Liminality by TJ Nichols

Like everyone in the Dark City, Elliot Brand never has enough coin for anything. There is never time for more than survival. He does everything he can to make sure he and his brother, Dayne, have a place to live. But when his brother goes missing, Elliot suspects the worst—that he has been taken by the Kin.
The Kin live in a castle on the lake that only appears at night. They trade with the rich up the hill. In exchange for giving the rich the orbs that light the city and healing nectar, the Kin are permitted to hunt humans in the Dark City.

Elliot swore on his mother’s death bed that he wouldn’t let harm come to Dayne, so he makes a plan to go to the castle and get his brother back. Dazzled by the wealth and the attentions of Umbra, a Kin with their own plans, Elliot gets caught up in machinations far beyond his wildest dreams.

But Dayne has tasted Kin blood and become one of the devoured. Elliot’s plans quickly unravel. If Elliot and Umbra are to survive, they’ll need one another.