Feather and Bone — Melissa Wright

A bargain with a fey lord cannot be unbroken. Frey holds the fate of both the fey and elven lands in her half-blood hands. The encroaching darkness has crept past the boundary and claimed a life as its toll. If she doesn't discover a way to end it, the next casualty could be one of her own. As her Seven rush to discover the source of the darkness, the changelings conspire to break themselves free, bringing forth a new danger--one that could end them all. Time is running out, and her only chance to defeat the darkness might be to embrace it.