Pandemonium

Pandemonium was the Great Wheel cosmology plane of chaotic neutral and chaotic evil alignment. Some characteristics of this plane were ascribed to the Deep Caverns in the World Tree cosmology model which was later renamed the Dismal Caverns by the World Axis cosmology model.

Description
Pandemonium was made entirely of rock pierced by innumerable tunnels and caverns large and small. All open space was filled with howling winds that carried the sound of every word, shout, or scream uttered in the place. Without adequate protection, beings with hearing were rendered temporarily deaf in a matter of seconds and permanently deaf in a few minutes. Normal fires could not survive and there was no natural light. Conversation could only be accomplished by shouting at close range.

Gravity in Pandemonium always pulled away from the center of any cavern so a traveler could walk on the floor, walls, or ceiling. Streams and rivers would flow along one surface of a cave, or inexplicably down the middle through the air. The larger caverns were hundreds to thousands of miles/kilometers deep, wide, and tall, and the winds there could lift up small humanoids and carry them far away before a dangerous landing.

Pandemonium had four known layers, each with slightly different conditions:

Pandesmos
The topmost layer with portals to Limbo, the Abyss, and Concordant Opposition was called Pandesmos and was the most habitable layer of the four. This layer had the largest caverns and the river Styx flowed through the rocky wastes. Nearly all great cities and citadels in Pandemonium were found here.

Cocytus
The second layer was called the layer of lamentation because the wind whistling through the tunnels carried the cries of anguish and the sounds of sorrow. The passages on this level were narrower making the noise shrill enough to drive insane anyone with unprotected hearing. Unlike the natural looking tunnels and caves of Pandesmos, the passages of Cocytus were carved by beings unknown to even the gods.

Phlegethon
The third layer was the layer of dark and damp. The walls were matte black and absorbed all light with no reflection&mdash;all light sources could only illuminate half as far as normal. Many rivers and streams cut through the black rock and the constant dripping of eons built up stalactites, stalagmites, and other cave formations.

Agathion
Agathion (a-GAY-thee-on ) was the deepest known layer of Pandemonium and was made of solid rock with no tunnels and only giant spherical bubbles of air or vacuum. Those bubbles with a barrier to Phlegethon were filled with huge windstorms capable of flinging even large humanoids around. Pockets without a connection to the next level were often the final resting places of things that should not be awakened.

Inhabitants
Pandemonium had no known indigenous life forms but many inhabitants that were either immigrants, visitors, exiles, marooned, or prisoners. Quasits and shadow demons were plentiful, as well as many low- to mid-level demons and the occasional demon lord-in-exile.

Afterlife
Only the spirits of the most merciless bugbears traveled to Pandemonium after death.

Realms

 * Talos the Destroyer had a realm here before he moved to the Towers of Ruin in the World Tree cosmology plane of Fury's Heart.
 * Auril the Frostmaiden dwelled in Pandemonium before she moved to her palace called Winter's Hall in the World Tree cosmology plane of Fury's Heart.
 * Ibrandul, the Lord of the Dry Depths, called the caverns of Phlegethon (or more specifically, his realms of Ibrandyllaran) home until he was slain during the Godswar by Shar, deep within Undermountain where his remains were left petrified.
 * Cyric, the Black Sun, moved his realm, the Shattered Castle (though he called it "The Castle of the Supreme Throne"), to Pandemonium after residing in Hades before.
 * Hruggek the god of bugbears had a great fortress decorated with grisly trophies of those who trespassed. He later moved to Hruggekolohk in the World Tree cosmology plane of Clangor.
 * Targus, the Netherese god of war, lived here before moving to the World Tree cosmology plane of Warrior's Rest where he was known as Garagos and claimed whatever land he occupied at the time to be his realm, Battle Guarde.
 * Hidden Betrayal was the realm in Phlegethon of the derro god Diirinka.
 * Kar'r'rga, the Dread God of the sartani resided here