Magna Medivh was the last Guardian of Tirisfal, part of a secret order and bestowed with great powers to protect the world from demonic corruption of magic, safeguard Azeroth from the demons and the Burning Legion. He is the son Magna Aegwynn, one of the greatest Guardians of Tirisfal, who fought and ultimately defeated the Lord of the Burning Legion, Sargeras.
Medivh was corrupted even before birth, and possessed by the avatar of Sargeras who used Medivh in his plan to conquer Azeroth. Sargeras manipulated Medivh to contact the warlock, Gul'dan, and together they opened the Dark Portal, which allowed the Burning Legion to begin its invasion of Azeroth. Khadgar, Medivh's young apprentice, would be the first to discover that it was Medivh who brought the orcs to Azeroth and Medivh was killed shortly after by his old friend, Lothar in the tower of Karazhan.
However after his death, Medivh's spirit escaped and foresaw the eventual Scourge invasion, the destruction of Lordaeron and with his mother's help he was reborn. Shortly before the beginning of the Third War, Medivh returned as a mysterious prophet, he manipulated events in order to arrange a alliance between the mortal races on Kalimdor, guiding them to victory at the Battle of Mount Hyjal.