Iron Bones by Yasmine Galenorn

Light and Dark Fae are both dying, caught in the grip of a plague that threatens to sweep through both courts. The deadly disease is spreading, and no one can pinpoint the origins, nor can an antidote be formulated until the details of the contagion are understood. As Cernunnos sets Ferosyn—his best healer—on the task, he also instructs the Wild Hunt to take on the investigation.

Now, Ember and Herne must race against time, seeking an answer before both Fae courts are decimated. Their search leads them deep into the mountains, to a hidden burial site where they discover a terrifying secret that could mean the extinction of the entire Fae race.