Gwydion is an intelligent and powerful magician. Once, Math son of Mathonwy was seeking a virgin as his wife he is offered Arianrhod, sister of the wizard Gwydion. As a test of her virginity, she is asked to step over a magic wand. But as she steps over the wand, a baby falls out of her, proving that she isn't a virgin. As she flees from the court, something much smaller, falls out of her. Gwydion seizes it and hides it to save his sister from further shame, and puts it in a chest at the bottom of his bed. Soon, he hears a scream from the chest and on opening it, he discovers a small baby.

Gwydion raises the child as his own, and the boy grows strong and healthy. One day, Gwydion takes the boy to Arianrhod's Court and introduces the boy to his mother. When she realises who he is, she vows the baby will never be given a name, weapons or a wife.

In the years that follow, Gwydion manages to deceive Arianrhod into giving the boy a name, Lleu Llaw Gyffes, and weapons. A wife is created for him out of flowers, called Blodeuwedd.