Ys was built on land reclaimed from the sea by Gradlon (Gralon in Breton), King of Cornouaille (Kerne in Breton), upon the request of his daughter Dahut (also called Ahes), who loved the sea. To protect Ys from inundation, a dike was built with a gate that was opened for ships during low tide. The one key that opened the gate was held by the king.
Princess Dahut had a lover for whom she threw a secret banquet and, under the influence of wine, she stole the key to the gate from her father and opened the gate, and the water submerged the entire city. Another version of the legend says that she stole the silver key to admit her lover, mistakenly opening the sluices in the dark.
St Gwénnolé who, according to one version, had foretold the city’s ruin due to its luxury, woke the king and commanded him to flee. He mounted his horse and took his daughter with him. As the water was about to overtake him, a voice called out: “Throw the demon thou carriest into the sea, if thou dost not desire to perish.” Dahut fell from the horse’s back, and Gradlon was saved.