The Endless War by Danielle L. Jensen

Newly crowned as king, Keris has watched powerless as his forbidden relationship with Zarrah is revealed. But when Zarrah is imprisoned by the Empress, Keris knows there is only one way to save to ally with the kingdom he nearly destroyed.
Imprisoned on the dreaded Devil’s Island, Zarrah faces two prove her loyalty to the Empress who condemned her or die a traitor. Yet as she struggles to survive among violent prisoners, Zarrah uncovers a third a rebellion to overthrow tyranny entwined with a destiny she must fight to claim.
While the Empress plots a war with devastating consequences, Keris and Zarrah must find their way back to each other. Yet their greatest adversary is the fiery passion between them. Unless they overcome the bitterness of betrayal, their love will not be the bringer of peace but rather the fuel that turns the Endless War into an inferno.