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.
List of items in Once Upon a Time
The Dark One's Dagger is a blade belonging to the Dark One's evolving victim, whose name is inscribed. It is also later revealed to be a broken half of the sword Excalibur. After Nimue turned dark, Merlin tethered her to it, turning the broken half of Excalibur into the Dark One's Dagger. Anyone who wields the Dark One's Dagger can control the Dark One. A Dark One can cleave themselves of the Dagger's control and retain their powers, so long as they have the Sorcerer's hat when it aligns with the night sky, and the heart of someone who knew them before they became the Dark One. The only other way for a Dark One to survive, but lose their powers is to be kissed by their true love; however, this doesn't work in the Land Without Magic. The more evil deeds a Dark One performs, the more their heart blackens, until eventually, their original self vanishes and the Dark One is all they are.