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 Welsh mythology, the "Dyrnwyn" ("White-Hilt"), one of the Thirteen Treasures of the Island of Britain, is said to be a powerful sword belonging to Rhydderch Hael, one of the Three Generous Men of Britain mentioned in the Welsh Triads. When drawn by a worthy or well-born man, the entire blade would blaze with fire. Rhydderch was never reluctant to hand the weapon to anyone, hence his nickname Hael "the Generous", but the recipients, as soon as they had learned of its peculiar properties, always rejected the sword.