Pandemonium

The Windswept Depths of Pandemonium is the outer plane where Chaotic Evil and Chaotic Neutral petitioners are sent after death. Pandemonium is a large, complex cavern that never ends. Compounding this problem are howling winds that drive most of its residents mad.
Pandemonium is one of a number of alignment-based outer planes that form part of the standard Dungeons & Dragons (D&D) cosmology, used in the Planescape, Greyhawk and some editions of the Forgotten Realms campaign settings.

Description

Pandemonium is the Howling Plane, the plane of madness, darkness, and deafening winds. It is where Chaos turns sick and damaged, becoming mentally ill, demented and deranged. It is a place of winding, mazelike tunnels with no rhyme or reason behind them.
The River Styx runs through Pandemonium in trickles and small streams, not yet joined into the great dark torrent that flows through the other Lower Planes. It also much less potent in Pandemonium than elsewhere.

Inhabitants

The gnoll deity Gorellik wanders throughout Pandemonium. Somewhere in the depths of the plane is a black tower haunted by undead, said to have been the abode of Vecna at one point.

Structure

As an outer plane, Pandemonium is spatially infinite, further consisting of four infinite layers (or sub-planes). Pandemonium’s first layer shares borders with the neighboring planes of Limbo, the Abyss, and the Outlands; travel is possible between Pandemonium and these planes at certain locations. The Outlands town of Bedlam actually contains a half-dozen gates to Pandemonium, and it's likely they lead to at least that many different places. Portals in Pandemonium are flat squares on the sides of caverns. Yellow ones usually lead to Limbo, red to the Abyss, and black to the Outlands, with a 1 in 5 chance of error.
The River Styx has its headwaters in Pandemonium, flowing from the blood, venom, suffering, and melting ice of the realm of Winter's Hall in trickles and streams until at last it becomes the dark artery of the Lower Planes.

Layers

Each of the four layers of Pandemonium is successively deeper within the caverns.

Pandesmos

Pandesmos (which borders the Outlands, the Astral Plane, Limbo, and the Abyss) is the largest and (relatively) most hospitable of Pandemonium's layers. The headwaters for the River Styx are found here.
Pandesmos is the location of a number of godly realms, including Loki and Auril's realm of Winter's Hall, Talos's realm of Towers of Ruin, and Ho Masubi's realm of Uchi-bi. Raxivort's realm, the Black Sewers, is located on this layer.
The Cavern of the Self is a cave where all the walls are as reflective as glass. Those who gaze into the walls see visions of themselves, displaying different emotions at different periods of their lives. (Planar Handbook, 171)
In a large cavern on the edge of Winter's Hall is Keener's Wall, which was founded by missionaries of a goddess of civilization. About half of the population is functional (as opposed to hopelessly insane), surviving by trading with the frost giants of Loki's realm. Bleakers from the Madhouse keep watch over Keener's Wall, looking for new arrivals with magical secrets. (The Plane Above, 117)
The Madhouse is a sprawling citadel under the care of the Bleak Cabal. It's an informal center of trade built around an old inn that has, over the centuries, become a kind of city hall. The ruler of the Madhouse, Maris Warrow, bears the title Matron of the Inn. (Planes of Chaos: The Book of Chaos, page 92)
Myriad of Gales is a bardic college dedicated to learning the music of the insane. Strangers are sometimes kidnapped and exposed to the plane's howling winds in order to tease secrets from their minds and lips. The bards of Myriad of Gales are rivals of the Bleak Cabal, and both groups would eliminate one another if they could. (The Plane Above, 117)

Cocytus

Cocytus is also known as the "layer of lamentation", for this is where the winds are the strongest. This layer was seemingly carved out by some ancient, maddened civilization.
Cocytus is the location of a number of godly realms, including the god Erythnul's realm, the Fields of Malice; the bugbear deity Hruggek's realm of Hruggekolohk; and Cyric's realm of the Shattered Castle.
The Harmonica is a cavern filled with many hundreds of cyclopean columns protruding from the walls on all sides, with spiral staircases built on a giant's scale and perforated with holes from which winds wail through unpredictably. The site is a magnet for rumors about the boons and curses that explorers might receive from the mysterious magic of the area. (Planes of Chaos: The Travelogue, page 34)
Howler's Crag is a jagged stone spike built on the grave of a dead phoenix god whose followers are said to have ruled the planes and controlled the secrets of reincarnation. A group of mystics called the Brotherhood of the Phoenix resided in burrows nearby, though some say they may have died out. The crag is covered in runes and letters, most of them indecipherable, and the tip glows with faerie fire. Anything shouted from the peak of Howler's Crag can be heard by its intended recipient anywhere in the multiverse, whether they're alive or dead. (Planes of Chaos: The Book of Chaos, page 96)
The Shrine of Acererak is a great statue of a humanoid skull, with jewellike stones in its eyesockets and in place of its teeth. Surrounding it are many other unmarked graves and tombs, and many, undead and otherwise, come to the shrine to learn the secrets of influencing the undead. (Planar Handbook, 177)
The Unseen Path is a cobblestone road that wanders unpredictably from floor to wall to ceiling, enchanted so that those who wander off it are befelled by a reverse gravity effect, and with a deeper darkness effect that inhibits sight. Those who make it the entire length of the path are said to gain a temporary darkvision ability. (Planar Handbook, 157)

Phlegethon

Phlegethon, darker and wetter than the other layers, is the location of a number of godly realms, including the Queen of Air and Darkness's realm, the Unseelie Court; Zeboim's realm, The Maelstrom; Erythnul's realm, the Citadel of Slaughter; and the derro deity Diirinka's realm, Hidden Betrayal. The City of Windglum, home to banished families of many races, can also be found here.
A fat, bloblike creature called the Ingress Mother forms a living portal from Phlegethon to the next layer, Agathion. Its children, the Ingress, will direct their Mother to take travelers where they want to go in exchange for food, shoving both food and travelers into the Mother's hungry maw.

Agathion

Agathion is a place of caverns sealed in an infinity of rock with no tunnels to connect them. This layer is a prison for artifacts and beings too terrible even for the gods to bear. The Lady of Pain once used it to imprison her enemies before she learned the dark of creating ethereal mazes.
Miska the Wolf-Spider and the fallen god Desayeus are imprisoned on this layer, as was the Wand of Orcus for a time.

Ecology

Flora

Fauna

Fauna native to Pandemonium include the howling dragon, howlers, and murska.

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