The Endless War by Danielle L. Jensen — book slip

Newly crowned as king of Maridrina, Keris finds himself in a precarious position when his forbidden relationship with Zarrah is exposed. With Zarrah imprisoned by the Empress, Keris must ally with a kingdom he nearly destroyed to save her.

Meanwhile, Zarrah, held captive on Devil's Island, faces a dire choice: prove her loyalty to the Empress or die a traitor. But Zarrah dares to seek a third path: rebellion against tyranny, entwined with a destiny she must fight to claim.

As they face impossible odds, Keris and Zarrah must find their way back to each other before it's too late.