Sometimes with the patronymic fab Ceidio. Probably historical 6th-century Welsh chieftain of the Old North (former Welsh-speaking petty kingdoms of what is now the Scottish Lowlands) slain at the Battle of Arfderydd (573/5). So traumatized by his death was his retainer Myrddin (or Merlin) that he went mad, became a wild man of the woods, and received the gift of prophecy. His name may be commemorated in the place-name Carwinley [Caer Wenddolau, Gwenddolou's Fort], near Longtown, Cumberlandshire. Geoffrey of Monmouth (12th cent.) latinized his name as Guennolous.
See Rachel Bromwich, Trioedd Ynys Prydain, rev. edn. (Cardiff, 1978), 379ff.