Waterdeep: Dungeon of the Mad Mage is an adventure module for the fifth edition of Dungeons & Dragons, released on November 20, 2018. It is the Immediate Sequel to Waterdeep: Dragon Heist, with Christopher Perkins serving as the lead designer of the module.
Taking place within the city of Waterdeep, the adventure centers on the exploration of Undermountain, the largest and most famous dungeon complex in the Forgotten Realms. As such, adventurers from all across Toril are drawn into the dungeon through its entrance within the Yawning Portal inn, hoping to return to the surface with riches and harrowing tales of bravery and danger. But through the depths of this labyrinthine dungeon, dark forces lie waiting to put the adventurers' mettle to the test. Among which is none other than Halaster Blackcloak, who madly reigns over Undermountain from his lair within the deepest level of the dungeon. Spanning across twenty-three levels, each one deeper than the last, only the bravest, luckiest, and most resourceful of adventurers would dare to challenge the dangers of Undermountain and live to tell the tale.
This adventure provides examples of:
- On the fourth level of Undermountain is a mage named Darribeth Meltimer, who has gone insane due to the demons encountered in the mountain. If cured of her sanity, she mentions wanting to return and see her wife Urgala Meltimer, a minor NPC from Storm King's Thunder.
- The module encourages this if the players previously ran Waterdeep: Dragon Heist. In particular, if the players' main threat in said module was the Xanathar Guild, then the DM is encouraged to use the outcome of the previous module to play into the story of the current one.
- Evil Versus Evil: Essentially what each level of Undermountain boils down to. There are usually two to three factions fighting each other for control over the area, with most aiming to either control the level, or expand further in. For example; the first level has the area divided into territory held by a group of bandits called the Undertakers, who pretend to be vampires to extort money from gullible adventurers, and the Xanathar Guild, a notorious Thieves' Guild in Waterdeep. Both groups fight each other in order to gain control over the area, and the module states at the end of each section for the level what happens if the adventurers involve themselves, ranging from one faction taking over the level, to the level being abandoned if both factions are killed.
- Fallen Angel: On the twenty-first level, adventurers can encounter Fazrian, a planetar who once followed Torm. After being summoned in Undermountain, he tried to stop Halaster's creations on said level, but grew insane as he did so. Now he has gone full Knight Templar and judges those who he finds, either sparing them if they show courage and a willingness to sacrifice themselves, or executing them if they do not meet his standards, all so he can make an army to cleanse Undermountain. Adventurers can convince him to judge himself if they are persuasive enough, at which point he realizes the severity of his actions and kills himself out of shame for falling from his duty to Torm.
- Mystical Cave: Halaster Blackcloak's influence over Undermountain has allowed him to alter magic so that any spell or effect that can transport or banish a creature while within the dungeon complex would immediately fail, as well as those that can alter or destroy the terrain of Undermountain. Although there are exceptions to this rule, such as Summon Magic, magic involving Pocket Dimensions, using a wish spell, etc.
- Space Base: Stardock is a hallowed-out asteroid situated along the Tears of Selûne that was converted into a space outpost by mind flayers, until the githyanki and their red dragon allies drove them out and repurposed the outpost into a shelter for raising and training young githyanki warriors known as Creche Kliir. From Toril, it can only be accessed through a portal found within the sixteenth level known as the Crystal Labyrinth, which is also under githyanki control as part of their campaign to exterminate the mind flayers within Undermountain.
- Villain Takes an Interest: One of the module's recommendations for Halaster's Evil Plan is to have him take an interest in the adventurers because they could become potential pupils that he can use. Any spellcaster who catches his eye will be someone he desires to take under his wing. This is not a good thing though, as Halaster will eventually get bored and leave them to a Fate Worse than Death.
- Wretched Hive: Skullport, the only real town in Undermountain. The city was once a Netherese enclave called Sargauth, but now is mostly controlled by the Xanathar Guild. While it has things like food and supplies, the city is not very welcoming to newcomers, and you are more likely to get attacked than to meet a friendly face.