Realm Breaker by Victoria Aveyard

A strange darkness is growing in the Ward. Even Corayne an-Amarat can feel it, tucked away in her small town at the edge of the sea.

Fate knocks on her door, in the form of a mythical immortal and a lethal assassin, who tell Corayne that she is the last of an ancient lineage - with the power to save the world from destruction.

Because a man who would burn kingdoms to the ground is raising an army unlike any seen before, bent on uprooting the foundations of the world. With poison in his heart and a stolen sword in his hand, he'll break the realm itself to claim it.

And only Corayne can stop him.

Alongside an unlikely group of reluctant allies, Corayne finds herself on a desperate journey to complete an impossible task, with untold magic singing in her blood and the fate of the world on her shoulders.