So that was how his parents had died . . . exactly like that spider. Had they been unblemished and unmarked too? Had they simply seen the flash of green light and heard the rush of speeding death, before life was wiped from their bodies?

Harry had been picturing his parents’ deaths over and over again for three years now, ever since he’d found out they had been murdered, ever since he’d found out what had happened that night: Wormtail had betrayed his parents’ whereabouts to Voldemort, who had come to find them at their cottage. How Voldemort had killed Harry’s father first. How James Potter had tried to hold him off, while he shouted at his wife to take Harry and run . . . Voldemort had advanced on Lily Potter, told her to move aside so that he could kill Harry . . . how she had begged him to kill her instead, refused to stop shielding her son . . . and so Voldemort had murdered her too, before turning his wand on Harry . . .

Harry knew these details because he had heard his parents’ voices when he had fought the dementors last year—for that was the terrible power of the dementors: to force their victims to relive the worst memories of their lives, and drown, powerless, in their own despair . . .

Moody was speaking again, from a great distance, it seemed to Harry. With a massive effort, he pulled himself back to the present and listened to what Moody was saying.

“Avada Kedavra’s a curse that needs a powerful bit of magic behind it—you could all get your wands out now and point them at me and say the words, and I doubt I’d get so much as a nosebleed. But that doesn’t matter. I’m not here to teach you how to do it.

“Now, if there’s no countercurse, why am I showing you? Because you’ve got to know. You’ve got to appreciate what the worst is. You don’t want to find yourself in a situation where you’re facing it. CONSTANT VIGILANCE!” he roared, and the whole class jumped again.

“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 . . . ”

They spent the rest of the lesson taking notes on each of the Unforgivable Curses. No one spoke until the bell rang—but when Moody had dismissed them and they had left the classroom, a torrent of talk burst forth. Most people were discussing the curses in awed voices—“Did you see it twitch?” “—and when he killed it—just like that!”