Melusina, a fairy of French legend, who married Raymond, a knight, on condition that on a particular day of the week he would not visit her, a stipulation which he was tempted to break, so that on a day of her seclusion he broke into her chamber, and found the lower part of her body from the waist downwards transformed into that of a serpent, upon which she straightway flew out at the window, to hover henceforth round the castle of her lord and only appear again on the occasion of the death of any of the inmates.