Medivh is The Last Guardian. He is partly responsible for allowing the Burning Legion to come through the Dark Portal who tried to destroy Azeroth in the past. He manipulated many events and people to form an alliance between the Humans, Orcs and Night Elves. Through these actions found some redemption for his sins and mistakes in the past.
In Warcraft III: Reign of Chaos Medivh was voiced by Michael Bell. In World of Warcraft: The Burning Crusade Medivh is voiced by Cam Clarke.