Flight of King Gradlon (1884), by Évariste Vital Luminais.
The mythical city of Ys, on the coast of Brittany in France, was ruled over by King Gradlon. A strong dike protected the city from the waves, and a locked gate could be opened to allow the tidal water to flow out. But only the king held the key.
Gradlon's daughter, Dahut, was responsible for the fall of the city. One night, while her father was asleep, she stole the key and opened the gate to let in her lover, who in some versions of the tale was the devil. The sea rushed in, flooding the city.
Gradlon seized his daughter, and they tried to escape to safety on his horse. But the waves were too great, and St. Gwénnolé (Winwaloe) told him that he had to throw Dahut to the waves. He did so, and managed to escape to found the city of Quimper.
Dahut survived, becoming a mermaid or morgen (a Breton water-spirit who lures men to their death).
