Shield of the Hidden Lord

The shield of the Hidden Lord was an item found in the possession of Thavius Kreeg. It contains Gargauth, a devil with a colorful history of rooting out an intruder in Hell's inner circle, who was once Archduke of Avernus himself before staging a coup against Asmodeus that failed. His sentence was to drag 13 cities to hell, and he was then cast out into the Prime Material Plane in the form of a shield in order to begin his duty.

Elturel may have been his first success; he won't say. He's been helping direct the actions of Zarielites in the city for a while, as well as posing as both the Dead Three and the demon Astaroth. He's essentially a demonic conman.

Traxigor suggests throwing him into the River Styx, and there may be something to it because he quickly changed his tune from gloating to attempting to be very helpful at that point, as well as trying to convince the party to leave him in the possession of Sylvira Savikas.