The House of Starling - The Sundering of Rhend, #1

Hatred divides the elves by the color of flesh and the sharpness of steel. Crossing the border is forbidden. Raemian Starling knows the consequences better than most. As a renowned fighter in the Shay army, she's fought the Bleck Larin for years. But when she's captured by the enemy, Rae finds the last thing she would have expected-kindness. Forced to reevaluate friend and foe, she puts her trust in the unlikeliest of places. Gastel, the youngest Bleck Larin prince, has never left his home, has never met a Shay, and is entirely unprepared when the Shay's prized warrior is not the monster he'd anticipated.
A decision to help Rae return home forces Gastel to face the true power he possesses and the reason his father has kept him hidden from the world. But time isn't on anyone's side. An unimaginable power threatens their world, one that cannot be defeated with a sword alone. Rae may be the only elf capable of confronting her queen and stopping the war, but she'll need Gastel's help to bring balance to elvenkind.  Lest their world cease to exist entirely.