Medivh

"I...am the reason for the Legion's return. Years ago, I brought the orcs to this world, and by doing so, I opened a path for the demons as well. For my sins I was murdered by those who I cared for most. Despite my death, war raged across the lands of the east for many years, leaving entire kingdoms devastated in its wake. Now at long last I have returned to set things right. I... am Medivh, The Last Guardian. I tell you now, the only chance for this world is to unite in arms against the enemies of all who live."
- 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. However, 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.