A Spanish giant who resided at Mont Saint Michel kidnapped Helen (Helena), the daughter/niece/wife of Hoel, King of Brittany.
Arthur and his knights, on their way to fight the Roman War, heard of the abduction and rode to rescue the lady. Arthur, Kay, and Bedivere arrived to find the lady dead (either killed by the giant or by herself to avoid rape). They crept to the giant’s lair and found him roasting children (or, in Geoffrey, piglets) on a spit. Arthur challenged and killed him. Arthur later ordered a church built at this site.
In the Norse Saga of Tristram and Ísönd, Tristan builds his famous Hall of Statues in the giant’s former cave.
According to the Middle English Parlement of the Thre Ages, Arthur fought a dragon, rather than a giant, on Mont St. Michel. The Prose Brut calls the location Saint Bernard’s Mount.
Wace names the giant Dinabuc.
Coat of the Giant of Mont Saint Michel | The Legend of King Arthur
Saint Michael’s Mount | The Legend of King Arthur