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.
Morgan le Fay
In the Post-Vulgate, where Morgan's explicitly evil nature is stated and accented, she also works to destroy Arthur's rule and end his life, but the reasons for her initial hatred of him are never fully explained other than just an extreme antipathy towards the perfect goodness which he symbolizes. The most famous and important of these machinations is introduced in the Post-Vulgate "Suite", where she arranges for her devoted lover Accolon to obtain the enchanted sword Excalibur as well as its protective scabbard, which has been previously confided to Morgan by Arthur himself as he had trusted her even more than his wife, replacing the real ones with fakes. In a conspiracy with the villainous lord Damas, Morgan plans for Accolon to use Arthur's own magic items against him in single combat, so she and her beloved Accolon would become the rulers. As part of her convoluted plan, both Arthur and Accolon are spirited away from their hunt with Urien by a magical boat of twelve damsels. Confident of her coming victory, Morgan also attempts to murder her sleeping husband Urien with his own sword, but in this act she is stopped by their son Yvain (Uwayne), who pardons her when she protests she has been under the devil's power and promises to abandon her wicked ways. After Arthur nevertheless mortally defeats Accolon in a duel arranged by Morgan, her former mentor Merlin, still having feelings for her, saves her from Arthur's wrath by enabling her to escape. To avenge Accolon's death, which caused her great sorrow, Morgan again steals the scabbard from the sleeping king. Pursued by Arthur for her betrayal, Morgan throws the scabbard into a lake, before temporarily turning herself and her entourage to stone, the sight of which makes Arthur think they have been already punished by God. That action of Morgan ultimately causes the death of Arthur, who would otherwise be protected by the scabbard's magic in his final battle. On her way out, Morgan saves Arthur's knight named Manassen (Manessen, Manasses) from certain death and enables him to kill his captor when she learns Accolon was Manessen's cousin. Now banished from Camelot, Morgan retires to her lands in Gore and then to her castle near the stronghold of Tauroc (possibly in North Wales). However, her treacherous attempts to bring about Arthur's demise in the "Suite" are repeatedly frustrated by the king's new sorceress advisor Ninianne (the Lady of the Lake). Such is the case when Morgan sends Arthur a supposed offering of peace in the form of a rich mantle cloak; Morgan's messenger maiden is made put on the cursed gift and it burns her to cinders. It is possible that this motif was inspired by classical stories like that how Medea killed her rival for Jason's affection or how Deianira sent a poisoned tunic to Hercules. In one of her castles, Tugan in Garlot, Morgan has hidden a magic book given to her by Merlin, which actually prophesied the deaths of Arthur and Gawain and who would kill them, but no one can read this passage without dying instantly.