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.
Excalibur
In Arthurian romance, a number of explanations are given for Arthur's possession of Excalibur. In Robert de Boron's "Merlin", the first tale to mention the "sword in the stone" motif, Arthur obtained the British throne by pulling a sword from an anvil sitting atop a stone that appeared in a churchyard on Christmas Eve. In this account, the act could not be performed except by "the true king," meaning the divinely appointed king or true heir of Uther Pendragon. As Malory writes: ""Whoso pulleth out this sword of this stone and anvil, is rightwise king born."" This sword is thought by many to be the famous Excalibur, and its identity is made explicit in the later Prose "Merlin", part of the Lancelot-Grail cycle.