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Undermountain is a vast labyrinth of caverns in the Underdark under the area of Waterdeep in the fictional world of the Forgotten Realms campaign setting for the Dungeons & Dragons fantasy role-playing game.
Undermountain was created by Ed Greenwood in 1975. It was the first dungeon for his fledgling Forgotten Realms campaign setting, and he used the Undermountain in both his Dungeons & Dragons and Advanced Dungeons & Dragons games.
Undermountain is at least nine levels deep and possibly deeper. The majority of Undermountain was created and expanded from the natural caverns already in the area by the powerful and ancient wizard Halaster Blackcloak, who lives in Undermountain and uses it as his personal playground and laboratory (307 DR–1370 DR). (Halaster Blackcloak was recently killed by an extremely powerful spell he attempted to perform.) There are ruins of an ancient Ghaunadaur-worshiping drow city in a part of Undermountain near the Promenade whose inhabitants were single-handedly driven away by Halaster while he was creating Undermountain. The lower three levels were delved by dwarves. Below that not much is known of Undermountain.
The upper levels house many famous sites such as Skullport, also known as the Port of Shadows, and Qilué’s Promenade of Eilistraee, a temple complex northeast of Skullport dedicated to the good-aligned followers of Eilistraee. Both are located on the third level.
Madgoth’s Castle also lies below the depths of Undermountain.