The Treason of the Long Knives[1] (Welsh: Brad y Cyllyll Hirion) is an account of a massacre of British Celtic chieftains by Anglo-Saxon soldiers at a peace conference on Salisbury Plain in the 5th century.
[2] According to the tradition, Vortigern, who had become a high king of the Britons in the wake of the end of Roman rule in Britain, called for Anglo-Saxons under Hengist and Horsa to settle on the Isle of Thanet in exchange for their service as mercenaries in battles against the Picts and Gaels in Scotland.
The settlers, however, exploit a drunken Vortigern's lust for Hengist's daughter into allowing them to increase their numbers and granting them more land, eventually including all of the Kingdom of Kent.
The story first appears in the much later Historia Brittonum, attributed to the Welsh historian Nennius, which was a compilation in Latin of various materials (some of which were historical and others mythic, literary or legendary) put together during the early 9th century, and surviving in 9th-century manuscripts – i.e., some 400 years after the supposed events.
The Treason of the Long Knives is also described in Book 6 of the Historia Regum Britanniae by Geoffrey of Monmouth, who wrote during the early 12th century and presumably used Nennius as his main source.
According to him, the incident took place at a banquet in modern-day Wiltshire, ostensibly arranged to seal a peace treaty, which may have been the cession of Essex and Sussex in exchange for intermarriage between Rowena, the daughter of Saxon chieftain Hengest, and Vortigern.
The story claims that the "Saxons" — which probably includes Angles and Jutes – arrived at the banquet armed with their long knives (seaxes) hidden on their persons.