Historical records of this early era are scant. Maelgwn appears in the royal genealogies of the Harleian genealogies, Jesus College MS. 20, and "Hengwrt MS. 202". His death in a "great mortality" of 547 is noted in the "Annales Cambriae". Tradition holds that he died of the 'Yellow Plague' of Rhos, but this is based on one of the "Triads" that was written much later. The record says only that it was a "great mortality", which followed the outbreak of the great Plague of Justinian in Constantinople by a few years.