Traditionally the son of Beli and the conqueror of the Island of the Mighty, thus disinheriting Manawydan fab Llyr, the brother and heir of Bendigeid Vran (Brān the Blessed), and cousin of Pryderi.

A separate Welsh tradition makes Caswallawn the brother of Llud, Llefelys and Nynniaw, thus having his origin in Cassivelaunus, the historical King of the Catuvellauni at the time of the first Roman incursion led by Julius Caesar.