Dark Spell of Corruption
This spell creates a living corruption in the brain of the victim which disrupts the normal brain pattern and makes the character wild and chaotic. Xander cast it on Clea and nearly defeated Doctor Strange by using her as his pawn. Any possession or control-breaking spell will eliminate it, as will the light of the Vishanti or of Agamotto (see these 2 entities and the Eye of Agamotto for more on their respective lights).
A player character so afflicted will become a villain and turn on his former fellow heroes. If and when he is reinstated his Karma will be as it was before he became evil, since it was not his desire to do so. His Popularity and reputation, however, will not be so easily straightened out.