The Mermaid and the Sea Witch

Peter is obsessed with the human world. His father swears his obsession is going to get him into trouble, but once he's caught sight of the human prince, Steve, there's no turning back. He makes a deal with the sea witch, Tony, to give him legs, but if he can't convince Steve to fall in love with him in time then he'll be cursed to live in Tony's undersea caves forever.

*This is just below Grimm's fairytales levels of dark. Proceed at your own risk.