Spirits of Light and Shadow by India Drummond

Book 1 of The Gods of Talmor:
When the most powerful senator in Talmor's second-largest city is cursed with dark magic, the other legislators secretly turn to his estranged son Korbin as a last, desperate chance to find a cure. With the assistance of a refugee schooled in the foreign magic of the One, Korbin seeks to help his father—and find redemption. But an ancient Spirit of Shadow sets the pair in her sights, corrupting their every hope into despair... and turning their quest for a cure into a fight for survival.