
 * Beshaba, the Goddess of Mischief and Maid of Misfortune once made her home in the Abyss.
 * Dagon controlled a watery layer filled with hezrou and aquatic dretch.
 * Demogorgon controlled several layers, each filled with dinosaurs, wild apes, and flying monsters as well as the usual demonkind.
 * Eltab lurked on this plane in the "Hidden Layer".
 * Fraz-Urb'luu had a realm that reportedly rivaled the beauty of the Twin Paradises or Arvandor but it was an illusion covering decay and rotting flesh.
 * Graz'zt had a great palace that covered an entire layer where he would marshal his armies to fight Orcus and Demogorgon.
 * Juiblex lived in a layer full of fungus and rot, where slimes and molds fed off the decaying matter of the plane, and each other.
 * Kostchtchie controlled only part of a layer, but his ice palace was filled with creatures of cold.
 * Laogzed, god of troglodytes had a realm here.
 * Lolth spun her webs on the 65th layer in the midst of a maelstrom that was the portal to her abode on the 66th layer before she became a greater god and removed her Demonweb Pits from the Abyss to join the fiendish planes. After the Spellplague her realm survived to float in the Astral Sea.
 * Obox-ob once had a realm here.
 * Orcus had an enormous palace made of bones, rising out of a field of bone dust. He employed many undead as his guards and servants.
 * Pale Night, the Mother of Demons, lived in a bone castle in the maze of Baphomet and controlled her own layer called Androlynne.
 * Pazuzu had no real home but controlled the skies above every layer of the Abyss. He, like Charon on the Styx, controlled a primary mode of travel in the Abyss and was respected if not feared.
 * The Queen of Chaos ruled the fourteenth layer called the Steaming Fen.
 * Sseth, the god of the yuan-ti, was imprisoned in the Abyss by Set.
 * Tharizdun was imprisoned in a remote layer of the Abyss.
 * Umberlee, the Bitch Queen once hailed from the Abyss.
 * Urdlen, the evil gnome deity carved his realm out of a layer of solid rock, crisscrossing it with tunnels filled with hate for beautiful things.
 * Vaprak, deity of trolls and father of ogres, had a realm here on a layer called Shatterstone.
 * Yeenoghu had a mansion the size of a city that rolled across barren salt flats, pulled by slaves driven by gnolls.
 * Zuggtmoy had several realms scattered throughout the layers, all echos of normal environments but bathed in a purplish light and being overtaken by fungus and fungoid creatures.