Spanning miles of caverns below Waterdeep. the City of Splendors. Undermountain is the deepest and deadliest dungeon in all the known Realms. To quote the famous sage Elminster. it is “the most famous battlefield in which to earn a reputation as a veteran adventurer-and the largest known grave of heroes in Faerun.” It predates the city itself. having been built in and around the ruins of several fortresses and hold fasts. including the Underhalls. a labyrinth built by the dwarven Melairkyn clan. A legendary archwizard called Halaster originally built Undermountain. along with his seven apprentices.
Halaster is as famous for his power as he is for his madness. He used the great dungeon primarily as safe storage for his magical treasures and as a testing ground for his various creations. many of which he derived from extraplanar fiends and his own magical experimentation. When adventurers began raiding the dungeon. Halaster dug deeper. leaving behind hordes of nightmare creatures and devilish traps to cover his tracks. He also delighted in watching the trials of would-be treasure hunters by magic. Sometimes inhabiting the very walls to spy upon intruders.
Sometime before the long-ago Spellplague that shattered the Realms, Halaster vanished leaving Undermountain completely uncontrolled. Monsters still abide there. and if the intervening century has done anything to the great delve. it has made it more dangerous indeed.