Unseen

The Unseen were a group of shapechangers, illusionists, assassins, thieves and evil creatures based out of Waterdeep. They were led by a greater doppelganger known as Hlaavin, who masqueraded as the Masked Lord, Nindil Jalbuck.

History
Originally a band of dopplegangers controlled by illithids, known by that species as a druuth. They were tasked with spying on Waterdeep for the Underdark city Ch'Chitl but when the elder brain of that city died, control of the druuth was lost. One of the dopplegangers was secretly half-illithid so this creature, called Hlaavin, took control.