Kill a king (use his madness against him)

There's a dungeon beneath the long, dim halls of Hogwarts, a magical cell that drains the core out of every witch and wizard, not designed to kill them but to leave them behind weak, helpless, a muggle. Hermione conceptualized it herself and helped to steady it, made the walls as hard as granite and there was no way for anyone to break out of her wards and spells. Not even him.