The Reaper by Melissa K. Roehrich

Before he was the Fire Court Third, he was the Reaper.
And before he was the Reaper, he was her Ash Rider.
Moranna. The Baroness of a colony hidden away on the Southern Islands. She works tirelessly to keep some of the most powerful Fae of the realm safe from those who seek to use them for their gifts.
Or that’s what Rayner has always believed.
When he learns the Baroness’s darkest secrets, he finds nothing is as it seems, and he realizes he has more to fight for than he ever thought. Vowing to put an end to her and everything she has created on the islands, he finds himself the center of a curse as punishment for his rebellion.
The path back to vengeance is wrought with violence, grief, and unexpected allies, and Rayner knows better than anyone: Some nightmares never end.