Undermountain

Undermountain was a multi-tiered dungeon, comprised of several inter-connected chambers and lairs located deep beneath the city of Waterdeep and its namesake mountain. Originally home to the Dwarves of Clan Melairkyn, the Underhalls were home to a grand dwarfhold before they were taken over by the "Mad Mage" Halaster Blackcloak and his seven apprentices.

“ Have you not heard the tales of the Mad Mage of Undermountain? The Dwarves built the passages under the mountain long ago, but they've belonged to Halaster for the last thousand years. He toys with anyone who enters the dungeon; the deeper you go, the more dangerous his games. ” — Durnan