They Kill. The Nyxia White Stories, #3

When monsters come hunting.she will be their nightmare. When Nyxia White transforms, she realizes she possesses a fearsome weapon and the power of the Darkin. A power that threatens her enemies and makes her a target. After rescuing Acheron, she understands her problems have only just begun. Someone wants them both dead and will tear their world apart to do it. She must learn to control her new power in time, before the forces rising against her attack. Together, they must face overwhelming odds and confront a sorcerous enemy that believes the only good Darkin is a dead Darkin.