The mythical city of Ys, France (pronounced eess) was built below sea level by King Gradlon for his daughter Dahut, who loved the sea. To protect the city from flooding, a bronze dike and a huge gate were built. The gate could be opened at low tide for ships to enter. Its only key was kept safe by the king himself.
Ys was the most beautiful city in the world, but Dahut’s orgies (after which she would often kill her lovers) soon won the city a reputation for sin. Saint Winwaloe warned of god’s wrath, but was ignored by Dahut and the populace.
One day, a red knight came to Ys. In the middle of the night, as a storm raged outside, he and Dahut celebrated together. When the princess was full with wine, the knight suggested she steal the key from her sleeping father. The knight then revealed himself to be the devil and opened the gate, allowing a wave as big as a mountain to crash down on the city.
The king took his daughter on his magical horse Morvarc’h, and they made their escape. Before they could leave Ys, however, Saint Winwaloe found them and cried, “Push back the demon sitting behind you!” The king reluctantly pushed Dahut from his horse and she was swallowed by the sea, where she became a mermaid.
It is said that when Paris is flooded, the city of Ys will rise up from under the waves.