Kingdoms Collide: Wisteria

Princess Kiera has never set foot beyond Wisteria's borders. Trained in secret to wield a blade but forbidden to ever use it beyond the palace walls, she has spent her whole life caged by her parents' fear, and by a past she's never been allowed to understand.
When the Sovereign Trials are announced; a legendary competition where each of the five kingdoms sends its ten most promising students to test their strength, skill, and worth. Kiera knows this is her one chance to prove she is more than a sheltered princess.

But selection isn't decided by the King and Queen. It's decided by the tree.
Every year, Wisteria's ancient, magical tree chooses ten souls it deems worthy. As the ceremony draws near, every eligible fighter in the kingdom to its roots, Kiera is forced to watch from the crowd, forbidden from stepping forward. Meanwhile, a mysterious girl moves like a shadow at the edge of the trees, watching Kiera far more closely than anyone should.

The first book in the Kingdoms Collide series - a slow-burn, forbidden-kingdoms romance where love and loyalty are about to collide.