Waterdeep: Dungeon of the Mad Mage Board Game 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 labyrinth dungeon known as Undermountain, domain of the mad wizard Halaster Blackcloak, who has seeded his lair with monsters, traps, and mysteries.