Caswallawn uab Beli, that is, son of Beli, is the usurper king of Britain at the opening of the Third Branch. He kills the regents who Bendigeidfran left behind when he took the armies of Britain to the Irish War.
Manawydan, brother of Bendigeidfran, has no stomach for more war on his return. He is therefore named as one of Three Humble Chieftains in a Triad. Instead he lives well away from British politics, in Dyfed, on his friend Pryderi’s bounty.
During Manawydan’s marriage feast to Rhiannon, Pryderi’s mother, Pryderi makes a journey to give homage to Caswallawn. This has been compared to the historical fealty sworn by Hywel Dda and other Welsh princes to Saxon kings.
Later Manawydan voices caution when he and the Dyfed family are facing conflict in towns outside Welsh territory. He is concerned that if the conflict escalates, Caswallawn will hear of it: clearly he is still a dynastic threat to Caswallawn’s rule. However he persuades the hotheaded Pryderi to
ride away from the conflict, and from Caswallawn’s attention.
The name Caswallawn may connect to the historical Cassivellaunos, king of the Belgae, who fought the Roman invaders. Roberts, historical credibility (1975: xiii) Romans are, significantly for dating purposes, not mentioned in the Mabinogi, but Caswallawn’s war with them is in Triad 35.