The Abyss

The Abyss embodies all that is perverse, gruesome, and chaotic. Its virtually endless layers spiral downward into ever more appalling forms.
Each layer of the Abyss boasts its own horrific environment. Although no two layers are alike, they are all harsh and inhospitable. Each layer also reflects the entropic nature of the Abyss. In fact, much of what one sees or touches on the plane seems to be in a decaying, crumbling, or corroded state.

Geography

It is possible that the number of layers is infinite, but no one can be certain. The first layer, named Pazunia after the demon lord Pazuzu, is 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.
Each layer has its own characteristics, which are as varied as the demons themselves. All types of terrain, atmosphere, weather, gravity, and material composition are represented in the layers.
A few known layers are as follows...
Layer #1 - Pazunia
Layer #21 - The Sixth Pyre
Layer #341 - Odorn's Stopover
Layer #586 - Prison of the Mad God
Layer #666 - The Heart of the Abyss

Fauna & Flora

Demons run amok in the Abyss. It is the source of these chaotic evil creatures. There are myriad types of demons, too numerous to catalog. Non-demonic life could be found in the Abyss but was most likely imported from other planes.

The Demon Lords

A few greater powers have carved out a realm in the Abyss. These demon lords were archfiends who gained great power and established a position of preeminence among demonkind. Over millennia, these demons have amassed tremendous power and authority over their domain. Each demon lord had a unique appearance and set of abilities and command over at least one layer of the Abyss, although they have no rigid hierarchy.

Baphomet

Civilization is weakness and savagery is strength in the credo of Baphomet, the Horned King and the Prince of Beasts. He rules over minotaurs and others with savage hearts. He is worshiped by those who want to break the bonds of their civility and unleash their bestial nature. Baphomet himself appears as a great, black-furred minotaur, 20 feet tall with six iron horns. An infernal light burns in his red eyes. He wields a great glaive called Heartcleaver, but sometimes casts it aside so that he can charge his enemies and gore them with his horns.
Baphomets lair is his palace, the Lyktion, which is on the layer of the Abyss called the Endless Maze. Nestled within the twisting passages of the plane-wide labyrinth, the Lyktion is immaculately maintained and surrounded by a moat constructed in the fashion of a three-dimensional maze. The interior of the palace is as labyrinthine as the plane it resides on and is inhabited by minotaurs, goristros, and quasits.

Demogorgon

Prince of Demons, and Master of the Spiraling Depths, Demogorgon is the embodiment of chaos, madness, and destruction, seeking to corrupt all that is good and undermine order in the multiverse, to see everything dragged howling into the infinite depths of the Abyss. The demon lord is meld of different forms, with a saurian lower body and clawed, webbed feet, as well as suckered tentacles sprouting from the shoulders of a great apelike torso, surmounted by two hideous simian heads, both equally mad. Their gaze bring madness and confusion to any who confront it.
Demogorgon makes his lair in a palace called Abysm, found on a layer of the Abyss known as the Gaping Maw. It is a place of madness and duality; the portion of the palace that lies above the water takes the form of two serpentine towers, each crowned by a skull-shaped minaret. The bulk of the palace extends deep underwater, in chill and darkened caverns.

Fraz-Urb'luu

All demons are liars, but Fraz-Urb'luu is the Prince of Deception and Demon Lord of Illusions. He uses every trick, every ounce of demonic cunning, to manipulate his enemies - mortal and fiend alike - to do his will. He can create dreamlands and mind-bending fantasies able to deceive the most discerning foes. Its true form is like that of a great gargoyle, some 12 feet tall, with an extended, muscular neck and a smiling face framed by long, pointed ears, and lank, dark hair, and bat-like wings are furled against his powerful shoulders. He can assume other forms, however, from the hideous to the beautiful.
Fraz-Urb'luu's home lies within the Abyssal layer known as Hollow's Heart, a featureless plain of white dust with few structures on it. The lair itself is the city of Zoragmelok, a circular fortress surrounded by adamantine walls topped with razors and hooks. Corkscrew towers loom above twisted domes and vast amphitheaters, just a few examples of the impossible architecture that fills the city.

Graz'zt

The appearance of the Dark Prince is a warning that not all beautiful things are good. A subtle wrongness pervades his beauty, from the cruel cast of his features to the six fingers on each hand and six toes on each foot. Graz'zt can also transform himself at will, appearing in any humanoid form that pleases him or his onlookers.
Graz'zt's principal lair is his Argent Palace, a grandiose structure in the city of Zelatar, found within his Abyssal domain of Azzatar. His den is adorned with finery and decorations so decadent that even the wealthiest of mortals would blush at the excess.

Juiblex

Called the Faceless Lord and the Oozing Hunger in ancient grimoires, Juiblex is demon lord of slime and ooze, a noxious creature that doesn't care about the plots and schemes of others of its kind. It exists only to consume, digesting and transforming living matter into more of itself. A true horror, Juiblex is a mass of bubbling slime, swirling black and green, with glaring red eyes floating and shifting within it.
It's lair is known as the Slime Pits, a realm which Juiblex shares with Zuggtmoy. This layer of the Abyss, which is known as Shedaklah, is a bubbling morass of oozing, fetid sludge. It's landscape is covered in vast expanses of caustic and unintelligent slimes, and strange organic forms rise from the oceans of molds and oozes at Juiblex's command.

Orcus

Orcus is the Demon Prince of Undeath, known as the Blood Lord. He takes some pleasure in the sufferings of the living, but far prefers the company and service of the undead. His desire is to see all life quenched and the multiverse transformed into a vast necropolis populated solely by undead creatures under his command. He has the lower torso of a goat and a humanoid upper body with a corpulent belly swollen with rot. Great bat wings sprout from his shoulders, and his head is like the skull of a goat, the flesh nearly rotten from it. In one hand, he wields his legendary wand.
Orcus makes his lair in the fortress city of Naratyr, which is on Thanatos, the layer of the Abyss that he rules. Naratyr is an eerily quiet and cold city, its streets often empty for hours at a time. The central castle of bone has interior walls of flesh and carpets made of woven hair.

Yeenoghu

The Beast of Butchery appears as a great battle-scarred gnoll, towering 14 feet tall. Yeenoghu is the Gnoll Lord, and his creations are made in his twisted image. He wants nothing more than slaughter and senseless destruction. The gnolls are his instruments, and he drives them to ever-greater atrocities in his name. They take pleasure in causing fear before death, and sow sorrow and despair through destroying beloved things.
Yeenoghu's lair in the Abyss is called the Death Dells, its barren hills and ravines serving as one great hunting ground where he pursues captured mortals in a cruel game. It is a place of blood and death, populated by gnolls, hyena's, and ghouls with few structures of signs of civilization.

Zuggtmoy

The Demon Queen of Fungi, Lady of Rot and Decay, Zuggtmoy is an alien creature whose only desire is to infect the living with spores, transforming them into her mindless servants and, eventually, into decomposing hosts for the mushrooms, molds, and other fungi that she spawns. Utterly inhuman, she can mold her fungoid form into an approximation of a humanoid shape, draped and veiled in mycelium and lichen. Indeed, much of her appearance and manner is a souless mockery of mortal life and its many facets.
Zuggtmoy's principal lair is her palace on Shedaklah. It consists of two dozen mushrooms of pale yellow and rancid brown. these massive fungi are some of the largest in existence. They are surrounded by a field of acidic puffballs and poisonous vapors. Sharing a layer of the Abyss with Juiblex, plus their mutual insatiable hunger, has made the two demon lords mortal enemies.

Type

Dimensional plane

Owner/Ruler

Talos