Melissa Service

One sister was born for the light. One for the dark. And Ellyce no longer knows which one she is. Her father is dead. Her twin married the boy who broke her. And the curse in Ellyce's blood is doing something worse than killing her-it's turning her into Sydney. Strand by strand, reflection by reflection, the line between the heir and her enemy is dissolving, and Ellyce can feel the darkness she swore she'd never become rising to meet her halfway.
She wants one thing now-revenge. And the only person who could help her get what she wants loves her too much to do it. Kasdaye would give her anything-anything but the one thing she's begging him for. He keeps telling her grace is the answer. But grace didn't save her father. Grace won't lift the curse. And grace has never felt further away than it does here, in the In-Between-the space between worlds, between mercy and vengeance, between the girl she was and the thing she's becoming.
Because the war her father warned her about was never about realms or crowns. It's about two sisters, one Promise, and a question with no safe answer. There will be a war. There will be a wedding. Choose whom you will serve.