Blood of a Daydreamer by Zinnia Keen

Amongst immortals, death is a sly beast, a rarity that causes more devastation than any war. And for the people of Reyhen, death has come. There is only one person who knows how to save them from its dark clutches — him.

Two kingdoms. Two royals. One Divide.
After spending the beginnings of her immortal life secreted away in the countryside, Eira Delengranz is — regretfully — returning to life at court for her initiation ceremony. The day the Virtuae Relic gifts her the powers it deems most useful for her royal life.

One letter is all it takes for her entire world to crack open, revealing a terrible, festering rot of a secret.
She has no option, but to place her faith in the man they say is a harbinger of death, the man they tell her lurks at the other side of the Valtayre Divide, biding his time, hungering for revenge. The man whose powers are unlike anything their Gods-given Relic would ever provide, his very existence sacrilege.

The Umbrian King. Eliaz Daegon.