Saint Cybi (Welsh), or Cuby (Cornish), was a 6th-century Cornish bishop, saint, and, briefly, king, who worked largely in Cornwall and North Wales: his biography is recorded in two slightly variant medieval 'lives'.

Maelgwn Gwynedd was king of Gwynedd during the early 6th century. Surviving records suggest he held a pre-eminent position among the Brythonic kings in Wales and their allies in the "Old North" along the Scottish coast. Maelgwn was a generous supporter of Christianity, funding the foundation of churches throughout Wales and even far beyond the bounds of his own kingdom. Nonetheless, his principal legacy today is the scathing account of his behavior recorded in De Excidio et Conquestu Britanniae by Gildas, who considered Maelgwn a usurper and reprobate. The son of Cadwallon Lawhir and great-grandson of Cunedda, Maelgwn was buried on Ynys Seiriol, off the eastern tip of Anglesey, having died of the "yellow plague"; quite probably the arrival of Justinian's Plague in Britain.