Son of Beli Mawr in the Mabinogi/Mabinogion who conquers Britain in Bendigeidfran's (Bran the Blessed's) absence, killing all but Caradog (2) and righting the wrongs of Branwen. His cloak of invisibility or veil of illusion allows him to wield his deadly sword without being seen. When he is crowned king in London, Pryderi pays homage. His characterization may have drawn from the historical figure Cassivellaunus. A brother of Arianrhod and sometimes a brother of Lludd. Rachel Bromwich (1954) has posited the existence of a lost saga dealing with Caswallon's wars in Britain.