High Voltage by Karen Marie Moning

There is no action without consequence.
Dani O’Malley was nine years old when the delusional, sadistic Rowena transformed her into a ruthless killer. Years later, Dani is tough and hardened, yet achingly vulnerable and fiercely compassionate, 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, dispensing justice swiftly and without mercy, determined to give to those she cares for the peace she has never known.

There is no power without price.
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 horrifying, 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.

There is no future without sacrifice.
With the lethal, immortal Ryodan at her side, armed with the epic Sword of Light, Dani once again battles to save the world, but her past comes back to haunt her with a vengeance, demanding an unspeakable price for the power she needs to save the human race. And no one—not even Ryodan, who would move the very stars for her—can save her this time.