Nihiloor

This strange and powerful creature was encountered in the sewers controlled by the Xanathar Guild, who had captured Floon Blagmar, believing him to be Renaer Neverember. It seemed to have some kind of authority over the other agents here, and used weird brain-like creatures known as Intellect Devourers  to assault the minds of it's foes. It also seemed to be able to project its own psychic attacks and thoughts.
Volothamp Geddarn would later identify this being as a Mind Flayer, but has no idea why such a creature would be in Waterdeep or be aligned with a criminal organisation.

Species

Illithid

Aligned Organization

Xanathar Guild