Part 4 of The Mabinogion, a collection of Welsh epic tales, this story gives all the good gossip on the royal family of Gwynedd. King Math can only survive during peacetime if a virgin holds his feet in her lap, but his nephew, Gilfaethwy, is crushing hard on the lady, Goewin. Gwydion helps Gilfaethwy hook up with Goewin, but Math gets mad and turns his nephews into animals. Gwydion then becomes the "fun uncle" to his nephew, Lleu, and helps him snag a hot girl—who then betrays him.
Check out this story for the private deets of Gwydion's life. Though his other friends get top billing here, we find out that Gwyddie learned all his magic tricks from his uncle Math. It also mentions the shenanigans Gwydion got up to with his nephew, Lleu.
Here, Gwydion faces off against Arawn, lord of the Underworld, master of evil. Gwyddie gets help from the powers of nature—he calls on the trees to help him conquer the bad guys—and from his brother Amaethon and nephew Lleu. He ultimately overcomes the powers of Bran, another villain, by guessing his name.
Want to visit Gwyddie, even though he's gone on to the Otherworld? The "Stanzas" provide all your info—minus directions of course. They say Gwydion's at rest at a place called "Morfa Dinlen," which is probably in modern Gwynedd in Wales.
Gwydion pops up in the twenty-first century in a play featuring Celtic characters galore. He chats with a woman-turned-pig named Cerridwen and hangs out with farm animals in the barnyard.