Within the city of Waterdeep rests a tavern known as the Yawning Portal, named after the gaping pit in its common room. On the bottom of this crumbling shaft is a labyrinthine dungeon shunned by all but essentially the most daring adventurers. Referred to as Undermountain, this dungeon is the domain of the mad wizard Halaster Blackcloak. Long has the Mad Mage dwelt in these forlorn depths, seeding his lair with monsters, traps, and mysteries-to what end is a continuing source of speculation and concern.This adventure picks up where Waterdeep: Dragon Heist leaves off, taking characters of 5th level or higher the entire approach to 20th level must they explore the entirety of Halaster’s home. Twenty-three levels of Undermountain are detailed herein, in conjunction with the subterranean refuge of Skullport. Treasures and secrets abound, but tread with care!