Binding ties The sentinel wars series, book 9.

They are the Sentinels. Three races descended from ancient guardians of mankind, each possessing unique abilities in their battle to protect humanity against their eternal foes: the Synestryn. Now a warrior must protect his onetime enemy-without succumbing to his darkest desires. Lyka Phelan is a Slayer, sent to live among her enemy to guarantee lasting peace with the Theronai. Yet she has a secret-and it could make her a slave to their powe...