The Autumn Empire #1 The Book of Fallen Leaves (original) A.S. Tamaki

Sen Hoshiakari is an exiled prince of a clan that lost everything in his father's failed rebellion. Deprived of his birthright, Sen is determined to restore his family's lands and honor at any cost.  Rui is a peasant girl who saved Sen's life on the night his family were put to the sword. But now, she is adrift and unsure of her place in the world, not knowing that the gods themselves have plans for her …

As civil war throws the empire into chaos, and demons seek vengeance on the living, Sen and Rui must fight for both their clan and their shared future … But vengeance demands a bloody price