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.
Avalon
Avalon (; , Old French: "Avalon", ; literally meaning "the isle of fruit [or apple] trees") is a legendary island featured in the Arthurian legend. It first appears in Geoffrey of Monmouth's 1136 pseudo-historical account "Historia Regum Britanniae" ("The History of the Kings of Britain") as the place where King Arthur's sword Excalibur was forged and later where Arthur was taken to recover from his wounds after the Battle of Camlann. Avalon was associated from an early date with mystical practices and figures such as Morgan le Fay. It is traditionally identified as the former island of Glastonbury Tor.