Following the death of Pwyll, Pryderi became ruler of the seven cantrefs (divisions of land) of Dyfed. He expanded his territories, conquering Ystrad Tywi (Carmarthenshire) and Ceredigion. He eventually settled down, marrying Cigfa, the daughter of Gwyn Gohoyw.
When Branwen, sister of Brân the Blessed king of Albion, is mistreated by her husband Matholwch, king of Ireland, Bendigeidfrân (a magical giant) raised an army, including Pryderi, and went to war against Ireland. Pryderi accompanies him.
Pryderi is one of only seven men to survive a violent battle. A mortally wounded Bendigeidfrân asks them to cut off his head and to return it to Albion. They arrive in Harlech, where they are entertained by Bendigeidfran's head, which continues to speak. They then moved on to Gwales (Grassholm Island). They lived there for eighty years without noticing the passage of time. Eventually, they take the now silent head to the Gwynfryn (the White Hill), where the Tower of London now stands and bury it facing France to ward off invasion.
Pryderi invites Manawydan, Bendigeidfran's brother and a fellow survivor, to live with him in Dyfed. He arranges marriage between Manawydan and his widowed mother Rhiannon. Pryderi, Cigfa, Manawydan and Rhiannon ascend a magical hill, but when they descend, Dyfed had turned into a barren wasteland with no inhabitants. Although, they travel to England for work, their superior skills are resented by tradesmen, and they are forced to move from one place to another.
They eventually returned to Dyfed and whilst hunting, Pryderi and Manawydan came across a white boar and followed it to a huge, towering fort. Ignoring Manawydan's advice, Pryderi enters and upon touching a huge golden bowl, is stuck and loses the power of speech. Rhiannon went after him and suffers the same fate as her son. Mist descends and Pryderi, Rhiannon and the fort vanish.
Manawydan's cunning eventually freed them from their imprisonment. It is revealed that their suffering was caused by an enchanter, Llwyd ap Cil Coed, in revenge for the humiliation of his friend Gwawl ap Clud by his father Pwyll who married his fiancé, Rhiannon. The enchantment is broken and Dyfed returns to its former glory.
Years later, Pryderi declared war on Gwynedd after otherworldly pigs given him by Arawn, king of Annwn, are stolen by the warrior magician Gwydion. After three defeats Gwydion and Pryderi engage in single combat and Pryderi is killed. His men return lamenting the death of their lord.