Wild Savage Stars by Kristina Pérez

Branwen has a secret powerful enough to destroy two kingdoms. Her ancient magic led to a terrible betrayal by both her best friend, the princess Essy, and her first love, Tristan. Now this same magic is changing Branwen. Adrift in a rival court, Branwen must hide the truth from the enemy king by protecting the lovers who broke her heart―and finds herself considering a darker path. Not everyone wants the alliance with Branwen’s kingdom to succeed―peace is balanced on a knife’s edge, and her only chance may be to embrace the darkness within…