Kestrel's Talon. The Stonewatchers, #1

Following the Prentish/Nemarri war, Kes is rejected by his homeland under the guise of religious purity laws. Though he's spared execution, the proud Nemarri's fate is only marginally more merciful than death when he is sold into sexual slavery at a prosperous pleasure house. Despite his stoic endurance, Kes knows he's reaching his breaking point, but there is nothing he can do-there is no path to freedom in the Holy Prentish Empire, only a lifetime of humiliating servitude.
That is, until a beautiful young slave and his formidable master approach Kes in the marketplace and make an astonishing offer to take him home with them. The only problem: "home" is the accursed Horthmont Castle from the scare-stories of Kes's childhood. Thrown into a world of living myth, powerful magic, and ancient gods, Kes learns the secrets kept hidden by Horthmont's thick blackstone walls. There he discovers something he thought he'd never know again: hope for the future.