The Sword-Witch’s Heart by Tavia Lark

Desire is his downfall.

Rejected by his ruthless family and the god he betrayed them for, Leth is desperate for a chance to prove himself. A purpose for his sword and magic. Anything beyond the legacy of restraint and repression that’s broken him down again and again.
But his new quest comes with an infuriatingly familiar companion.
Evain draws power from human desire. Every heated glance or tender touch feeds the flames of his magic. He has little patience for a frigid sword-witch who hates anything demonic—but somehow, traveling with pretty Leth makes him feel stronger than ever.
On the rare occasion Leth deigns to look at him.
Wild magic and winter nights throw them together, and cracks start forming in their first impressions. Leth wants more than a purpose from Evain, and Evain wants more than heated glances from Leth. But Evain’s hiding a secret that could shatter their fragile trust: he isn’t human.
And Leth can’t afford weakness when his family’s out for blood.