The Darkest Road — Guy Gavriel Kay

And so the time of prophecy has come at last, the final days for those who dwell in Fionavar, first of all worlds. Even as the Unraveller's armies march to battle, even as his rain of death unleashes plague upon the lands, the warriors of Light are calling upon the most ancient of powers to aid in their struggle.

But despite all that can be done by mage lore and earth magic, despite the sacrifices made by those brought from our own world to take on powers no mortal was ever meant to encompass, despite the deadly fury of the Wild Hunt, and the eternal Warrior's might--all may come to naught because of one child's choice. For he has been born of Darkness and Light, and as the fate of all the worlds hangs in the balance, he alone must walk THE DARKEST ROAD