Hailing from one of the most well-known Western folk legends, Excalibur is the famous sword of King Arthur of Camelot, and is often attributed with magical powers and/or symbolizing the rightful king of England. It and its influence has made its way into many video games, particularly titles featuring swords as weapons.
Some games with a historical focus, such as the Tomb Raider series, may directly reference the Arthurian legend, and implement the sword as a historical or magical plot device rather than a weapon that the player can use. Others, particularly fantasy-themed RPGs, typically include Excalibur as a powerful late-game weapon. And still other games with no tie to the legend at all may merely use the weapon's name to allude to its formidable history in battle.
In most versions of the Arthurian legend, King Arthur obtained the magical sword Excalibur either by drawing it from the Stone (and establishing himself as the rightful King of England) or by accepting it from the Lady of the Lake; various versions of the story differ on whether the Sword in the Stone and Excalibur were the same weapon, and this is why the sword is sometimes attributed to the king's reign.
Some versions of the story ascribe magical powers to Excalibur, such as the ability to cleave through armor, or blind enemies' eyes with the glare of its blade. Excalibur's scabbard is often said to have powers of its own, protecting the wielder from death via blood loss. However, the scabbard was later stolen by Morgan le Fay and thrown into a lake.
When King Arthur was mortally wounded during his battle against the traitor Sir Mordred, in his dying moments he instructed his knight Sir Bedivere to throw Excalibur back into the lake so that the Lady would retrieve it. Twice, Bedivere was unable to cast aside such a priceless sword and lied to his king, but Arthur knew the truth because Bedivere had failed to mention the Lady. On the third attempt, the Lady's hand caught the blade as it fell and it was forever lost to myth and legend.