The story of King Arthur is one of the most enduring pieces of culture in Western civilization. The Arthurian legend comes in many different variations and has inspired all sorts of folklore (and terrible films) about divinely chosen leaders fending off the forces of evil using mythical weapons and brave sidekicks. The exact origins of the legend are shrouded in mystery, and very little concrete evidence of such a king is known to exist. Still, the legend persists throughout Western culture and folklore.
Though earlier tales contained similar stories and characters, the modern iteration of the legend that most of us are familiar with owes itself to the 12th-century Historia Regum Britanniae (History of the Kings of Britain) by Geoffrey of Monmouth.