Blackwarden by Ciara Hartford

Rosalin Greene knows one thing for sure: the Dark Fae are monsters.

When her younger sister is selected to be the town’s sacrifice to the Unseelie Court Rosalin volunteers in her place. She’s dragged to a creepy mansion where she’s completely surrounded by magic for the first time in her life and left with nothing but the Gatehouse and its secretive Gatekeeper, Keres, as company.
But there’s something off about Keres. It’s not just that he’s an unnaturally beautiful, terrifying Dark Fae. He’s hiding something, and she fears it’s dreadfully important. Her curiosity grows with every question he avoids, but she’s determined to figure out what’s going on before she faces her final fate.
Keres has long since given up on trying to break the curse that imprisons him. He accepted his fate centuries ago. He must take a human maiden through the portal every five years, whether he likes it or not. But there’s something different about Ms. Greene. Something that makes it very hard for him to keep her at arm’s length. In eight days he’ll use his shadow magic to take her through to the Unseelie Court like all the others, but this time, he has a decision to make.

One that will quite literally determine the rest of his life and likely hers as well.