Culhwch and Olwen (Welsh: Culhwch ac Olwen) is a Welsh tale about a hero connected with Arthur and his warriors that survives in only two manuscripts: a complete version in the Red Book of Hergest, ca. 1400, and a fragmented version in the White Book of Rhydderch, ca. 1325. Certain linguistic evidence indicates it took its present form by the 11th century,[citation needed] making it perhaps the earliest Arthurian tale and one of Wales' earliest extant prose texts. The title is a later invention and does not occur in early manuscripts.

Lady Charlotte Guest included this tale among those she collected under the title The Mabinogion. Besides the quality of its storytelling it contains several remarkable passages: the description of Culhwch riding on his horse is frequently mentioned for its vividness (a passage reused to similar effect in the 16th century prose "parody" Araith Wgon, as well as in 17th century poetic adaptations of that work), the fight against the terrible boar Twrch Trwyth certainly has antecedents in Celtic tradition, and the list of King Arthur's retainers recited by the hero is a rhetorical flourish that preserves snippets of Welsh tradition that otherwise would be lost.

Synopsis
Culhwch's father, King Cilydd son of Celyddon, loses his wife Goleuddydd after a difficult childbirth. When he remarries, the young Culhwch rejects his stepmother's attempt to pair him with his new stepsister. Offended, the new queen puts a curse on him so that he can marry no one besides the beautiful Olwen, daughter of the giant Ysbaddaden Pencawr. Though he has never seen her, Culhwch becomes infatuated with her, but his father warns him that he will never find her without the aid of his famous cousin Arthur. The young man immediately sets off to seek his kinsman. He finds him at his court in Celliwig in Cornwall; this is one of the earliest instances in literature or oral tradition of Arthur's court being assigned a specific location.[citation needed]

Arthur agrees to help, and sends six of his finest warriors to join Culhwch in his search for Olwen. These warriors include Cai (known to later literature as Sir Kay, Arthur's foster brother), Bedwyr (Sir Bedivere) and Gwalchmei (Sir Gawain, Arthur's nephew). The group meets some relatives of Culhwch's that know Olwen and agree to arrange a meeting. Olwen is receptive to Culhwch's attraction, but she cannot marry him unless her father agrees, and he, unable to survive past his daughter's wedding, will not consent until Culhwch completes a series of about forty impossible-sounding tasks. Fortunately for Culhwch (and the reader), the completion of only a few of these tasks is recorded and the giant is killed, leaving Olwen free to marry her lover.

The story is on one level a typical folktale, in which a young hero sets out to wed a giant's daughter, and many of the accompanying motifs reinforce this (the strange birth, the jealous stepmother, the hero falling in love with a stranger after hearing only her name, etc.). However, for most of the narrative the title characters go unmentioned, their story serving as a frame for other events. Culhwch and Olwen is as a whole more than simply a folktale.

In fact, the majority of the writing is taken up by two long lists and the adventures of King Arthur and his men. The first of these occurs when Arthur welcomes his young kinsman to his court and offers to give him whatever he wishes. Culhwch, of course, asks that Arthur help him get Olwen, and invokes some two hundred of the greatest men, women, dogs, horses and swords in Arthur's kingdom to underscore his request. Included in the list are names taken from Irish legend, hagiography, and sometimes actual history.

The second list includes the tasks Culhwch must complete before Ysbaddaden will allow him to marry Olwen. Only a fraction are recounted, but several that are of great significance. The longest episode, the hunt for the boar Twrch Trwyth, is possibly related to the boar hunt in the Irish stories of Diarmuid Ua Duibhne and is probably based on a story mentioned in the Historia Britonum. The rescue of Mabon ap Modron from his watery prison has numerous parallels in Celtic legend, and the quest for the cauldron of Diwrnach the Irishman may well be related to the tales of Bran the Blessed in the second branch of the Mabinogion and the poem The Spoils of Annwn in the Book of Taliesin, possibly linking it to the Grail Quest.