Solus Incomitatus

Snape stops Voldemort from killing Harry during the Battle of Hogwarts, thus compromising his position as a spy. Put under the Imperius Curse by The Dark Lord, Harry is forced into being a Death-Eater, possessed by Voldemort, and Severus realizes that his proximity to Potter allows the latter to regain control of his body. The result? Voldemort wants Severus' head. (full sum. ins.)