The game utilizes a stripped down version of the 3rd Edition AD&D ruleset, with only four basic character classes. It is not a port of the original game, but it does include practically the same story and set pieces.
Dark times are upon the city of Waterdeep, as an unspeakable evil is lurking under the city. Four adventurers are sent by the lords of Waterdeep to explore the underworld beneath, and bring the evil entity to justice. The game begins in the sewers, just as the party realizes they are trapped inside, thanks to a huge cave in caused by Xanathar, the beholder whom the title of the game refers to. The party has to keep going down, deeper and deeper beneath the city, through Dwarf and Drow settlements, towards the lair of Xanathar, to destroy this evil once and for all.