Excalibur is a legendary sword that featured prominently in the Arthurian legends - according to most tales this mystical sword could only be welded by the rightful king of Britain and first appeared as the famous "sword in the stone".
Many people tried to free the sword from the stone but it wouldn't move until a young man by the name of Arthur Pendragon took the sword in his hands and freed it: the crowd saw this as a sign from heaven that Arthur should become king and thus the now famous King Arthur was crowned.
Excalibur is second only to the Holy Grail in significance when it comes to medieval romance and is the centre of innumerable stories, legends and fiction - in some accounts Excalibur will kill anyone who is not pure of heart while in other stories it will only display its seemingly miracolous powers when wielded by King Arthur or one of his descendents.
According to the original Arthurian tales Excalibur was eventually tossed into a lake - where the magical maiden known as the Lady of The Lake proceeded to spirit it away: presumably to the mystical Otherworld (an aspect of Celtic mythology that remained popular even after Christianity took over Britain as the dominant religion).
Excalibur is involved in numerous modern fiction stories, including The Extordinary Adventures of Alfred Kropp, and in The Alchemist series of books.