Gilfaethwy, nephew to the Venedotian king, Math fab Mathonwy, falls in love with his uncle's virgin foot-holder, Goewin.

To this end, Gwydion steals a number of otherworldly pigs from the Demetian king, Pryderi, leading to an invasion of Gwynedd.

The two contenders meet at a place called Y Velen Rhyd in Ardudwy, and "because of strength and valour and magic and enchantment", Gwydion triumphs and Pryderi is killed.

When Math hears of the assault on Goewin, he turns his nephews into a series of mated pairs of animals: Gwydion becomes a stag for a year, then a sow and finally a wolf.

During the test, she gives birth to a "sturdy boy with thick yellow hair" whom Math names Dylan and who takes on the nature of the seas until his death at his uncle Gofannon's hands.

Ashamed, Arianrhod runs to the door, but on her way out something small drops from her, which Gwydion wraps up and places in a chest at the foot of his bed.

Gwydion, however, tricks his sister by disguising himself and the boy as cobblers and luring Arianrhod into going to them in person in order to have some shoes made for her.

The boy uses an unspecified missile to strike a wren "between the tendon and the bone of its leg", causing Arianrhod to make the remark "it is with a skillful hand that the fair-haired one has hit it".

He reveals to her that he can only be killed at dusk, wrapped in a net with one foot on a cauldron and one on a goat and with a spear forged for a year during the hours when everyone is at mass.

The poem Kateir Kerrituen ("Song of Cerridwen") states that he fell at Arllechwedd, whilst also giving him a son, Minawg fab Lleu.

Within the narrative, the character of Taliesin states: When I return from Caer Seon From contending with Jews I will come to the city of Lleu and Gwydion.