In the Mabinogion, he seized the thone of Britain from Bran the Blessed when the latter was fighting a war in Ireland. In the Welsh Triads, his horse is named as Meinlas and his lover, whom he abducted from Caesar in Gaul, as Fflur, and he is said to have left Britain with 21,000 men in pursuit of Caesar and never returned.
The second combined expedition was conducted by Caswallawn the son of Beli, and grandson of Manogan, and Gwenwynwyn and Gwanar, the sons of Lliaws, the son of Nwyvre and Arianrhod the daughter of Beli, their mother.
The first was Caswallawn the son of Lludd, son of Beli, son of Mynogan; the second was Caradog, son of Bran, son of Llyr Llediaith; and the third was Owain the son of Maximus.
The first was Caswallawn son of Beli, for Flur, ab Mygnach the dwarf, and he went for her as far as the land of Gascony against the Romans, and he brought her away, and killed 6,000 Caesarians; and in revenge the Romans invaded this Island.
Cassivellaunus appears in the Welsh Triads, Mabinogion, and Welsh versions of Geoffrey's Historia, as Caswallawn, son of Beli Mawr.
Caswallawn's decision to allow the Romans to land in Britain in exchange for a horse called Meinlas ("slender grey") is one of the Three Unfortunate Counsels of the Island of Britain.
Caswallawn is named as one of the Three Golden Shoemakers of the Island of Britain, disguising himself as a shoemaker when he went to Rome to seek Fflur.