Undermountain is a dungeon complex located beneath Waterdeep.
The dungeon was created by the archmage Halaster Blackcloak, who is said to still reside within its deepest levels, continually expanding and altering its layout as players progress through it.
The complexity of Undermountain provides a nearly endless series of challenges for adventurers, filled with traps, monsters, and treasure. Its levels vary greatly in theme and difficulty, offering everything from ancient, ruined cities to deadly, trap-filled corridors.
The dungeon connects to many different places, including other planes of existence, making it a nexus for adventure and exploration in the Forgotten Realms.