Not mentioned in medieval chronicles, he is the subject of possibly legendary or fictional writings from the sixteenth century onwards,[2] the oldest surviving report being that of the Tudor antiquary John Leland.
Einion may have been the brother or son of the historical figure Cadifor ap Collwyn (Cedivor in some anglicisations),[1] Lord of Dyfed.
Both Lewis Glyn Cothi and Gwilym Tew say that Einion originated from Gwynedd, while other writers link him to Ednowain ap Bleddyn of Ardudwy and claim he was a relative of Iestyn, perhaps a great-nephew.
He persuaded Robert FitzHamon, historically lord of the honour of Gloucester, and twelve other knights, to bring a great army to the aid of Iestyn.
They jointly spoiled Morgannwg, but the Normans took the rich Vale of Glamorgan for their own share and left Einion only the mountainous areas of Senghenydd and Miscin, while the sons of Iestyn were rewarded for their acquiescence in their father's fate by the lowland lordship of Aberafan.
This full and elaborate story is first found in the "Brut y Tywysogion" (first printed in the second volume of the Myvyrian Archaeology, and later with a translation by Aneurin Owen for the Cambrian Archæological Association in 1863).
But the original manuscript of this "Brut" is believed not to be older than the middle of the sixteenth century, and therefore not much earlier than David Powel's History of Cambria (1584), in which the story of the conquest of Glamorgan also appears at length, varying from the above account in only a few details.