Pandemonium

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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. People of all stripes are sometimes banished to Pandemonium, including demons fleeing the wrath of the Abyssal lords, slaadi grown too grim and terrible for Limbo, and mortals banished by evil spellcasters or their own psychoses.

Pandemonium has four known layers: Pandesmos (which borders the Outlands, the Astral, Limbo, and the Abyss), Cocytus (which was seemingly carved out by some ancient, maddened civilization), Phlegethon (even darker and stranger), and Agathion, which is a prison for artifacts and beings too terrible even for the gods to bear.

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.

Sources: 
Planes of Chaos
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