Graz'zt, called The Dark Prince, is a demon lord and one of the most powerful fiends in the Abyss.
You haven't seen him yet and you don't know much about him. You know that he is at war against Demogorgon and Orcus for the title of "Prince of all demons". And you are at the middle of that war.
You also know that he is Verin's boss and mentor.
The only hint you have on his physical form is the wall painting in his fortress.