Legendary king of Britain in pre-Roman times; foundational ancestor of multiple Welsh royal lineages traced through his sons Lludd Llaw Eraint (eponymous foundational-king of London — the Mabinogi tradition derives the name London / Welsh Lludd from him, though the actual etymology is from Latin Londinium), Caswallawn (the Welsh form of Cassivellaunus, the historical Briton-king who fought against Julius Caesar — illustrating the Welsh medieval-historical connecting of the legendary-Beli-Mawr lineage to the Roman-period historical record), and Nynniaw / Nyniaw. The Beli-cognate Continental-Celtic deity-name Belenus / Belinus (a healing-and-solar deity attested across Roman-period Continental Celtic territories) suggests an underlying deity-tier substrate, with the Welsh Beli rationalized into a mortal-king-figure in the medieval-Welsh tradition. Per the Mabinogi-genealogical tradition, also fathered Penarddun (mother of the Branch-2 Llŷr trio); the Beli-Penarddun-Llŷr connection links the Branch-2 Llŷr family to the foundational Welsh royal-ancestor lineage through the maternal line.