(Sorry about the working title, brainstorming left me empty handed) Sybil never finished the prophecy. While she only catches to first section, it was the last verse that Voldemort knew is the key. "The one with the power to vanquish the Dark Lord approaches... born to those who have thrice defied him, born as the seventh month dies... and the Dark Lord will mark him as his equal, but he will have power the Dark Lord knows not... and either must die at the hand of the other for neither can live while the other survives... the one with the power to vanquish the Dark Lord will be born as the seventh month dies....and so shall the one with the power to vanquish him... For she will be given power unseen in decades, raised by those faithful to the dark one... And she alone has the power to stop the fight once and for all... "

The title says it all, what do i gotta say?