To this end, Gwydion employs his magic powers to steal a number of otherworldly pigs from the Demetian king, Pryderi, who retaliates by marching on 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.

The boy throws a stone and strikes 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.

A large tradition seems to have once surrounded the Battle of the Trees, a mythological conflict fought between the sons of Dôn and the forces of Annwn, the Welsh Otherworld.

Amaethon, Gwydion's brother, steals a white roebuck and a whelp from Arawn, king of the otherworld, leading to a great battle.

Gwydion fights alongside his brother and, assisted by Lleu, enchants the "elementary trees and sedges" to rise up as warriors against Arawn's forces.

The alder leads the attack, while the aspen falls in battle, and heaven and earth tremble before the oak, a "valiant door keeper against the enemy".

", while in the same corpus, the poem Kadeir Cerridwen relates many familiar traditions concerning Gwydion, including his creating of a woman out of flowers and his bringing of the pigs from the south.

In the first three books of the five-novel series, Gwydion defeats the Horned King by shouting his real name, leads the quest to secure the Black Cauldron, and helps Taran, Fflewddur Fflam, Gurgi and Prince Rhun rescue Princess Eilonwy from the sorceress Achren.

As a member of the Royal House of Don, he often wears a pendant depicting a simple golden disk meant to represent the sun.

Patricia Kennealy-Morrison's Keltiad series—Irish, Welsh and Scottish legends translated to an interstellar, Star Wars style context—has a character named Gwydion Prince of Don as its co-protagonist.

He is a gifted bard, sorcerer and warrior, close to (and descended from) the magical Sidhefolk of Keltia, and is portrayed as Aeron's true and loving partner and her equal in most things.

He is also included in Alan Garner's novel The Owl Service through the character of Huw Halfbacon (the last name a reference to stealing pigs).