Shadowfever (Fever, #5)

Alina is dead and her sister Mac has returned to Ireland, the country that expelled them, to hunt her sister's murderer. But after discovering that she descends from a bloodline both gifted and cursed, she descends into an epic battle between humans and Fae immortals as the Sinsar Dubh turns on her, and begins mowing a deadly path through those she loves. Mac's journey will force her to face the truth of her exile, and make a choice that will either save the world... or destroy it.