The Breton legend of Ker-Ys (The Fortress of the Deep) is one of a number of Brythonic legends of drowned cities, like the Cymric Cantre'r Gwaelod or the Cornish Lyonesse. Though the legend of Ker-Ys is perhaps the most complete extant account.

The story starts with King Gradlon, a Cornishman, who was in possession of a large fleet of boats that plied the waters between Kernow and the cold north. Warships which he also used to oppose his enemies. An excellent strategist and naval leader, Gradlon became wealthy from the plundered riches of his opponents. Though over the years his sailors and warriors wearied of the constant battles and one day, as they were commanded to attack yet another northern fortress they rebelled. Many of them died in the terrible winter that ensued and the survivors decided to return to their boats and make for the land of Brittany to find wives, raise children and live in peace.

For his part, Gradlon allowed them to leave and found himself alone, facing the coldness of the night. Through all the battles and adventures he had faced he had finally been defeated, not by a stronger foe, but by the desertion of his own men. Bereft, he felt a deep despondency settle upon him.

Suddenly Gradlon felt a presence around him; a spirt more powerful than he had ever encountered before. Rising his head he beheld a pale figure standing above him. Her complexion was pale as a bone moon and about her chest she wore a silver cuirass that shone with the wan light of the northern stars. Her head and shoulders were framed by thick strands of flame-red hair. This striking beauty was Malgven, Queen of the North; sovereign of the hyperborean lands.

Extending a hand she brought Gradlon towards her and offered him this proposal: “I know thee, Gradlon, for thou art courageous and skilful in battle; young and vigorous. Unlike to my husband who is old, his sword rusty with disuse. Come, together we shall dispatch him and I shall return with you as your wife to your land of Kernow”. Enchanted by this magical being, Gradlon fell in love with her instantly and together they killed the old King of the North. They filled a single chest with gold and as Gradlon had lost all his boats to his deserting men they took Malgven's steed, Morvarc'h. A night-black stallion that blew fire from its nostrils and which galloped atop the foamy crests of the sea's waves. The steed charged as swift as the wind itself and soon they had caught-up with Gradlon's men's boats.

Gradlon took charge of the flagship and the flotilla sailed east and then south-east towards the wild seas surrounding the Misty Isle. It was then that the storm struck, a violent tempest that scattered the boats to the far-reaches of the compass points. Tempest-tossed Gradlon and Malgven were blown north-west to realms unknown. They were a full year on the sea before they found the course back to Kernow. Upon this epic voyage Malgven gave birth to a girl-child whom they called Dahut; though soon after the birth Malgven became ill and died. Gradlon, having lost Malgven was a broken man. Though they found their way back to Kernow, Gradlon ensconced himself in his castle and never came out. Over the intervening years Dahut grew and transformed into a young woman as beautiful as her mother had been. Though she reminded him of this wife, Gradlon adored his daughter and liked to play with the long curls of her golden hair. Like her mother Dahut was very fond of the sea and one day she asked her father to build for her a citadel; a home by the sea.

So devoted to his daughter was Gradlon that he readily agreed. Many thousands of architects, stonemasons, carpenters, blacksmiths and labourers were assembled and set the task of creating this new city set in a bay off the coast of Brittany. To an observer it seemed that the domes, spires and rooftops of the city were emerging from the very sea itself. To prevent the city from being over-run by the waves it was completely enclosed by an enormous, impenetrable, wall of stone. Its only entrance being through an unique gigantic bronze door to which Gradlon alone had the key. This magnificent construction was the Fortress of Ys, Ker-Ys in the Breton tongue.

Each evening the fishermen who passed by the new city they would see a beautiful woman on the shore. As she combed her long golden hair she would sing to the waves that lapped at her feet:

Ys became a place of excess, a realm full of sailors where each day saw the advent of new games, feasts and dances.

True to the words of her song Dahut would take a new sailor to her bed. During the day she would feast and carouse with him. At the evening's festivities she would place a black mask upon his face and take him to her bedchamber. All night she would use the young sailor for her sport. Then as the sawn sun rose and the meadowlark began to sing the black mask would extend itself around the throat of Dahut's companion of the night before and slowly it would begin to suffocate him. Once the young man was dead the mask would fall off and Dahut would summon a rider who would take the body on his steed to cast it as an offering to Ocean at a place known as the Bay of the Dead (Trepasses).

