Magus Medivh

Magus or Magna Medivh was the last Guardian of Tirisfal, an ancient line of protectors bestowed with great powers to do battle with the agents of the Burning Legion. He was corrupted even before birth, and possessed by the spirit of the demonlord Sargeras who used Medivh in his plan to conquer Azeroth. Medivh contacted the orcish warlock, Gul'dan, and together they opened the Dark Portal, which allowed the Burning Legion to begin its destruction of Azeroth. He was slain shortly after by a party led by one of his friends, Lothar.
Shortly before the beginning of the Third War, Medivh returned as a mysterious prophet, manipulating events in order to arrange the formation of a last-ditch alliance between the mortal races on Kalimdor, guiding them to victory at the Battle of Mount Hyjal. The prophet's name means "keeper of secrets" in Thalassian. He possessed Atiesh, Greatstaff of the Guardian.