According to Welsh tradition, Derfel Gadarn (English: Derfel the Mighty) was a valiant warrior who served under King Arthur at the Battle of Camlann and is recorded as one of its seven survivors. He was reputedly born around 566 and is one of the few Arthurian characters generally agreed to be a historical, rather than mythological, personage.

Following Arthur's final battle, Derfel became a Christian monk and is said to have founded the shrine of Llandderfel in Gwynedd. During his religious life, he served as the abbot of Ynys Ensi, Bardsey Island and died of natural courses on April 6, 660.

In Bernard Cornwell's The Warlord Chronicles, the story is related by the character, Derfel Cadarn, who carries out many of the legendary deeds of Bedivere (such as throwing Excalibur into the Lake; or in Cornwell's story, the sea).