Melusine is a French legend. She's a water fae who agrees to marry a knight and bring him riches and fame, as long as he keeps a list of weird conditions which change depending on the telling of the legend. I think she has a snake's tail instead of legs and possibly wings in her true shape, though she appears human. And of course the knight breaks the contract and is cursed. Or she turns into a dragon and devours him and/or the town.