Book #5 (Book #11 in Mac's series)
Book #1 Iced, Book #2 Burned, Book #3 Feverborn, Book #4 Feversong.

Dani O'Malley was nine years old when Rowena transformed her into a ruthless killer. Years later, Dani is living alone by her own exacting code. Despite the scars on her body, and driven by deeper ones carved into her soul, no one is more committed to protecting Dublin. By day she ensures the safety of those she rescues, by night she hunts evil, determined to give to those she cares for the peace she has never known. When the Faerie Queen used the dangerously powerful Song of Making to heal the world from the damage done by the Hoar Frost King, catastrophic magic seeped deep into the earth, giving rise to unforeseen consequences-and now deadly enemies plot in the darkness, preparing to enslave the human race and unleash an ancient reign of Hell on Earth. With the immortal Ryodan at her side, armed with the Sword of Light, Dani once again battles to save the world, but her past comes back to haunt her, demanding a price for the power she needs to save the human race. And not even Ryodan can save her this time.