Ys is a mythical city, said to have been situated on the coast of Brittany. Ys was created by Gradlon, King of Cornouaille, at the request of his beautiful daughter Dahut (or Dahud). Dahut tries to steal the key to the city from her father, (in some versions to give to a knight who turns out to be the devil). The city gate is opened and a giant wave then crashes down on the city, (either because God or the sea is angry at Dahut's sin, or because Dahut summoned it with sorcery), swallowing the city and sinking it beneath the sea. King Gradlon escapes on his magical horse Morvarc'h, but pushes Dahut into the sea where she becomes a mermaid. There are many different versions of the story, each with varying details and a different slant on the moral of the tale or who was to blame for the disaster.
In Greek mythology, maenads are the female followers of the god Dionysus (or the Roman Bacchus), who would fall into a frenzy and hunt down and tear to pieces all animals, men and children that they came across. Maenads translates as 'the raving ones'.
Charlotte Mary Yonge (1823 - 1901), was an English novelist. She was extremely prolific in her own time but now most of her works are sadly out of print. Angela Brazil (1868 - 1947), was an English writer of modern schoolgirl fiction, with many of her stories set in boarding schools, and acknowledging current issues and concerns.
Melusine, whose husband spies on her in her bathroom and sees her transformed into a mermaid or half-serpent.