Barbas is the shape shifting hound of Clavicus Vile. He is occasionally sent to the mortal realm by Vile for various reasons, often as a punishment and to assist adventurers who have made deals with vile.
Barbas is often the more ‘level headed’ and ‘better half’ of vile and discourages others from making deals with Vile knowing they’ll come to regret it. His advice is often ignored by Clavicus Vile such as when he warned against taking into possession the sword Umbra, foretelling the trouble it would cause. Vile seems to harbor a strong hatred for Barbas and asked the player to kill him in the fourth era.
Barbas’s relationship with Vile is not well understood and some believe Barbas to literally be the ‘better half’ of Vile and not a separate entity due to Clavicus Vile being severely weekend by the absence of Barbas who houses a portion of his power.