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.
Carneddau
Llyn Ogwen was reputed to be the place from which Sir Bedivere failed to draw King Arthur's sword Excalibur. Another legend has it that the two small lakes nestling below the towering cliffs to the east of Carnedd Llywelyn were haunted and that deformed fish with heads but no bodies lurked in their depths. A more recent legend records that the two great boulders known as the "Meini Gwynedd" near the summit of Carnedd Llywelyn were lifted there bodily in 1542 from the banks of one of these lakes. Henry VIII is said to have ordered the investigation of this claim and later proclaimed that it was true.