Ys

Legends tell of the fall of the city of Ys, that once stood in the Bay of Douarnenez, built by King Gralon the Great for his daughter Dahut, who loved the sea.
However where Gralon was pious, Dahut was wayward and she opened the dykes protecting the city in a drunken stupor. All drowned but the King, who rode to land after being told by God to leave his daughter to her fate and save himself.
Other versions of the legend tell that Ys was drowned more than 2,000 years before Gralon's reign when foul magic was performed in the city on the eve of its destruction, of summoners attempting to invoke the power of elder races.