And in Deathly Hallows, why didn't the Killing Curse rebound when Voldemort tried killing Harry with the Elder Wand when Harry was undefended?

TL;DR: The reason that the killing curse did not rebound was because it found a target - Voldemort's last piece of soul that was latched onto Harry.


“He took your blood believing it would strengthen him. He took into his body a tiny part of the enchantment your mother laid upon you when she died for you. His body keeps her sacrifice alive, and while that enchantment survives, so do you and so does Voldemort’s one last hope for himself.” (Deathly Hallows, Chapter 35 "King's Cross")

To understand why there was no rebound, we need to investigate just how Avada Kedavra works.

  • Given pristine conditions of the AK victims' bodies as discussed elsewhere in the books, it seems that the curse either destroys the person's soul, or severs the soul's connection to the body.

  • The former seems NOT to be the case since Harry saw the spirits of his parents when he used Resurrection Stone. So their souls were not destroyed. Just the links to the bodies.

    However, even if this bullet is wrong and the former IS the case (the soul is destroyed instead of the link), the reasoning below works in either case, except the real world analogy is adjusted somewhat.

  • Lily's "love enchantment" is not some magical Force Field a la Star Trek. What it does is, seemingly, very specific - it prevents Avada Kedavra from ripping Harry's soul from his body. What it did NOT do, was prevent Voldemort's last split-off soul piece from being separated from Harry's body.

  • To use a very crude analogy, pretend that the body is a log of wood, the soul is a solid ball, and it is connected to the wood by a thread.

    • Avada Kedavra is equivalent to the knife being thrown to sever that thread.

    • Note: if my original reasoning is slightly wrong and A.K. destroys the soul instead of the connection, just change the analogy so that the soul is a fragile glass ball, with no thread.

    • Lily's protection is equivalent to covering the thread (or glass ball in version 2) with steel. If you throw a knife at it, and the thread is under high tension, the knife will actually bounce off of protective covering and either fly back, or release kinetic energy as an explosion, or both.

    • Now, Voldemort's soul piece was another ball on a thread - except it wasn't a protected thread.

So when Voldemort cast the second Avada Kedavra, it DID find a valid target (unprotected thread) and severed it. Magical energy was expended as designed. Nothing to rebound.

  • I don't think those were his parents' souls. Weren't they just shadows/echoes/something like that? (Perhaps I'm just confused with Priori Incantatem?) – Izkata Sep 5 '12 at 23:10
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    @Izkata - see the latest edit. It is not a certainty so I have a (not very impactful) tweak to account for the fact that this guess could be wrong. – DVK-on-Ahch-To Sep 5 '12 at 23:12
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    If AK worked on souls directly, wouldn't it leave people in the same state as a Dementor's Kiss, i.e., alive (for a while) but mindless? In DH, when Hermione is explaining how a Horcrux is the opposite of a human being, she says that killing someone doesn't affect their soul at all. – Joe White Sep 6 '12 at 3:39
  • All the points seemed valid enough except the one about the luv enchantment cos I Thot it broke when Harry left privet drive foreva – Myers Luke Sep 6 '12 at 19:14

Harry didn’t die since his blood in the Dark Lord tied him to life.

Harry asks Dumbledore the same question - how is he alive when the Dark Lord used the Killing Curse, and no one protects him from it by dying for him?

“But if Voldemort used the Killing Curse,’ Harry started again, ‘and nobody died for me this time – how can I be alive?”
- Harry Potter and the Deathly Hallows, Chapter 35 (King’s Cross)

Dumbledore explains to him that since the Dark Lord took his blood, Lily’s sacrifice lives on in the Dark Lord and tied Harry to life, which was the reason Harry survived the Killing Curse that time.

“He took my blood,’ said Harry.

‘Precisely!’ said Dumbledore. ‘He took your blood and rebuilt his living body with it! Your blood in his veins, Harry, Lily’s protection inside both of you! He tethered you to life while he lives!”
- Harry Potter and the Deathly Hallows, Chapter 35 (King’s Cross)

The enchantment that the sacrifice created then lived on inside the Dark Lord, tying Harry to life by keeping a part of that enchantment alive outside of Harry’s own body. This kept him alive so that when the Killing Curse hit him, it only killed the piece of the Dark Lord’s soul but not Harry himself.

“He took your blood believing it would strengthen him. He took into his body a tiny part of the enchantment your mother laid upon you when she died for you. His body keeps her sacrifice alive, and while that enchantment survives, so do you and so does Voldemort’s one last hope for himself.”
- Harry Potter and the Deathly Hallows, Chapter 35 (King’s Cross)

It wasn’t that the Killing Curse had already killed the piece of the Dark Lord’s soul, and therefore had used itself up - if Lily’s sacrifice contained in his blood hadn’t been in the Dark Lord tying Harry to life, Harry would have been killed by that Killing Curse as well.

The curse didn’t rebound because Harry meant to let it kill him.

The reason that the Killing Curse didn’t rebound that first time was because Harry didn’t resist it. He meant to die, so that the piece of the Dark Lord’s soul in him would die along with him.

“But …’ Harry raised his hand instinctively towards the lightning scar. It did not seem to be there. ‘But I should have died – I didn’t defend myself! I meant to let him kill me!’

‘And that,’ said Dumbledore, ‘will, I think, have made all the difference.”
- Harry Potter and the Deathly Hallows, Chapter 35 (King’s Cross)

Since Harry intended to die, it wouldn’t rebound. The Elder Wand wouldn’t be conflicted as its true master’s will was to be hit by the Killing Curse. Harry wasn’t fighting back, and he didn’t cast any spell that could repel it. If he’d resisted, presumably the Killing Curse would have rebounded as it does the second time the Dark Lord tries to use it on him with the Elder Wand. However, this would mean it’d never hit Harry, so the piece of the Dark Lord’s soul in Harry would have remained alive within him and the Dark Lord would still be tied to life through that piece of soul.

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