On the night of his birth Rhiannon's ladies-in-waiting fell asleep, and the tiny infant disappeared before he could be named. To save themselves from being blamed, the women smeared Rhiannon with blood and claimed that she had killed the baby and disposed of the body. Pwyll believed her innocent and refused to divorce her. Rhiannon, rather than fight the testimony of the women, did penance by carrying all those who would accept her offer into the castle.
The small baby was discovered by Teyrnon Twryf Liant (Teirnon). He adopted the boy as his own and named him Gwri Golden Hair. A year later, struck by the likeness of the boy to Pwyll, Teyrnon Twryff Liant concluded that the boy he had been looking after was none other than the lost child, so he took him to Pwyll. Pwyll and Rhiannon called the boy Pryderi and placed him in the care of Pendaran Dyfed, under whose guidance he grew up to be handsome, courteous and brave, a great warrior who was loved by his people. He married Cigfa, the daughter of Gwynn Gohoyw.
Pryderi was among the seven survivors of the expedition to Ireland led by Bendigeid Vran, and one of those who, after eighty-seven years' journeying, buried the severed head of Bendigeid Vran under the White Mount in London. The others who returned with him were Manawyddan fab Llyr, Gluenu Eil Taran (Glifieu), Taliesin, Ynawc (Ynawag), Grudyen (Gruddieu) and Heilyn. On his return to Wales he found that his cousin Manawydan fab Llyr had been disinherited by Caswallawn, the son of Beli. To compensate him for his loss Pryderi gave Manawydan fab Llyr his own mother, Rhiannon, as a wife, along with the seven cantrefs of Dyfed (literally meaning 'one hundred', a cantref was an administrative district).
One night, as Pryderi, Manawydan fab Llyr, Rhiannon and Cigfa were feasting, they heard a huge clap of thunder. This was followed by a cloud, out of which emanated a brilliant, blinding light, which enveloped them. As the light faded, they found themselves quite alone in Dyfed, for all their houses, men and animals had mysteriously disappeared. The four then became hunters, for the woods were well stocked with game and the rivers teemed with fish. For two years they lived quite happily, but Pryderi and Manawydan fab Llyr grew tired of their lonely existence and decided to travel from town to town to earn a living.
Some time later they were in the woods, hunting with their dogs, when the animals disappeared into a caer. Pryderi, against the better judgement of Manawydan fab Llyr, followed them, meaning to lead them out again, but he too was trapped inside. Manawydan fab Llyr waited until dusk for his cousin to reappear and then returned to Rhiannon to tell her what had happened. She immediately went to the caer and there saw a door that had remained invisible to Manawydan fab Llyr. She, too, entered the caer, whereupon it disappeared, taking Rhiannon, Pryderi and the dogs with it.
Cigfa was very frightened when she realised that only she and Manawydan fab Llyr were left. He, however, behaved honourably towards her, and swore to provide for her in her husband's absence. Having no visible means of doing this, Manawydan fab Llyr became a cobbler, a trade at which he excelled. Within a year he had prospered sufficiently to establish three crofts. These he sowed with wheat, but when the first crop was ripe and he came to reap it, he found that every ear had been stripped clean. The same happened with the second, so he sat and watched over the third. At night a host of mice appeared and started to eat the wheat.
When Manawyddan fab Llyr appeared, the mice scattered, but he managed to catch one, which he swore he would solemny hang the next day for theft. Cigfa tried to dissuade him from this ridiculous act, but Manawydan fab Llyr was adamant that the mouse should pay for its crime. As he was preparing the tiny gallows, a poor clerk came by, the first person other than Cigfa he had seen for a year, and offered Manawydan fab Llyr a sum of money if he would spare the mouse. Manawydan fab Llyr refused. Next a richly dressed priest came by and significantly increased the offer of the clerk, but again the offer was refused. Finally a bishop accompanied by his entire retinue stopped and offered Manawydan fab Llyr a king's ransom for the life of the mouse. Once more the offer was refused, but the bishop persisted, offering instead to give Manawydan fab Llyr that which he desired more than anything else.
Manawydan fab Llyr said that he wanted the return of Rhiannon and Pryderi, and the spell on their land to be lifted. The bishop agreed and revealed himself to be Llwyd fab Cil Coed, the husband of the mouse that Manawydan fab Llyr had captured. He also confessed that it was he who was responsible for the spell, which he had cast to avenge Pwyll's treatment of his friend Gwawl fab Clud. Good as his word, Manawydan fab Llyr released the mouse, the spell was immediately broken, Rhiannon and Pryderi appeared, and Dyfed was restored to its former state.
Pryderi's death came about through the trickery of Gwydion fab Dôn. Pryderi owned a herd of magic swine, which had been given to Pwyll by Arawn. Gwydion fab Dôn told his master Math fab Mathonwy about them and promised to secure them for him. Along with eleven companions, all disguised as bards, Gwydion fab Dôn travelled from Gwynedd to Dyfed, where he and his companions were warmly received by Pryderi.
Gwydion fab Dôn explained the purpose of his visit to Pryderi and promised the next morning to show him a fair exchange for the swine. After consulting his advisers, Pryderi agreed. That night Gwydion fab Dôn magically created twelve stallions, twelve greyhounds with golden collars and twelve golden shields. Pryderi accepted them and Gwydion fab Dôn hurriedly drove off the swine for he knew that his enchantment would fail after two days.
When it did, Pryderi and his men set out in hot pursuit to meet the army of Mat fab Mathonwy, who feared for the safety of his kingdom. This was exactly what Gwydion fab Dôn wanted, for the purpose in obtaining the swine had been to intice the conflict and thus get Math fab Mathonwy out of the way for long enough to allow him and his brother Gilfaethwy to abduct and ravish the foot-holder Goewin.
After two indecisive battles, Pryderi and Math fab Mathonwy called a truce and decided that the argument should be settled in single combat between Gwydion fab Dôn and Pryderi. The two met at Maen Tyriawg, where Gwydion fab Dôn used his magic to overcome and kill Pryderi.