The story of Culhwch and Olwen forms part of what has become known as the Mabinogion that features in both the White Book of Rhydderch and the Red Book of Hergest. (Although the White Book only has the first two thirds of the story.)
The tale is as much about Arthur as it is about Culhwch. (Olwen, of course doesn't feature much at all apart from as an object of desire.) Culhwch is effectively cursed by his stepmother, and obliged to marry one Olwen. Her father Yspaddaden Pencawr moreover is similarly operating under a curse, as his life is prophesised to end with the marriage of his daughter. Yspaddaden Pencawr is therefore, not over keen on letting Olwen get hitched and requires Culhwch to accomplish all manner of seemingly impossible tasks in order to obtain her hand in marriage.
Dating from the 11th century, (and quite probably based on an oral tradition older still) it is believed to be one of the earliest contributions to Arthurian Mythology. In contrast to the Arthur of the Historia Brittonum (who is portrayed as a 6th century warrior chieftain leading the fight against the Saxons), this Arthur is a fully fledged king of Britain with a complete retinue of warriors, wife, magic sword, shield and dagger, the works.
The story with its emphasis on the collection of various wonders from around the country (as in the Thirteen Treasures of the Island of Britain) is believed to have been the inspiration behind the later tales of Arthur's quest for the Holy Grail.
The text is based on the translation by Lady Charlotte Guest except that I have modernised some of the language; (After all, there are only so many thee and thous that one can take at one sitting.) and is included as follows;