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.
Artus-Excalibur
Years pass and Uther wages a bloody war with King Loth of Orkney for control of the country. Uther is killed in battle. After the devastating battle, the surviving knights mourn their fallen comrades ("Das Feld der Ehre / The Field of Honor"). Merlin appears and produces the magic sword, Excalibur. The wizard rams the sword into a massive stone, proclaiming that who ever pulls Excalibur from the stone will be crowned the true King of England. ("Der Heiler / The Healer").