A british chieftain, Geoffrey says he was the King of Scotland, who died at the battle of Arfderydd in around 573 AD, fighting a combined force of the sons of Eliffer – Gwrgi and Peredur – and King Rhydderch of Cumbria. Merlin was belived to be his bard and adviser, and events at the battle drove Merlin insane. Gwenddolau’s warriors continued the battle for a month and a half despite their lord’s death.
The Welsh Myrddin poems say that he was Myrddin’s (Merlin’s) lord at the battle, his retinue being described as one of the six faithful companies of Britain, for it continued to fight on for six weeks after the death of Gwenddolau.
One of the Trioedd ynys Prydein elaborates the story, saying that he had birds that were tethered by a yoke of gold and that ate two corpses for dinner and supper. They were killed by Gall, the son of Dysgyfdawd.
The Vita Merlini contradicts all the earlier sources by saying that Gwenddolau was a British prince who fought on the opposing side to Merlin at Arthuret. Fighting against his cousins Gwrgi and Peredur, he was killed in the fracas.
Various; all Myrddin poems | 12th century to 15th century
Annales Cambriae | c. 960-980
Triads of the Island of Britain (Welsh ”Triads”) | 11th century to 14th century
Vita Merlini | Geoffrey of Monmouth, c. 1150