From the product page:
Beneath the streets of Waterdeep, a sinister dungeon awaits adventurers brave enough to face its perils. Created ages ago by the wizard Halaster, the sprawling dungeon is a lair for terrible monsters and ruthless villains. In the dark chambers, they hunger for victims and plot the downfall of those who dwell on the streets above. Who will unravel the twisting labyrinth to reveal its secrets, claim its treasures, and stop the foul machinations of the dungeon’s denizens?
Halls of Undermountain is a 96-page hardcover book that presents a ready-to use dungeon detailing over eighty locations. The adventure offers a way for Dungeon Masters participating in the recent D&D Encounters season, The Elder Elemental Eye, to extend the adventure. In addition, this book provides all the ingredients to run a classic dungeon delve, including enticing treasures, deadly traps, a wide assortment of monsters, and two double-sided full-color poster maps.