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 you say Expecto Patronum. For the honor of House Malfoy, and not just because you promised me a favor. Show me you didn't lie to me that day in the train station when you told me Lucius was a good father. Show me you can do what Salazar Slytherin could do."
And Harry stepped backward, behind Draco, out of Draco's field of vision, so that Draco only faced the dusty old teacher's desk and blackboard at the front of the unused classroom.
Draco cast one look behind him, that strange look still on his face, and then turned away to face forward. Harry saw the exhalation, the inhalation. The wand twitched once, twice, thrice, and four times. Draco's fingers slid along the wand, exactly the right distances -
Draco lowered his wand.
"This is too -" Draco said, "I can't think this right, while you're watching -"
Harry turned around and started walking toward the door. "I'll come back in a minute," Harry said. "Just hold to your happy thought, and the Patronus will stay."
From behind Draco came the sound of the door opening again.
Draco heard Harry's footsteps entering the classroom, but Draco didn't turn to look.
Harry didn't say anything either. The silence stretched.
Finally -
"What does this mean?" Draco said. His voice wavered a bit.
"It means you love your father," Harry's voice said. Which was just what Draco had been thinking, and trying not to cry in front of Harry. It was too right, just too right -
Before Draco, on the floor, was the shining form of a snake that Draco recognized; a Blue Krait, a snake first brought to their manor by Lord Abraxas Malfoy after a visit to some faraway land, and Father had kept a Blue Krait in the ophidiarium ever since. The thing about the Blue Krait was that the bite wouldn't hurt much. Father had said that, and told Draco that he was never allowed to pet the snake, no matter who was watching. The venom killed your nerves so fast that you didn't have time to feel pain as the poison spread. You could die of it even after using Healing Charms. It ate other snakes. It was as Slytherin as any creature could possibly be.
That was why a Blue Krait head had been forged into the handle of Father's cane.
The bright snake darted out its tongue, which was also silver; and seemed to smile somehow, in a warmer way than any reptile should.
And then Draco realized -
"But," Draco said, still staring at the beautifully radiant snake, "you can't cast the Patronus Charm." Now that Draco had cast it himself, he understood why that was important. You could be evil, like Dumbledore, and still cast the Patronus Charm, so long as you had something bright left inside you. But if Harry Potter didn't have a single thought inside him that shone like that -
"The Patronus Charm is more complicated than you think, Draco," Harry said seriously. "Not everyone who fails at casting it is a bad person, or even unhappy. But anyway, I can cast it. I did it on my second try, after I realized what I'd done wrong facing the Dementor my first time. But, well, my life gets a little peculiar sometimes, and my Patronus came out strange, and I'm keeping it a secret for now -"