Firebird Vengeance

When eighteen-year old half-demon Alexandra Gold uses her firebird spirit to save the soul of a murdered girl, it brings her to the attention of the most powerful beings in her parallel universe.

At first, she believes this is a blessing. When she discovers that she has been the unwitting pawn of an ancient vengeance that intends to destroy every living human and demon in both universes, she realizes that she's been cursed.

This ancient vengeance has the power to re-animate the dead and use them to murder the living—and for each of the thirty-six most righteous human beings it murders, the more powerful it becomes. 

Alex is determined to protect the thirty-six and save both worlds, but she cannot defeat Vengeance alone. All she has is her demon blade, her spirit fire, and her wits—to prevail, she must convince those humans and demons who are terrified and blame her for this catastrophe to help her.