Brydw was one of the sons of Vortigern, the great usurper of the British High-Kingdom. He is chiefly known for his appearance amongst the names inscribed on the 'Pillar of Eliseg': a 9th century memorial which still stands near Valle Crucis Abbey in Powys. The inscription has now gone and it was fragmentary even when first recorded in the 17th century, but it once proclaimed the illustrious ancestry of the Kings of Powys and their victories over the Saxons. Brydw was evidently mentioned in a missing portion which then continues, "Britu, moreover, the son of Guorthigern, whom Germanus blessed and whom Severa bore to him, the daughter of Maximus the King." He presumably met St. Germanus alias Garmon during his famous reprimandary visit to Vortigern's Court.

When Vortigern was finally ousted by his rival, Ambrosius, his sons were allowed to retain their father's central powerbase in Powys and the surrounding areas. Here they set up a number of petty kingdoms but, despite his dynasty being recorded, exactly which portion fell to Brydw is unknown. EBK suggests that he ruled Maelienydd (north-east Radnorshire) - which may have been named after his grandson, Mael ap Camuir - and possibly Elfael just to the south.