https://forgottenrealms.fandom.com/wiki/Pandemonium
Once the realm of Tharizdun, the screaming wind-filled tunnels of Pandemonium are now inhabited almost entirely by outsiders (known as the banished); other residents include members of the Doomguard and the Bleak Cabal, intangible undead, berbalangs, vampires, and monstrous cults. Almost all of them are quite, quite mad, having been driven insane by the darkness and the screaming winds.
Despite this, there are a few specks of light in the deep. Keener's Wall is a failed missionary-city founded by Erathis worshippers, which remains the largest civilized place in Pandemonium today. The Myriad of Gales is a college made up of mad bards who seek inspiration in Pandemonium's screaming winds. Finally, it is a matter of common knowledge that Vecna maintains a stronghold somewhere in the depths here, though precisely where, only he knows.
Uniquely, Pandeomium is presented as an Astral Dominion that, whilst dangerous, could be visited by adventures from Heroic through to Epic tier; as a result, the "Adventure Hooks" section provides one major hook for each of the three tiers. The Environmental Features list adds a new danger - the Maddening Wind - and a new Hazard, in the form of the Mindscouring Gust (level 13 lurker).