The Blood Knight by Greg Keyes

The legendary Briar King has awakened, spreading madness and destruction across a land devastated by a royal family’s fall from power through treachery and dark magic. Half-remembered prophecies may point to the young princess Anne Dare, rightful heir to the throne of Crotheny, as the world’s only hope. But a mysterious assassin stalks her, so skilled that even Anne’s friend and protector Cazio cannot stand against him, nor can her sworn defender, the young knight Neil MeqVren. Though Anne herself is the conduit of fearsome powers beyond her understanding and control, it is time for girl to become woman, princess to become queen. Anne must stop running and instead march at the head of an army to take back her kingdom . . . or die trying.