Rift (Nightshade Prequel #1) by Andrea Cremer

Sixteen-year-old Ember Morrow is promised to a group called Conatus after one of their healers saves her mother's life. Once she arrives, Ember finds joy in wielding swords, learning magic, and fighting the encroaching darkness loose in the world. She also finds herself falling in love with her mentor, the dashing, brooding, and powerful Barrow Hess. When the knights realize Eira, one of their leaders, is dabbling in dark magic, Ember and Barrow must choose whether to follow Eira into the nether realm or to pledge their lives to destroying her and her kind.

With action, adventure, magic, and tantalizing sensuality, this book is as fast-paced and breathtaking as the Nightshade novels.