According to the legend, Ys was built below sea level, protected by a dam from being engulfed. The only keys of the gate in the dam were held by Gradlon, but Satan made Dahut steal them and give them to him. He then opened the gate and Ys was flooded. In some versions of the story, Satan was sent by God to punish the city, whose inhabitants were becoming decadent.
The only survivors were the King Gradlon, who was advised to abandon his daughter by Saint Guénolé, and Saint Guénolé himself. Gradlon then founded Quimper and on his death, a statue representing him on horseback looking in the direction of Ys was erected on the Saint Corentin Cathedral and still stands there.
It is said that Ys was the most wonderful city in the world, and that Lutèce was renamed Paris after Ys was destroyed, because "Par-Is" in Breton means "Similar to Ys".
This deluge legend differs from others because the location of Ys is well defined: the statue of Gradlon looks at it, most of the localities mentioned exist, several Roman roads actually lead into the sea (and are meant to lead to Ys), and this myth could in fact depict the engulfment of a real city during the 5th century.
This history is also sometime viewed as the victory of Christianity (Gradlon was converted by Saint Guénolé) over druidism (Dahut and most inhabitants of Ys were worshipers of Celtic gods).