 In the accounts where he is credited as a king, he is reported to have lost his realm to an invasion by his brother Cadell, King of Ceredigion.Merfyn's death may be connected to the incursion into Anglesey by the Viking Ingimundr in the first decade of the 10th century.The drowning of his son Haearnddur, or "Haardur", was reported by both the "Chronicle of the Princes" and the "Annals of Wales".