Promise of Shadows by Justina Ireland

Zephyr Mourning has never been very good at being a Harpy. She’d rather watch reality TV than learn forty-seven ways to kill a man, and she pretty much sucks at wielding magic. Zephyr was ready for a future pretending to be a normal human instead of a half-god assassin. But all that changes when her sister is murdered—and she uses a forbidden dark power to save herself from the same fate.

Zephyr is on the run from a punishment worse than death when an unexpected reunion with a childhood friend (a surprisingly HOT friend) changes everything. Because it seems like Zephyr might just be the Nyx, a dark goddess made flesh that is prophesied to change the power balance. For hundreds of years the half-gods have lived in fear, and Zephyr is supposed to change that.

But how is she supposed to save everyone when she can’t even save herself?