"The Killing Curse… not nice. Not pleasant. And there's no counter-curse. There's no blocking it."
–Barty Crouch Jr. (As Alastor Moody)
The powerful Blasting Curse sent Potter flying until he impacted with a headstone. He would have screamed in agony if he didn't have the breath knocked out of him. A high, cold voice laughed not twenty feet away.
"I expected you to put up more of a fight, Potter. You are supposed to be Dumbledore's favourite, are you not?" Lord Voldemort said, his mouth twisting into a cruel, cold sneer. "If you are, then all I say is… how disappointing."
The Dark Lord turned to his Death Eaters. "There are some among you that even now thinks that this boy, that Dumbledore's chosen, could possibly be my equal or perhaps my superior. I shall leave no doubt in your mind how mistaken that belief is. I have allowed him to fight. I have even allowed him flight. And he is still defeated. And now he will die.
"Any last words, Potter?" He said, his red eyes looking down at his so-called nemesis. "No? Very well. Avada Kedavra!"
The lethal green flash, the 'Death-light' as some called it, sped toward the boy—
—And Harry Potter lifted his wand.
A huge, green-lit explosion nearly knocked the unsuspecting Death Eaters off their feet. Voldemort looked bewildered behind the hastily conjured shield that protected him from the shockwave.
The Killing Curse was said to be unblockable, unstoppable. It wasn't. Voldemort and his closest lieutenants learned through repeated usage that it could be blocked by a physical barrier, such as Summoned or Conjured objects. But Voldemort could see clearly that Harry Potter conjured nothing, summoned nothing, into the curse's flight path. The boy had somehow managed the impossible, and blocked the deadly green jet of light with a spell.
The Death Eaters, stunned, could only watch as Harry Potter slowly climbed to his feet. His emerald eyes, previously obscured by his hair, were now clearly visible behind his glasses. They shone an unnaturally cold green, so much like the Killing Curse itself.
"Avada Kedavra!" Voldemort snarled again.
All eyes followed the trajectory of the second curse as it flew toward its target, and Harry Potter jabbed his wand forward again. A spell raced from the boy's wand and collided with the most lethal curse in existence in midair. A second explosion, a second burst of green sparks, and when the smoke cleared and the dust settled the boy remained standing. A second miracle. There was no denying it now: the boy had done it. He had conquered the Killing Curse.
They all had the same question on their minds.
How?
Harry Potter took one step forward, and instinctively, involuntarily, Voldemort took a step back. And in that one instant, the self-proclaimed Dark Lord knew that he had lost—this engagement, and perhaps everything he ever had. For in that one tiny step, the man who had gone further than any other in his quest for power and immortality, Tom Marvolo Riddle who had removed his humanity and became Lord Voldemort—had shown fear, and weakness, and worst of all he had done it in front of his followers. There was no turning back; even if he killed Potter in the next second the damage was already done, the seed of doubt sowed.
"You have taken too much, far too much, from me and mine." Voldemort blinked at the boy's whisper. "You will die. Now."
There could be only one answer to that, Voldemort thought. "Avada Kedavra!"
A Killing Curse erupted from Voldemort's wand for the third time tonight. Potter's response gave the Dark Lord hope, for the boy dodged this time; whatever he did before must have strained him magically. Potter retaliated with a Fire Whip, but it missed his opponent by a wide margin, so wide that Voldemort didn't even bother to dodge—until he realized that he wasn't Potter's target at all. Three horrifying screams among his Death Eaters told him that Potter had claimed his first victims with the unexpected attack.
"How dare you!" Rage filled Voldemort's voice. He was not at all bothered by the loss of three of his servants; what angered him was that Potter had the gall to dismiss him in the face of his assault. Another round of Killing Curses was sent flying, another series of emerald explosion, but this time, at this range, Voldemort was finally able to discover just what spell Harry Potter used to thwart the most lethal curse in the world.
Of course. Of course.
It should have been obvious, Voldemort thought as a jet of light emerged from the boy's wand and leaped toward the momentarily dazed Dark Lord. He was too slow to cast the counter, even if he knew the secret now. Harry Potter's curse slammed into the Dark Lord, tearing the life and soul from his body once more. In the last moments of his life, this life, green light filled his eyes.
After all, the Killing Curse could not be blocked by any other spell.
A/N: An idea that started with "What happens when two AK's hit each other".