Life continued like this for many years and Ker-Ys became rich with the bounty brought to it by Ocean. All the result of Dahut's pact. Then, one spring morning a strange rider approached Ys. He rode a magnificent black steed and was dressed head to foot in scarlet. As he entered the city Dahut spied him and saw that he was handsome, his hands long and elegant with nails that were curved and pointed into talons. Dahut smiled at him but the stranger neither returned the smile nor deigned even to look at her as he strode haughtily into the heart of the city. Undaunted, however, Dahut arranged for the stranger to be seated near her during the evening’s feast. During the feast the stranger acquiesced to sit beside Dahut and as the evening progressed and she regaled him with tales and legends he entwined his taloned fingers in her golden hair and whispered in her ear.

Suddenly there was a roaring from the direction of the sea and a howling gust of wind knocked at the ramparts of the city. Dahut placed a reassuring hand on the stranger's arm and noted: “Though the wind may roar and the sea may rage the doors of the city are strong and un-breachable for my father, King Gradlon is in possession of the only key which is hung about his neck”.

“Ah...” the stranger whispered “but your father the king sleeps at this hour, if you wish you can easily take the key from him”. Gazing into the strange knight's eyes Dahut was minded to do as he wished and excusing herself from the table she crept into her father's bedchamber where she spied the key upon its silver chain about his neck. Bending down over her father's slumbering form she detached the key. At that very instant a monstrous wave, higher than the tallest mountain, rose up and crashed down upon the city of Ys. The sound immediately brought Gradlon awake and seeing his eyes flutter open Dahut implored: “Father, quick, the sea has breached the walls, let us take Morvarc'h and escape”. Taking his daughter's hand Gradlon made for the stables and mounting his steed, Morvarc'h they reared up to the surface of the waves. But the sea bubbled madly beneath them, threatening to drag them down in the undertow.

Dahut desperately clung to her father and cried out to him “Save me my father!” At this instant a jagged bolt of lightning rent the sky and a voice commanded: “Gradlon, drop they princess...”

A figure, pale as a moonlit corpse, and clothed in a mud-brown habit emerged from the frothing waters. This was Saint Guenole who reprimanded the princess with the words: “Shame and misfortune on thee, witch, for thou hast tried to take the key from the fortress of Ys”.

Dahut responded with “Save me, bring me from this end of the world!”. But the steed, Morvarc'h was trapped by the frothing waters and was slowly sinking whereupon Gradlon repeated his command to the king: “Drop the princess!” An enormous wave rolled right up to their steed's neck and Dahut, caught by the waters, slipped from her perch by the pommel. As she fell away from her mount she raised her hands imploringly, but under Guenole's insistence Gradlon pushed her under the waves and turned his horse away. The waves closed over the princess' head and the sea claimed the city of Ys, drowning all its inhabitants. However, Morvarc'h managed to find safe waters and cresting the waves it reached the safety of the beach. Galloping all night, through forest and across dale, Gradlon arrived at a city where was found the confluence of two rivers sited between seven hills. This was the city of Quimper and here Gradlon decided to make his new capital. Here Gradlon lived-out the remainder of his life and when he died a statue of him was hewn from granite. A statue which survives to this day. The statue represents Gradlon upon his steed, Morvarc'h, as he gazes out in the direction of the vanished city of Ys.

Some say that Dahut was transformed by the waters into a morverc'h (a mermaid) who appears, combing her long golden hair, to fishermen on nights where there is a full moon. It is also said that on quiet nights the bells of Ker Ys may still be heard tolling gently on the wind.

The following is a Breton song:

According to legend the fortress of Ys was located in the Bay of Douarnenez and Pouldavid, a few kilometres to the east of Douarnenez is the French form of Poul Dahut (the Pool of Dahut) and indicates the place where the princess was drowned by the encroaching waves.

Breton legend also claims that Ker Ys was the most beautiful and nicest capital in the world and that the French capital was called Paris from Par Ys (Like unto Ys). Two claims that are attested by the Breton proverbs: Abaoue ma beuzet Ker Is. N'eus kavet dan par da Paris (Since Ker Ys was drowned her equal was not seen, even in Paris) and Pa vo beuzet Paris, ec'h adsavo Ker Is (When Paris is swept away Ker Ys will re-emerge).