Dark Waters by Michael Merriam
Richard Martz, a broken mage cursed by his former colleagues, has removed himself from the world of magic. He lives anonymously until he encounters Holly Ellefson. She is not the human woman everyone sees, but a half-blood dark fey, who recognizes Richard as a mage. Holly’s mother, a powerful human mage, placed a spell on Holly, hiding her from the local mages and fey courts, as Holly is both part-fey and a potential mage, a bloodline that is taboo by magical treaty. The death of Holly’s father, her last living blood-relative, has caused the magic grounding the spell to collapse, exposing Holly to those who would destroy her. Holly begs Richard for help. He agrees, but in return she must kill him once her safety is secured.
Thus begins a deadly battle of arcane hide-and-seek . . . but which ones will live, and which will die?