Xanathar is the name given to the beholder crime lord that lives in the dungeons under Waterdeep. It isn't the first beholder to claim this mantle, nor will it be the last.
Like all beholders, Xanathar is a paranoid tyrant that charms and bullies its minions into servitude. The Xanathar Guild is made up of some of Waterdeep's most disreputable folk, as well as monsters forced into subservience or drawn to the beholder by the promise of treasure, food, or power.
1 The Xanathar