"It's wingardium leviOsa, not leviosAAH".
What happens when you mispronounce a spell? A mistake, of course. You could grow a man wings with a simple healing spell. You could make his clothes set on fire by trying to casting Rictusempra, a tickling spell. But what would happen if you mispronounced the killing spell?
This is exactly what Wormtail had done, hardly over a year ago. He thought he killed Cedric Diggory, the same thing that Harry Potter was so sure of when he was spoken to by his ghost. Even his father, Amos Diggory, buried a son he thought to be dead.
But something happened. Something that shouldn't have happened but did. Cedric Diggory arose from his grave, as a vampire turned as one by untraditional means.
He ran away, as far away as he could, knowing that Voldemort would follow him, just like he did with Harry. He had painfully abandoned his previous life and adopted a new name and a new family. A whole new life, not as a wizard boy but as a strange vampire with an American accent.
But the unprecedented meeting with Harry Potter in Forks shattered his new life and forced him to face the inevitable return of his past and future.
What will happen to him and his family? Will Voldemort seek after him too? Moreover, what will happen to Renesme?
This little story was never meant to happen... (And it never did... did it?)