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 Broken Kingdom
In the early years before Camelot, we meet the stable boy who would later become the once and future King, Arthur, who was motivated to fulfill his prophecy of finding Excalibur at a very early age, which he would follow through as an adult years later (33 years before the Storybrooke residents' arrival, including 28 years of Regina's Curse), when he pulled the sword out the stone. While he proclaimed to his kingdom that he had found the entire sword, Arthur, like Merlin, only spoke in half truths, as the King left out the part about the sword being split in half, after he shows a portion from the case. He even tells Guinevere that he has studied the translations of the Camarthen Scroll, which contain the three elements: the star, the eye, and the sun, but he can't decipher the clues. Arthur's quest to reunite Excalibur has led to his long running obsession to seek out the missing sword piece, which in turn leads to shutting Guinevere out of his life, which in turn leads to her turning to Lancelot, who plans a party for her and makes her feel loved, and would stay behind to watch Guinevere as Arthur began his search for the missing dagger.