Culhwch and Olwen

Probably composed c. 1100 AD. Culhwch's father, King Kilydd of Kelyddon, lost his wife. Before she died, she made Kilydd promise not to marry until a two-headed thorn grew on her grave. Seven years later, Kilydd remarried. The new wife wanted Culhwch to marry her own daughter, and Culhwch refused, citing his youth as an excuse. Offended, his stepmother swore that in that case, he would never marry, unless he wed Olwen, the daughter of a giant named Ysbaddaden Pencawr. Kilydd told his son to seek out this Olwen, but also said he would need the help of his cousin, King Arthur.

Arthur agreed to help (along with some two hundred other heroes), but no one left Ysbaddaden's realm alive. Culhwch fell in love with Olwen at first sight, but she refused to marry him unless her father agreed, knowing that her father would die if she married anyone. Ysbaddaden at first refused entirely, but then agreed that Culhwch could marry Olwen if he completed over forty impossible tasks. One of the more important tasks was to rescue Mabon ap Modron from prison.

Eventually, Culhwch completes all the labours and a shepherd whose brothers had been killed by Ysbaddaden kills the giant, so Culhwch and Olwen can marry.