"Now...those three curses - Avada Kedavra, Imperius, and Cruciatus - are known as the Unforgivable Curses. The use of any one of them on a fellow human being is enough to earn a life sentence in Azkaban. That's what you're up against. That's what I've got to teach you to fight. You need preparing. You need arming. But most of all, you need to practice constant, never-ceasing vigilance. Get out your quills...copy this down..."