In the Celtic mythology of Brittany Ker Ys (Breton: Kêr-Is) was a City lost under the sea. The City was said to have been built by Gradlon, legendary 5th century King associated with the area of Cornouaille in Brittany. King Gradlon was said to be a great warrior and was engaged in warfare against lands in the far north. 
He had many ships which he used to conduct his attacks. Although successful his sailors became tired of fighting and decided to return home leaving Gradlon alone in the north. It was then that Gradlon met the magician and Queen of the North, Malgven.
Impressed by his prowess as a warrior, she persuaded Gradlon to join her in killing her husband, the King of the North. The pair then took Morvarc'h, Malgven’s magical black horse, who is described as being able to ride on the sea and breathed fire from his nostrils, One version say’s that they caught up with the fleet of boats carrying Gradlon’s sailors, but that a storm separated them from the main fleet. Another says that the fleet scattered out of fear of the site of Morvarc'h. The result of either was that Gradlon and Malgven were adrift for many months. On the long journey Malgven gave birth to their daughter, Dahut. In some versions of the story Malgven died during childbirth. In others, she did not die but decided to return to her own land having established that Gradlon loved and cared for the child. She asked Gradlon to drop her off on an island and told him that Dahut would always have Malgven’s face to remind him of her.
