"The Fairy Melusina"

 Melusina (or Melusine) is a fairy or water spirit of European legend. As punishment for a wrongdoing, Melusina was condemned to take the form of a mermaid, (or, in some versions of the story, become a serpent from the waist down) every Saturday. A mortal man, Raymond of Poitou, fell in love with her, and she agreed to marry him on the condition that he would never enter her chamber on a Saturday. They married, and had several children, and each child had a strange deformity. Raymond kept his promise to his wife for many years, but eventually he spied on her whilst she was taking a bath, and saw her altered form. Melusina, her form once again altered to become either a  specter or a dragon, leaves her husband, never to return.

Several different versions of the Melusina story can be found here.