This follows on from Waterdeep: Dragon Heist, 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.

Storyline:  Hundreds of years ago, long before Waterdeep had been built, Halaster Blackcloak carved a vast dungeon beneath Mount Waterdeep and stocked it with all manner of creatures from across the planes. Over time Halaster has gone quite insane and the dungeon he carved, now dubbed Undermountain, is a labyrinthine death trap for those bold enough to venture within. Precious few have returned from Halaster's home, and frequently they're not who they were when they started out. One thing is certain however. Wealth unimaginable exists within the halls of Undermountain.