The Treachery of the Long Knives () was a pseudohistorical myth and legend of a massacre of British Celtic chieftains by Anglo-Saxon soldiers at a peace conference on Salisbury Plain in the 5th century. The story is not included in any contemporary accounts, but does feature centuries later in the semi-mythological histories of the Historia Brittonum and the Historia Regum Britanniae. Though a popular cautionary tale in medieval Europe, no historical evidence for The Treachery of the Long Knives exists, and the story has been widely debunked as a purely literary construction by historians.