"Grindelwald possessed an ancient and terrible device," said Dumbledore. "While he held it, I could not break his defense. In our duel I could not win, only fight him for long hours until he fell in exhaustion; and I would have died of it afterward, if not for Fawkes. But while his Muggle allies yet made blood sacrifice to sustain him, Grindelwald would not have fallen. He was, during that time, truly invincible. Of that grim device which Grindelwald held, none must know, none must suspect, there must be not a single hint. And therefore you must not speak of it, and I will say no more for now. That is all, Harry. There is no moral to it, and no wisdom. That is all there is."
Harry slowly nodded. It wasn’t entirely implausible, by the standards of magic…
"And then," Dumbledore’s voice went on, even quieter, almost as though he were speaking to himself, "since it was I who felled him, they obeyed me when I said he should not die, though they cried by the thousands for his blood. So he was imprisoned in Nurmengard, in the prison that he built, and he abides there until this day. I went to that duel without any intent to kill him, Harry. Because, you see, I had tried to kill Grindelwald once before, a long time ago, and that… that was… it proved to be… a mistake, Harry…" The old wizard was staring now at his long dark-grey wand where he held it in both hands, as though it were a crystal ball out of Muggle fantasy, a scrying pool within which answers could be found. "And I thought, then… I thought that I should never kill. And then came Voldemort."
The old wizard looked back up at Harry, and said, in a hoarse voice, “He is not like Grindelwald, Harry. There is nothing human left in him. Him you must destroy. You must not hesitate, when the time comes. To him alone, of all the creatures in this world, you must show no mercy; and when you are done you must forget it, forget that you ever did such a thing, and go back to living. Save your fury for that, and that alone.”
In that office there was silence.
It lasted for some many long seconds, and finally was broken by a single question.
"Are there Dementors in Nurmengard?"
"What?" said the old wizard. "No! I would not have done that even to him -"
The old wizard stared at the young boy, who had straightened, and his face changed.
"In other words," the boy said, as though talking to himself without any other people in the room, "it’s already known how to keep powerful Dark Wizards in prison, without using Dementors. People know they know that."
"No," the boy said. The boy looked up, and his eyes were blazing like green fire. "I do not accept your answer, Headmaster. Fawkes gave me a mission, and I know now why Fawkes gave that mission to me, and not to you. You are willing to accept balances of power where the bad guys end up winning. I am not."
"That too is not an answer," the old wizard said; his face showed nothing of his hurt, he had long practice in concealing pain. "Refusing to accept something does not change it. I wonder now if you are simply too young to understand this matter, Harry, despite your outward airs; only in children’s fantasies can all battles be won, and not a single evil tolerated."
"And that’s why I can [DATA EXPUNGED] and you can’t," said the boy. "Because I believe that the darkness can be broken."
The old wizard’s breath stopped in his throat.
"The phoenix’s price isn’t inevitable," the boy said. "It’s not part of some deep balance built into the universe. It’s just the parts of the problem where you haven’t figured out yet how to cheat."