The Silver Knight by Kate Cotoner

As the Prince Bishop's chief summoner, Sufyan is accustomed to dealing with dangerous men. When he hears tales of a wild blood-fiend and the solitary silver knight who does battle with the monster each year, Sufyan decides there's more to the situation than meets the eye. Suspecting trickery, he investigates the haunted church and meets the silver knight, Everard de Montparnasse.

Everard has been battling the blood-fiend for years. He's startled when Sufyan offers to help, and together the two men manage to drive back the horrifying monster for one night. But Everard knows the blood-fiend will strike again. He doesn't want to lose Sufyan -- but the secret Everard is hiding could mean the death of them both.