The Scot's Blood Warrior (Shadows of the Dark Hollow) by Keira Montclair

Her sight. His blood. One faery hill that should never have been opened. Edan MacRuari has already lost his wife. He refuses to lose his daughter. When little Heilyn is snatched from her cradle during a strange, rainless storm, Edan turns to the legendary Clan Grant on the Isle of Mull for help. He expects an army. Instead, he gets Ailith. Quiet, fiercely empathetic, and burdened with the power to see the past, Ailith uncovers a terrifying truth: Edan’s father broke a bargain with the Unseelie court, cursing Edan with "iron blood"—a magic that burns the fae but threatens to collapse the very underworld where Heilyn is trapped. With the full moon approaching and the children's souls at risk of being hollowed out forever, Ailith and Edan must brave a realm of shadow warriors, mourning banshees, and deceitful bogles. But as the darkness closes in, they soon discover that the only thing stronger than Edan's iron blood is the undeniable, fierce bond forging between them. Read more