Imperius Curse

The Imperius Curse is one of the three Unforgivable Curses of the wizarding world, and Harry Potter and the Goblet of Fire introduces it in a Defence Against the Dark Arts lesson the fourth years do not forget. Cast with the incantation Imperio, it places the victim wholly under the caster's control, floating in a blissful emptiness while obeying any instruction, and an onlooker may notice nothing wrong at all. That is what makes it feared: it turns anyone into an instrument. Use of it on a human being carries a life sentence in Azkaban. Alone among the Unforgivable Curses it can be fought, since a strong enough will can refuse the command, and the lesson that introduces it soon has the class practising exactly that resistance.