Publisher's blurb:
Halaster Blackcloak, the Mad Mage of Undermountain, always has a contingency in place. As long as Undermountain remains beneath the city of Waterdeep, Halaster can never truly die within its walls, for the dungeon rebuilds the mage’s body on the rare occasions when adventurers “kill” Halaster.
Halaster realized that if adventurers destroyed all of Undermountain (a nigh impossible feat more difficult than even slaying the mage), he would lose his immortality. To further protect his life, Halaster took some of the rooms and halls of Undermountain and flung them across the multiverse, including other worlds of the Material Plane. With Undermountain spread across existence, the task of permanently killing Halaster is even more difficult.
Unfortunately for the mage, a party adventurers recently killed him in his dungeon beneath the City of Splendors. When Halaster’s body reformed, he found himself in Falx, another world of the Material Plane inhabited entirely by enormous, carnivorous monsters with endless appetites. These scaly bipeds are closely related to the tarrasque, an infamous monstrosity feared across the worlds of the Material Plane. Halaster, now obsessed with punishing the adventurers who killed him, began to hatch a plot. If one tarrasque could level an entire city, what could a planet’s worth of the beings do to Faerûn?