A legendary early Briton, thought originally to have originated in the god Bel. King of Britain in several non-Arthurian Welsh sources. His daughter or sister was Penardun who, by Llyr, was the mother of Bendigeid Vran.
According to The Dream of Macsen, Macsen conquered Britain from Beli; in Lludd and Llefelys, however, Beli’s son Lludd inherits the rule of the island after Beli’s death. Beli’s father was Manogan. His other sons were Llefelys, Caswallawn, and Nyniaw (Nynniaw).
In Welsh mythological genealogies, he is the grandfather of Bran the Blessed and Modron, and he is the brother-in-law of the Virgin Mary. He may be the same character later used as the father of Rheidwn or Rhun. He could be a reflection of the pseudo-historical Belinus.
According to Henry of Huntingdon (c. 1084-1155), Beli Mawr was the brother of the first-century British King Cunobelinus (Cunobel) or Cybeline. Welsh legend makes him the father of Avalloc, grandfather of Modron and Lord of the Otherworld.
Bendigeid Vran was thought to be an ancestor of Arthur in both his paternal and maternal pedigrees. His name is also thought to have given rise to Belinant, who features in the Arthurian legends as the father of Dodinel, as well as of Pellinore (Beli Mawr to Pellinore does not need too much imagination).
Beli Adver | The Legend of King Arthur