Skyshade by Alex Aster

Love kills kingdoms…

Back in Grim’s castle on Nightshade, Isla is reeling in the wake of a brutal battle and the devastating truths it exposed. Her future—and the fate of the world—now hinge on a heart split in two.
Past Isla, who fell in love with the ruler of Nightshade, fights to resist feelings she considers treasonous. The Isla of the present, who has seen the ruin her powers can cause, will do anything to save Lightlark and its king.
As the line between enemy and friend is tested, Isla is more desperate than ever to understand the oracle’s final prophecy and change her heartrending fate. But a storm is coming. And with it, a long-buried evil greater than anything the realms have faced before.

With the clock ticking on her destiny and the survival of two warring kingdoms hinging on her own shattered heart, Isla Crown will either save the world—or destroy it.