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 Excalibur Alternative
The story begins when a 14th-century English army is abducted by aliens of the Galactic Federation to serve as mercenaries on planets where only low-tech weaponry is legal. The aliens are bound by a Galactic Federation law that states that advanced weapons can not be used on primitive worlds. Another Guild had previously abducted a legion from the Roman Empire and used it to obtain victory in many confrontations and thereby increase its commercial empire. Other aliens were inspired to try the same tactics and came to Earth to find their own army to fight for them. As the plot progresses, the English army continues to fight for the aliens, until the Baron in charge of the English is approached by a dragon-like alien under the heel of the Guild Aliens. The dragon-like creature tells the Baron the story of the Federation.