This is also a new interpretation of a very old legend from France, Luxembourg, and The Low Countries.
The legend is the legend of Melusine, or Melusina. In legend she is associated with wells and rivers. In this case, I have further fused the mermaid lore with her by making her a mermaid.
Melusine of legend is traditionally depicted as half fish like a mermaid, just one of fresh water. I have brought her back to the ocean, and quite far from the region that invented her.
it is said many ancient European dynasties can run their lineage back to Melusine the mermaid of fresh water. Limburg luxemburg dynasty, holy roman empire (1308 to 14something.) Bohemia, Hungary, the house of Anjou, House of Plantagenet, and several others.
it is said, Melusine, is the daughter of a fairy and an old king of Scotland. Her fairy mother, left her king of Scotland spouse, and fled with her children to the Isle of Avalon, after her spouse, breaks a promise to her never to view her and their daughter (Melusine) while they are bathing. Later, Melusine herself will marry a nobleman, and demand the same promise only to have him fail to keep it and of course she follows her mother's example. She simply flies away.
Jean D'arres (1382-1394) gathered stories told by women at their spinning wheels. Evidently, the tale of Melusine was a common one. From there the story was translated to German, and finally English. There is also a Dutch translation as well from the middle ages.
As the story goes, an ancient king of Scotland, went hunting in the woods during the time period of the crusades. He found a beautiful woman in the woods, she extracted a promise and then agreed to marry. They had 3 children, one of them Melusine, who would later marry a noble under the same extracted promise. But first, she would grow up in Avalon with her mother the fairy and her 2 sisters. Until age 15 when she would ask why they left and came to Avalon. her mother explained about the broken promise. Melusine, in anger, with her sisters went to find daddy dearest. They imprisoned him in a mountain. Their mother had a melt down in quite a hissy fit. She punished her girls. Melusine's punishment, was to have a mermaid tail every saturday for life. And so, she too had to extract a promise from the man she wed. To never seek her on saturdays.
Like her mother, Melusine met some fancy promise breaker in the forest, in Poitou France. He promised to leave her alone on saturdays. Then he went and found her in her mermaid form. (in my story it is a monthly event that revolves around the full moon.) Anyway, later he called her a serpent infront of the court. She turned into a dragon, gave him 2 golden wings and flew away never to be seen again.
I have taken a great deal of creative license, and created a variation on the old story. A major variation. But this is the inspiration and loose roots of this story.