Harry Potter and the Methods of Rationality

Then Draco understood, and shoved himself up and away from his desk in a single angry motion. That was it. It was over. "Like Dumbledore's servants," he spat.

"Like Salazar Slytherin," Harry said steadily.

Draco almost stumbled over his own feet in the middle of his first stride toward the door.

Slowly, Draco turned back toward Harry.

"I don't know where you came up with that," said Draco, "but it's wrong, everyone knows the Patronus Charm is a Gryffindor spell -"

"Salazar Slytherin could cast a corporeal Patronus Charm," Harry said. Harry's hand darted into his robes, brought out a book whose title was written as white on green, and so almost impossible to read in the green light; but it looked old. "I discovered that when I was researching the Patronus Charm before. And I found the original reference and checked the book out of the library just in case you didn't believe me. The author of this book doesn't think there's anything unusual about Salazar being able to cast a Patronus, either; the belief that Slytherins can't do that must be recent. And as a further historical note, though I don't have the book with me, Godric Gryffindor never could."

After the first six times Draco had tried calling Harry's bluff, on six successively more ridiculous occasions, he'd realized that Harry just didn't lie about what was written in books. Still, when Harry's hands opened the book and laid it out to the place of a bookmark, Draco leaned over and studied the place where Harry's finger pointed.

Then the fires of Ravenclaw fell upon the darkness that had cloaked the left wing of Lord Foul's army, breaking it, and it was revealed that the Lord Gryffindor had spoken true; the fear they all had felt was not natural in its source, but coming from thrice a dozen Dementors, who had been promised the souls of the defeated. At once the Lady Hufflepuff and Lord Slytherin brought forth their Patronuses, a vast angry badger and a bright silver serpent, and the defenders lifted their heads as the shadow passed from their hearts. And Lady Ravenclaw laughed, remarking that Lord Foul was a great fool, for now his own army would be subject to the fear, but not the defenders of Hogwarts. Yet the Lord Slytherin said, "No fool he, that much I know." And the Lord Gryffindor beside him studied the battlefield with a frown upon his face...

Draco looked back up. "So?"

Harry closed the book and put it into his pouch. "Chaos and Sunshine both have soldiers that can cast corporeal Patronus Charms. Corporeal Patronuses can be used to convey messages. If you can't learn the spell, Dragon Army will be at a severe military disadvantage -"

Draco didn't care about that right now, and told Harry so. His voice was sharper than it probably should have been.

Harry didn't blink. "Then I'm calling in the favor you owe me from that time I stopped a riot from breaking out, on our first day of broomstick lessons. I'm going to try to teach you the Patronus Charm, and for my favor, I want you to do your honest best to learn and cast it. I trust to the honor of House Malfoy that you will."

Draco felt that certain weariness again. If Harry had asked at any other time, it would have been a fair return on favor owed, given that it wasn't actually a Gryffindor spell. But...

"Why?" Draco said.

"To find out whether you can do this thing that Salazar Slytherin could do," Harry said evenly. "This is an experimental test, and I will not tell you what it means until after you have done it. Will you?"

...It probably was a good idea to discharge that favor on something innocuous, all the more so if it was time to break with Harry Potter. "All right."

Harry drew a wand from his robes, and laid it against the globe. "Not really the best color for learning the Patronus Charm," Harry said. "Green light the exact shade of the Killing Curse, I mean. But silver is a Slytherin color too, isn't it? Dulak." The light went out, and Harry whispered the first two phrases of the Continual Light enchantment, recasting that part of it, though neither of them could have cast the whole thing by themselves. Then Harry tapped the globe again, and the room lit with a silver radiance, brilliant but still soft and gentle. Color returned to the desks and chairs, and to Harry's slightly sweaty face beneath his shock of black hair.

It took that long for Draco to realize the implication. "You saw a Killing Curse cast since the last time we met? When - how -"

"Cast the Patronus Charm," Harry said, looking more serious than ever, "and I'll tell you."

Draco pressed his hands to his eyes, shutting out the silver light. "You know, I really should remember that you're too weird for any normal plots!"

Within his self-imposed darkness, he heard the sound of Harry snickering.

Harry watched closely as Draco finished his latest run-through of the preliminary gestures, the part of the spell that was difficult to learn; the final brandish and the pronunciation didn't have to be precise. All three of the last runs had been perfect as far as Harry could see. Harry had also felt an odd impulse to adjust things that Mr. Lupin hadn't said anything about, like the angle of Draco's elbow or the direction his foot was pointing; it could have been entirely his own imagination, and probably was, but Harry had decided to go with it just in case.

"All right," Harry said quietly. There was a tension in his chest that made it a little hard to speak. "Now we don't have a Dementor here, but that's all right. We won't need one. Draco, when your father spoke to me at the train station, he said that you were the one thing in the world that was most precious to him, and he threatened to throw away all his other plans to take vengeance on me, if ever you came to harm."

"He... what?" There was a catch in Draco's voice, and a strange look on his face. "Why are you telling me that?"

"Why wouldn't I?" Harry didn't let his expression change, though he could guess what Draco was thinking; that Harry had been plotting to separate Draco from his father, and shouldn't be saying anything that would bring them closer together. "There's always been just one person who matters most to you, and I know exactly what warm and happy thought will let you cast the Patronus Charm. You told it to me at the train station before the first day of school. Once you fell off a broomstick and broke your ribs. It hurt more than anything you'd ever felt, and you thought you were going to die. Pretend that fear is coming from a Dementor, standing in front of you, wearing a tattered black cloak, looking like a dead thing left in water. And then cast the Patronus Charm, and when you brandish the wand to drive the Dementor away, think of how your father held your hand, so that you wouldn't be afraid; and then think of how much he loves you, and how much you love him, and put it all into your voice when