Eliezer Yudkowsky - Harry Potter and the Methods of Rationality

"So you planned it all along, Harry?" Dumbledore said.

"Of course," Harry said. "Note how I'm doing this as soon as I wake up, rather than pausing to think of it."

Harry drew the hood of the Cloak over his head, and glanced back up at the wall clock he'd surreptitiously glanced at earlier.

It had then shown around twenty-three minutes after eight, and now it was five minutes after nine.

Minerva stared as the boy put himself into the dueling stance, his wand held low. For a second Minerva wondered if Harry might possibly - no, that was completely ridiculous, it was Mad-Eye Moody and that was beyond impossible. Of course that was what she'd thought about his partial Transfiguration, too...

"Let's go, then," Harry said and fell over.

Severus gave a single chuckle. "Mr. Potter has his points, I must confess," the Potions Master said. "Though I would never say it while he was awake, and if you repeat the words I shall deny them, for the boy's ego is quite large enough already. Mr. Potter does have his points, Mad-Eye, but duelling is not among them."

Mad-Eye's own chuckle was lower and grimmer. "Oh, yes," said Mad-Eye. "Only fools duel. Standing like that and waiting for me to attack, what was the boy thinking? Why, I ought to give him a scar, to remember this occasion -"

"Alastor!" barked Albus, just as she cried "Stop!", Severus dashed forward, and Mad-Eye Moody deliberately leveled his wand on Harry Potter's body.

" Stupefy! "

Mad-Eye's body seemed to almost flicker as he spun on his wooden foot like lightning, faster than she'd ever seen anyone move without magic, the red Stunning Hex passing through the suddenly empty air and barely missing Severus to crash into the opposite wall, and by the time her eyes jerked back to Moody there were seventeen radiant orbs in the pattern of a Sagitta Magica, visible for only an instant before they streaked brilliance and struck something that fell to the floor with a thud -

"Hello again, Harry," said Dumbledore.

"I cannot believe that guy's reaction time," Harry said, brushing off his Cloak as he stood up from where he'd been lying invisible on the floor, unseen by his previous self. "I can't believe his movement speed either. I'm going to have to figure out some way to zap him without speaking an incantation that gives it away..."

- and then Mad-Eye ducked hard and fast, his hands hitting flat on the floor. She almost didn't see the two tiny white threads passing through the space he'd been, but her eyes went to the blue spark when the threads impacted on one of the Headmaster's devices, and by the time she managed to turn her eyes back, Mad-Eye had spun smoothly up to his feet, his wand was dancing unseeably fast and there was another thudding sound -

"Hello again, Harry."

"Pardon me, Headmaster, but could you let me go down your stairs, and then come back up again, before I make the final jump backward? This is going to take longer than one hour of preparation -"

Minerva gaped at Mad-Eye Moody, who hadn't lowered his wand in the slightest; and Severus had a look on his face that was almost like shock.

"Well, boy?" said Mad-Eye Moody. "What else have you got?"

Harry Potter's head appeared, floating in midair as an invisible hand drew back the hood of his invisibility cloak.

"That eye," said Harry Potter. There was a strange fierce light in the boy's eyes. "That isn't any ordinary device. It can see right through my invisibility cloak. You dodged my Transfigured taser as soon as I started raising it, even though I didn't speak any incantations. And now that I've watched it again - you spotted all my Time-Turned selves the moment you Flooed into this room, didn't you?"

Mad-Eye Moody was smiling, the same teeth-bared grin she'd seen him wear as they'd faced off against Voldemort himself. "Spend a hundred years hunting Dark wizards, and you see everything," said Moody. "I once arrested a young Japanese who tried a similar trick. He found out the hard way that his shadow replica technique was no match for this eye of mine."

"You see in all directions," Harry Potter said, that strange fierce light still in his gaze. "No matter where that eye is pointing, it sees everything around you."

Moody's tiger-grin grew wider. "There's no more of you in this room, now," Mad-Eye said. "Think that's because you'll give up after this time, or because you'll win? Any bets, boy?"

"It's my final attempt because I decided to stake my last three hours on one shot," said Harry Potter. "As for whether I win -"

There was a blur filling the whole air of the Headmaster's office. Mad-Eye Moody leapt to one side with blinding speed and an instant later Harry's head darted backward as he cried " Stuporfy! "

Three shimmers in the air went past Harry's moving head, just as a red bolt erupted from Harry's location, shooting past Moody as he dodged in yet another direction -

If she'd blinked, she would have missed it, the red bolt making an angled turn in midair and slamming into Moody's ear.

Moody fell.

Harry Potter's floating head dropped to the height of a first-year on their hands and knees, then dropped further to the ground, his face showing sudden exhaustion.

Minerva McGonagall said, "What in Merlin's name just -"

"So you went to Flitwick, then," Moody said. The retired Auror was now sitting in a chair, drinking long draughts from a restorative in a bottle he'd taken off his belt.

Harry Potter nodded, now sitting in his own chair instead of perched on an armrest. "I tried the Defense Professor first, but -" The boy grimaced. "He... wasn't available. Well, I'd decided it was worth risking five House points, and if you say a risk is worth it, you can't complain when you have to pay up. Anyway, I figured that if you had an eye that saw things other people couldn't see, then as Isaac Asimov pointed out in Second Foundation , the weapon to use is a brilliant light. Read enough science fiction, you know, and you'll read everything at least once. Anyway, I told Professor Flitwick that I needed a Charm that would make a huge number of shapes, bright and flickering and filling the whole office, but invisible, so only your eye could see them. I had no idea what it would even mean to cast an illusion and then make it invisible, but I figured if I didn't mention that out loud, Professor Flitwick would just do it anyway, and he did. Turns out there was no spell like that I could cast myself, but Flitwick Charmed me a one-time device for it - though I had to persuade him that it wasn't cheating, since nothing could possibly be cheating against an Auror who'd lived long enough to retire. And then I still didn't see how I could hit you, when you were moving that fast. So I asked about targeted spells, and that was when Flitwick showed me that hex I cast at the end, the Swerving Stunner. It's one of Professor Flitwick's own inventions - he's a champion duellist as well as a Charms Master -"

"I know that, son."

"Sorry. Anyway, the Professor says he left the duelling circuit before he got a chance to use that spell, since it only works as a finishing move on an unshielded opponent. The hex gets as close to the target as possible along its original trajectory, and then once it detects that the target is getting more distant again, the hex turns in midair and heads straight for the target. It can only swerve once - but the incantation sounds very close to 'Stupefy' and the hex is the same red color, so if the enemy thinks it's a regular Stunning Hex and tries a normal dodge, that midair retargeting will finish them off. Oh, and the Professor requested that none of us talk about his special move, just in case he does get a chance to use it during competition someday."