Ceridwen was a witch — or a goddess, or both — of the Welsh tradition, who dwelt by Lake Tegid (Bala Lake in Gwynedd) with her husband Tegid Foel and their children. She had two children of her own: Creirwy, the most beautiful girl in the world, and Morfran ("Sea Crow"), the ugliest boy. To make up for the boy's ugliness, Ceridwen determined to make him the wisest man in the world, and to this end she brewed in her great cauldron the awen, the inspiration of poetry — a brew that must boil for a year and a day, with proper herbs added at the proper hours.

She set Gwion Bach, a small boy, to stir the cauldron, and Morda the blind man to tend the fire beneath. At the end of the year, three drops flew from the cauldron and fell upon Gwion's thumb; he put the thumb in his mouth, and the wisdom of awen passed into him instead of into Morfran. The rest of the brew turned at once to deadly poison, splitting the cauldron and ruining the year's work. Gwion fled, and Ceridwen pursued him.

What followed was the great chase of Welsh tradition. Gwion turned himself into a hare; Ceridwen became a greyhound. Gwion became a fish; Ceridwen became an otter. Gwion became a bird; Ceridwen became a hawk. At last Gwion became a single grain of wheat in a heap of grain on a barn floor; Ceridwen became a black hen, picked the grain from the heap, and swallowed him. Nine months later she gave birth to a beautiful boy; she could not bring herself to kill him, and instead set him in a leather bag and cast him into the river Dyfi. The bag floated downstream, was caught in the salmon-weir of King Gwyddno Garanhir, and was opened by the king's son Elffin — who, finding the radiant child, named him Taliesin ("Shining Brow"). And Taliesin grew up to be the greatest of the Welsh bards.