A fallen clock tower holds components from an ancient artifact. Upon discovery of the components, competing agents look to launch expeditions to uncover additional components scattered across distant places—and times. The now-lost Temple of Moloch and the construction site of the legendary Tomb of Horrors are two of those locations. However great the risks, finding the components demands the effort, for whoever does so can reassemble the fabled Infernal Machine of Lum the Mad!

Welcome to Waterdeep! A fantastic treasure trove is yours for the taking in this adventure for the world's greatest roleplaying game. Famed explorer Volothamp Geddarm needs you to complete a simple quest. Thus begins a mad romp through the wards of Waterdeep as you uncover a villainous plot involving some of the city’s most influential figures. A grand urban caper awaits you. Pit your skill and bravado against villains the likes of which you’ve never faced before, and let the dragon hunt begin!

In the city of Waterdeep rests a tavern called the Yawning Portal, named after the gaping pit in its common room. At the bottom of this crumbling shaft is a labyrinthine dungeon shunned by all but the most daring adventurers. Known 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 constant source of speculation and concern. This adventure picks up where Waterdeep: Dragon Heist leaves off, taking characters of 5th level or higher all the way to 20th level should they explore the entirety of Halaster’s home. Twenty-three levels of Undermountain are detailed herein, along with the subterranean refuge of Skullport. Treasures and secrets abound, but tread with care!