The caged queen

"Once there were two sisters born with a bond so strong that it forged them together forever. Roa and Essie called it the hum. It was a magic they cherished--until the day a terrible accident took Essie's life and trapped her soul in this world. Dax--the heir to Firgaard's throne--was responsible for the accident. Roa swore to hate him forever. But eight years later he returned, begging for her help. He was determined to dethrone his cruel father, under whose oppressive reign Roa's people had suffered. Roa made him a deal: she'd give him the army he needed if he made her queen. Only as queen…