Children of Sun and Moon (Skyfall #1) by Matt Larkin
The Lunar King bargained his daughter away in marriage to end generations of war between the two dynasties of the Skyfall Isles. The King sends his niece Chandi along as handmaid to his daughter. Chandi has two tasks: watch over her cousin, and spy on the Solars. Still seething over the death of her lover during the war, Chandi accepts the task he gives her. The Solars cost her everything she cares about, and now she wants nothing more than proof of their treachery so she can go home.
She knows little of spying, but the blood of the Moon God running through her veins gives her powers mortals can’t match, powers that let her slip into places she’s not supposed to be. Of course, the more she uses her powers, the faster she becomes a lunatic.
When she discovers a Solar soldier, Naresh, watching her, she decides to return the favor and stick close to him. But as he shows her the wonders of the domed underwater city, she begins to realize the Solars are not what she thought. Soon, she’ll have to choose between loyalty to her people and her own heart.