Diablo II: Izual
Izual is an angel and used to be a lieutenant of Tyrael. However, he was captured by the demons during an attack on the Hellforge. His spirit was bound to a monster and has roamed the Plains of Despair ever since.
Tyrael will give you the Fallen Angel quest where you have to slay Izual. This is one of the best quests in the game as it will reward you with two skill points.