Excalibur
Geoffrey of Monmouth, in his "Historia Regum Britanniae" ("The History of the Kings of Britain", c. 1136), Latinised the name of Arthur's sword as "Caliburnus" (potentially influenced by the Medieval Latin spelling "calibs" of Classical Latin "chalybs", from Greek "chályps" ["χάλυψ"] "steel"). Most Celticists consider Geoffrey's "Caliburnus" to be derivative of a lost Old Welsh text in which "bwlch" (Old Welsh "bulc[h]") had not yet been lenited to "fwlch" (Middle Welsh "vwlch" or "uwlch"). In the late 15th/early 16th-century Middle Cornish play "Beunans Ke", Arthur's sword is called "Calesvol", which is etymologically an exact Middle Cornish cognate of the Welsh "Caledfwlch". It is unclear if the name was borrowed from the Welsh (if so, it must have been an early loan, for phonological reasons), or represents an early, pan-Brittonic traditional name for Arthur's sword.
The Mists of Avalon
After Uther dies in battle against the Saxon invaders, Arthur claims the throne of Britain despite questions about his legitimacy (he had been conceived within days of Igraine's marriage to Uther Pendragon). Since Arthur must now defend Britain against the Saxons, Viviane has Morgaine make him an enchanted scabbard that will prevent him from losing blood and gives him the sacred sword Excalibur. With the combined force of Avalon and Caerleon, Arthur repels the invaders.