This is the fourth book in the Blood Prince series by Jennifer Blackstream. It’s a twist of the classic fairy tale, The Little Mermaid.
Patricio is consumed with the desire to consume the black souls of evil men. He was cursed by a witch to have the family of those he’s killed become enamored with him and worship him for his deeds. He’s disgusted with their adoration and goes out of his way to avoid them so he doesn’t have to deal with it. Marcela is a mermaid that witnessed Patricio kill her brother and because of the curse falls in love with him. She goes to the sea witch and makes a deal to get legs to follow Patricio. They struggle with finding out what is really love or just the curse, a supposed fiancee and a way to break the curse so they can have their HEA.