Rooming with his arch nemesis, managing Slytherin politics, and fighting his own personal demons was not how Harry expected to spend his sixth year. But when Harry casts a Sectumsempra against Draco Malfoy, Housemaster Severus Snape gets to set the punishment, and the Potions Master does not play nice. Snape-mentors-Harry, Resorting, Slytherin!Harry, abusive Dursleys