The Replacement de Brenna Yovanoff

The Replacement 

Though he lives in the small town of Gentry, Mackie comes from a world of tunnels and black, murky water, a world of living dead girls ruled by a little tattoed princess. He is a replacement - left in the crib of a human baby sixteen years ago when it was stolen away by the fey. Now, because of fatal allergies to iron, blood and consecrated ground, Mackie is slowly dying in the human world. Mackie would give anything just to be normal, to live quietly amongst humans, practice his bass guitar and spend time with his crush, Tate. But when Tate's baby sister goes missing, Mackie is drawn irrevocably into the underworld of Gentry, known as Mayhem, where he must face down the dark creatures and find his rightful place - in our world, or theirs.