Eye of the Beholder - Advanced Dungeons & Dragons

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The lords of the city of Waterdeep hire a team of adventurers to investigate an evil coming from beneath the city. The adventurers enter the city's sewer, but the entrance gets blocked by a collapse caused by Xanathar, the eponymous beholder. The team descends further beneath the city, going through Dwarf and Drow clans, to Xanathar's lair, where the final confrontation takes place.