Rift

                          She can battle darkness, but she can't fight love.
Rift by Andrea Cremer is the astounding prequel to the incredible Nightshade series. This novel how ever does not follow the story of Calla Tor as did the first 3 novels, it follows the story of Ember Morrow a beautiful, brave girl with a heart of gold. Rift is an astounding fast paced, action packed, romantic thrill ride that will have readers on the very edge of they're seats. The terrors Ember has to face will haunt you're nightmares forever.® 
 As she thought only of the horrors that might be creeping outside her door, waiting to rend her flesh. She would not live a life as the hunted; she would be the hunter.
Summary from book jacket:
Ember Morrow was promised to the knights of Conatus at birth, her life traded for her mother's. So when the horsemen come for her, soon after her sixteenth birthday, Ember's not surprised. What does surprise her, though, is the magic the knights wield, the demons they hunt, and the feelings she has for Barrow Hess, the powerful man she serves. When the knights realize that one among them is dabbling in dark magic, danger heightens and Ember must choose whether to follow the path to darkness or to escape the order and pledge her life to destroying her fellow knights.