Laisrathera (Her Instruments 3). Her Instruments, #3

The Queen of the Eldritch has offered Reese Eddings a life out of a fairy tale, one beyond the imagination of a poor girl from Mars who'd expected to spend her life eking out a living with a rattletrap merchant vessel. Unfortunately, the day Reese reached out to accept Liolesa's offer, Hirianthial's enemies betrayed him--and his entire planet--to a race of sociopathic shapeshifters with dreams of conquest.
Now the only thing between Reese and a castle of her very own is a maniacal alien despot, his native quisling and all the Eldritch dead-set on preventing the incursion of aliens at any cost, including the ousting of their current usurper, who happens to be an alien himself... Reese, Hirianthial and the crew of the Earthrise have been battling these pirates since Hirianthial's capture inspired their fateful meeting, but to beat them Reese will have to own the power she's always denied herself, and Hirianthial must make peace with his bloody past and uncertain future.
The stakes have never been higher, and this last time will count for all. The final battlefield awaits. Laisrathera, Book 3 of Her Instruments, concludes the trilogy begun with Earthrise and continued in Rose Point.