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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DinoTuesday has played this adventure and would recommend it.
I ran this as my first adventure. As a newly minted DM, it was a significant challenge to operate in Roll20 with such large maps, but I dearly wanted a megadungeon of my own. And a glorious megadungeon it was! Me and my friends had a blast exploring this dungeon. It required a ton of up front prep work before I felt like I was ready, and I mixed in material from various sources like Skerple's Monster Menu-All, and Wyatt's Dungeon of the Mad Mage Companion Guides (on the DM's Guild) which were INVALUABLE. I learned a bunch in the process, so honestly I'm glad that Undermountain was such a fertile environment to try building my first monster encounters, factions, magic items, traps, and so on. It is not particularly well formatted so I annotated the massive maps in Roll20 and broke them into manageable segments. My players would walk from one to the next and I would "load" the next map region and drop them there. We got through the first 2 levels before scheduling ended the game. I wouldn't run it again, but it had some great elements and it sparked my love of megadungeons even more.
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.