This is a story I first gave some serious attention when seeing it featured in two works of modern fiction The White Queen by Philipa Gregory and Possession by A S Byatt. Melusina legends and stories are more well known on continental Europe than the UK and are frequently set in North West France or the Low Countries. Her fame is demonstrated by various European royal houses claiming her as an ancestress, including the house of Plantagenet. A summary of the main legends and literature associated with Melusina can be found on Wikipedia. The details of the stories vary and I have tried to summarise the main strands below. Usually they involve a nobleman (often the historical Raymond of Poitou or other historical nobleman) and a companion in a thick forest, in some accounts boar hunting. The nobleman comes across a beautiful woman and female companions in a watery setting, sometimes after accidentally killing his companion. The woman agrees to marry the nobleman if he will promise to give her one day of absolute privacy in her chamber a week (in many stories a Saturday) where he must on no account see her, or in other versions see her in the bath or when bathing her children. The couple are married and have children, who in many variants are monstrous or very ugly. Often Melusina magically gifts the nobleman with magnificent buildings and general air of wisdom is associated with her, although in some versions her works are always flawed. Prohibitions in fairy tales are there to be broken so of course the nobleman spies on his wife on the forbidden day. He sees her enter a bath where she is either transformed into a being whose lower half is a serpent or a mermaid, sometimes with two tails like the Starbucks logo. She then vanishes and is never seen by the nobleman again. She continues to haunt her descendants, appearing in the family home at times of crisis or when a death is imminent, usually in the form of a woman or serpent.
The first literary version of Mesulina-The Romans of Partenay or of Lusignen: Otherwise known as the Tale of Melusine was written by Jean d’Arras in 1382-1394, although because of her connections to European royal houses, she makes a passing appearance in other earlier works relating to royalty such as that of the 12th century Gerald of Wales. From then onwards to the present day many literary versions have been produced or the myth is referred to in other works of fiction (sometimes by well known authors such as Walter Scott or more modern authors such as Philipa Gregory or A S Byatt). Another notable version is the translation into German by Thüring von Ringoltingen in 1456, this was later translated into English in the 16th century. A nineteenth century version on which modern Melusina stories and theories are often based, is in Curious Myths of The Middle Ages Vol 2, by the extraordinarily prolific Victorian folklorist Rev Sabine Baring Gould. Melusina is an example of the fairy/animal bride folklore motif. These stories vary but they always include some kind of prohibition which the husband always breaks resulting in his wife leaving him and their children, who are often unusual and or gifted. A tale that is similar in many essentials according to Sabine Baring Gould is the 12th century Knight of the Swan, although this unusually for the genre involves a fairy/animal groom rather than a bride.