Coll is remembered first through his family and craft. His uncle, Rudlwm the Dwarf, teaches him one of the Three Great Enchantments of the Island of Britain. The Welsh Triads do not reveal the enchantment's name or its full power, but they place Coll in the company of Math son of Mathonwy and Uther Pendragon, the other masters of British magic. The honour is extraordinary. Coll is not a king or a druid with a title. He is a keeper of swine, yet in Welsh tradition the swineherd often stands close to the Otherworld.

His master is Dallwyr Dallben, whose pigs graze in the Glen of Dallwyr in Cornwall. Among them is Henwen, an ancient sow filled with wonders. A prophecy warns that her womb-burden will bring harm to Britain. King Arthur, the legendary ruler of the island, gathers the army of Britain and rides to destroy her before she can give birth. The hunt begins in Cornwall.

Henwen runs. Coll runs beside her, his hand fixed to her bristles wherever she travels, by sea or by land. At Penrhyn Awstin she plunges into the sea, and Coll goes into the water with her. They emerge at Aber Tarogi in Gwent Is-coed, and the first birth comes at Maes Gwenith, the Wheat Field. Henwen drops a grain of wheat and a bee. From that day the place is the best in Britain for wheat and bees, turning the sow's threatened evil into the island's first harvest blessing.

The flight continues to Llonion in Pembroke, where Henwen drops barley and a bee. That valley too becomes famous for barley. Then she climbs to Riw Cyferthwch in Eryri, Snowdonia, and gives birth to a wolf cub and an eagle. Coll does not kill them. He gives the eagle to Brynach the Irishman of the North and the wolf to Menwaedd son of Arllechwedd. The gifts become famous in their own right: the Eagle of Brynach and the Wolf of Menwaedd, creatures carried from the womb of a sow into the lore of warriors.

At Maen Du in Llanfair in Arfon, Henwen drops one final creature: a kitten. This time Coll throws the tiny animal into the Menai Strait. The act seems sensible. The prophecy has promised harm, and the sea should swallow it. Instead the kitten reaches the Isle of Anglesey. There it grows into Cath Palug, the Cat of Palug, a monstrous beast that rises from the water to become one of the Great Oppressions of the island. The creature Coll discards becomes the most feared survivor of Henwen's brood.

Coll disappears from the surviving Triads after Hen Wen's journey, and they do not record his death or later career. Chaucer's House of Fame later mentions a conjuror called 'Colle tregetour' performing an impossible-looking feat with a windmill and a walnut shell. Some later writers have connected that Colle with the Welsh Coll, but the identification is not secure and should be treated as a possibility rather than part of Coll's established medieval biography.

Coll's story survives because it gathers the world of Welsh magic into one journey. He is servant, nephew, student, keeper, guide, and accidental author of monsters. He carries the fruitful wheat and the dangerous cat in the same hand. He cannot stop Henwen, but he follows her to the end, and the island is changed by everything she drops.