Who is Belinus and Brennius from The History of the Kings of Britain and what is their importance?

Belinus and Brennius are Dunvallo's sons. The two engage in a battle over who will rule and without the intervention of their mother..... would have killed each other. Afterward, the brothers sail to Gaul with the intention of conquering it together. Belinus later returned to Britain, where he ruled until his death. Belinus founded new cities and rebuilt others, which served to bring a temporary peace to the island. Brennius, however, stayed in Italy, where he ruled with violence.