He married Rhiannon, widow of Pwyll of Dyfed and mother of Pryderi, and went to live there. One day, all of Dyfed turned into a wasteland, and only Rhiannon, Manawydan, Pryderi, and his wife Cigfa, were spared. Manawydan and Pryderi out hunting followed an enormous white boar into a caer, where Pryderi saw a golden bowl; when he touched it, he was enspelled. Rhiannon went after him and fell under the same spell; the caer then vanished, taking them with it. Manawydan, turning to farming, lost his crops to an army of mice which were clearly faery mice. He managed to catch one, and despite being begged by important passersby (priests in the Christianized version) threatened to hang it. Finally, Llwyd appeared and confessed to being behind all the trouble, in vengeance for Gwawl, Rhiannon's original suitor.
Manawydan ap Llyr, son of Llyr and Penarddun and brother of Branwen and half brother of Nisien and Efnisien. Manawydan was a scholar, a magician, and a peaceful man. But when Bran learned of the slavery imposed upon his sister Branwen by her Irish husband Matholwch, he joined the expedition to rescue her. Matholwch was terrified at the sight of a forest approaching Ireland across the sea: Bran's navy, and Bran himself wading through the water. He sued for peace, they built a house big enough for Bran, and Matholwch agreed to settle the kingdom on Gwern, his son by Branwen. Some Irish lords objected, and hid themselves in flour bags to attack the Welsh. But Efnisien, scenting Irish treachery, cast them into the fire, and then cast Gwern himself in (avoiding the geas against shedding kinsmen's blood thereby). A war broke out, and the Irish replenished themselves through the cauldron. Efnisien, repenting, sacrificed himself by feigning death and being thrown into the cauldron, which he then broke, dying in the process. Only seven Welshmen survived, and Bran was fatally wounded. His head, which remained alive and talking, was returned to Wales and buried, and soon afterwards Branwen sailed to Aber Alaw and died. Manawydan inherited after Bran's death, but surrendered the kingdom to his cousin to avoid warfare.