She was in the court of Math ap Mathonwy, a king who needed to rest his feet in the lap of a virgin unless he was at war, or wanted to be. When one of his virgins, Goewin, was raped by Gilfaethwy, she was replaced with Gilfaethwy's sister, Arianrhod. She immediately gave birth to a young boy (Dylan) and a blob.
Dylan was a sea-creature who immediately moved into the ocean. The blob, however, was placed in a chest by Gwydion, Arianrhod's brother. Arianrhod created three geases: only she could give him a name; only she could give him weapons; he would have no human wife. Arianrhod denied him the three aspects of masculinity. Gwydion raised him anyway, even without a name. Later Arianrhod saw him killing a wren with a single stone. She said that he was a bright lion with a sure hand and he took the name Llew Llaw Gyffes ("bright lion with a sure hand"). Gwydion then tricked her into arming him. Llew created his own woman out of flowers, Blodeuwedd.