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!
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Zuishin has played this adventure and would not recommend it.
Each level of the dungeon is (for the most part) narratively discontiguous, and as such it doesn't really hold up as a straight-run adventure. That being said, there are a ton of great ideas, NPCs, magic items, and dungeon ecologies that can be borrowed from DotMM and transplanted in your own campaign.
Zane K has played this adventure and would recommend it.
This adventure is a fantastic source of maps, monsters, encounters, and more that you can put it into your campaign setting with only a few of changes. It’s fantastic for when you need a quick dungeon map to use for a one shot, or to use as many smaller dungeons (although each level is still quite sizable on its own) scattered around the campaign.