Ysbaddaden Pencawr, whose name means "Chief Giant," ruled from a castle in Cornwall that was called the fairest in the world. He was a figure of grotesque scale and cruelty. His eyelids were so heavy that they drooped over his eyes, and he kept them propped open with golden forks or spears so that he could see. He was the father of Olwen, the maiden whose beauty was described in the most famous passage of Welsh literature, and a prophecy had foretold that he would die on the very day she took a husband. To prevent this fate, Ysbaddaden kept Olwen hidden and guarded, determined that no suitor would ever win her hand. He was also a kinslayer of terrible proportions: he had stripped his brother Custennin, a humble shepherd, of his lands and had murdered twenty-three of Custennin's children, leaving only the twenty-fourth, a boy named Goreu, alive. Custennin's wife was the sister of Goleuddydd, the mother of Culhwch, and it was through this connection that the hero would one day come to the giant's door.

Culhwch arrived at Ysbaddaden's castle with a warband drawn from the court of his cousin King Arthur at Celliwig in Cornwall. The party included Cei, the sharp-tongued seneschal who would later become Sir Kay, Bedwyr, the one-handed warrior who would become Sir Bedivere, and Gwalchmei, Arthur's nephew who would become Sir Gawain. For three days they approached the castle, and for three days Ysbaddaden threw poisoned stone-spears at them as they turned to leave. On the first day, Bedwyr caught the spear and hurled it back, wounding the giant in the knee. On the second day, the enchanter Menw caught the spear and pierced the giant's chest. On the third day, Culhwch himself caught the spear and threw it with such force that it passed through Ysbaddaden's eye and out the other side. Weakened and cornered, the giant at last agreed to hear the suitor's request, but he laid down a condition: Culhwch must complete a series of about forty impossible tasks, the anoethau, before he could claim Olwen.

The tasks Ysbaddaden set were not random cruelties but a single elaborate preparation for a wedding feast and a grooming ceremony that the giant himself would attend. Culhwch must plough a great thicket, burn it to charcoal, sow it with seed, and harvest it by morning so that it could be made into food and drink for the wedding guests. He must obtain the two oxen of Gwlwlydd Wineu, yoked together, to pull the plough. He must win the basket of Gwyddneu Garanhir, the hamper of Gwyddneu Long-Shanks, and the cauldron of Diwrnach the Irishman. He must capture the tusk of Ysgithyrwyn, the White-Tusked Chief of Boars, to shave the giant's beard. And above all, he must hunt Twrch Trwyth, a former Irish king named Taredd who had been transformed by God into a monstrous boar as punishment for his sins, and retrieve the comb, razor, and shears that lay hidden between the boar's ears, entangled in his venom-dripping bristles. The hunt for Twrch Trwyth alone consumed half the tasks, for Ysbaddaden specified not only the boar but the dogs, the leashes, the collars, the chains, and the huntsmen needed to bring him down, including Mabon ap Modron, a prisoner who had been stolen from his mother when he was three nights old and must be freed before he could handle the hound Drudwyn.

Arthur and his warriors accomplished the impossible. They crossed the sea to Ireland, pursued Twrch Trwyth and his seven piglet-warriors across the island, then chased them through Wales and down to Cornwall, losing many men in the chase. They seized the razor and scissors at the mouth of the River Severn and the comb in Cornwall, while the boar was driven into the sea and drowned. They freed Mabon from his prison, obtained the tusk of Ysgithyrwyn, gathered every vessel and ingredient the giant had demanded, and returned to the castle with the full tally complete. Ysbaddaden, bound by his own promise, had no choice but to yield. Culhwch and his company fell upon the giant. Caw of Pictland shaved Ysbaddaden's beard, flesh, and skin down to the bone, and both ears were taken completely. The giant, accepting his humiliation, declared that it was high time to take away his life. Goreu, the twenty-fourth son of the shepherd Custennin, stepped forward and avenged his twenty-three murdered brothers by striking off Ysbaddaden's head. The head was placed on the spike of the citadel, Goreu claimed his uncle's lands as his own, and Olwen was free to marry the man who had moved heaven and earth to win her.

Ysbaddaden belongs to one of the oldest patterns in world folklore: the giant's daughter tale, in which a hero must win a bride by completing impossible tasks set by her monstrous father. The pattern stretches from the Welsh Mabinogion to the Greek tale of Jason and Medea, where King Aeetes set tasks to prevent Jason from taking the Golden Fleece and his daughter together. A parallel runs between Ysbaddaden and Balor, the one-eyed giant of Irish mythology whose eye was kept closed with a lid propped up by pegs, and who was fated to be killed by his own grandson. The forked eyelids, the prophecy of death through marriage, and the tyrannical guarding of a daughter link the two figures across the Irish Sea. Ysbaddaden's name has been connected to the Welsh word for "hawthorn," yspadadden, and some have suggested that the giant was originally a vegetation deity or a lord of the wild wood, whose death was necessary for the harvest and the turning of the seasons. Whether as a folktale giant, a Celtic god, or the monstrous father of the fairest maiden in Britain, Ysbaddaden stands as one of the most formidable obstacles ever placed in the path of an Arthurian hero.