Custennin was born into a world of giants and curses, the son of Mynwyedig and the husband of a daughter of Amlawdd Guletic, a princess of such lineage that she was sister to both Eigyr, the mother of King Arthur, and Goleuddydd, the mother of Culhwch. This kinship made Custennin the uncle of the two greatest heroes in Welsh tradition, though he lived far from their courts in the shadow of a fortress that no army had ever taken. His domain was the great plain that stretched before Ysbaddaden's castle, the fairest of all caers in the world, where he tended an enormous flock of sheep that belonged to the chief giant. He stood upon a mound to watch his flock, clad in a jerkin of skins against the wind, accompanied by a dog larger than a stallion whose breath was so hot that it could burn up any dead tree or bush on the plain. The dog had never lost a lamb, much less a grown sheep, and it would allow no company to pass without doing them harm or slaying those who belonged to it.

The tragedy of Custennin's life was Ysbaddaden's hatred. The chief giant had stripped him of his lands and murdered twenty-three of his twenty-four sons, leaving the youngest, Goreu, hidden in a stone chest at the end of the hearth to keep him from sharing the same fate. Custennin's wife, a woman of immense strength who could squeeze a log until it appeared as a twisted twig, lived in daily fear that the last of her children would be taken. When Culhwch and Arthur's warriors arrived on the plain, seeking Olwen the daughter of Ysbaddaden, Custennin greeted them with a jest that carried the weight of years of grief: "There is no affliction to do me harm save my wife." The words were bitter, for the affliction that had done him the most harm was the giant who had destroyed his family because of that very wife.

The meeting that changed everything began with a gold ring. Culhwch gave it to Custennin as a token of kinship, but the ring was too small to fit the shepherd's giant finger, and Custennin placed it instead within the finger of one of the gloves he wore. When he returned to his dwelling and told his wife that Culhwch son of Cilydd had come to ask for Olwen, the woman felt two things at once: joy at the coming of her nephew, and sadness because she had never seen anyone go with his soul still with him who had come requesting that. She opened the stone chest at the end of the hearth, and out came Goreu, a youth with curly yellow hair, the last of twenty-four brothers. Cai, Arthur's most loyal warrior, looked at the boy and said, "Let him keep companionship with me, and we will not be killed unless we are killed together." From that moment, Goreu was no longer a prisoner in a chest but a knight in the making.

Custennin's greatest service to the quest was not his sword but his knowledge. He knew the ways of Ysbaddaden's fortress, the habits of the nine porters and the nine watchdogs, and the path that Olwen took every Saturday when she came near to his dwelling to wash her head. The maiden left her rings behind in the bowl each time, never asking for them back, and Custennin and his wife supported themselves in part by selling these gifts of gold. When Culhwch needed to meet Olwen, it was Custennin who arranged the secret encounter on the plain, and it was Olwen herself who agreed to lead the strangers to her father's castle. Without the shepherd's help, the quest would have ended before it began, for Ysbaddaden's fortress was as unreachable by force as it was by stealth without a guide who knew its secrets.

The end of the story brought the vengeance that Custennin had waited years to see. After Arthur and his knights completed the forty impossible tasks that Ysbaddaden had set, the chief giant was shaved and humiliated, dragged from his hall by the very son he had tried to kill. Goreu laid hold of Ysbaddaden by the hair of his head, dragged him behind him to the mound, and cut his head off, placing it on the stake of the rampart. Then Goreu overcame his uncle's caer and his territory, reclaiming the lands that Ysbaddaden had stolen from his father. The Welsh Triads remember Goreu as one of the three who rescued Arthur from imprisonment beneath the Stone of Echymeint, a sign that the boy who had been hidden in a chest grew into one of the most trusted warriors of the Round Table. Custennin the Shepherd disappears from the tale after the beheading, his name never spoken again, but the flock he tended and the plain he watched were his again, and the twenty-fourth son had survived where the twenty-three had fallen.