Melusina
The most famous of the fays, or fairies, of medieval French
legend. Being condemned to turn into a serpent from the waist
downward every Saturday, she made her husband, Count Raymond
of Lusignan, promise never to come near her on a Saturday.
This prohibition finally excited his curiosity and suspicion,
and he hid himself and witnessed his wife’s transformation.
Melusina was now compelled to quit her mortal husband
and was destined to wander about as a specter until the day of
doom. She became the Banshee of Lusignan. It is said also that
the count immured her in the dungeon of his castle.
Sources
Briggs, Katherine A. A Dictionary of Fairies. London Penguin
Books, 1976. Reprinted as An Encyclopedia of Fairies, Hobgoblins,
Brownies, Bogies, and Other Supernatural Creatures. New York
Pantheon Books, 1976