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.
Accolon
Accolon's next battle is with a mysterious swordsman who is actually Arthur, wielding a sword he believes to be Excalibur but which was actually a fake given to him by Morgan. Neither man recognises the other but Accolon gains the upper hand due to wielding the real Excalibur. During the battle Arthur's sword broke; he quickly realized this was not the real Excalibur and that he had been betrayed. Rather than giving up, Arthur continued to fight with what was left of his shield. Nimue, the Lady of the Lake, arrived and used her magic to cause Excalibur to fly from Accolon's hand. Arthur then wrestled the scabbard back and dealt Accolon a mortal wound.