Medieval Welsh tradition held that he was related to Hywel, a legendary Brythonic king of Brittany.
Tudur Penllyn wrote: According to Lewys Glyn Cothi: After Camlan, Derfel is unanimously held in Welsh tradition to have entered the religious life.
The Llandderfel image was removed and dismantled by order of Thomas Cromwell during the English Reformation and used to burn a Catholic priest, John Forest, at Smithfield in London in 1536.
[4] Part of the image, a wooden statue of Derfel's horse, survives to the present day at Llandderfel.
[3] A fictionalized Derfel Cadarn is the main character in Bernard Cornwell's historical fiction/historical fantasy novel trilogy The Warlord Chronicles, retelling the story of King Arthur in Dark Age Britain.