Excalibur is an ancient fey sword. Its best known holder was King Arthur who defended Britain from anglo-saxons invasions in the early medieval period.
Arthur Pendragon, you will lead your people in the defence of your homeland. Take this sword and show it to your people. You will be known as King Arthur!
The sword was forged by the Fey of Western Europe long before Humans arose there. Its original purpose was that of a weapon wielded by the guardian of the European Fey Order's castle. Guardian's duty was to keep Humans out of the castle until the order deems them ready. Humans were deemed ready around the time of the height of the Roman Empire. No one knows why it was that moment. Then, Excalibur was put away to the castle's armory never to be seen again. The sword resurfaced in the 5th century CE in Britain when Lady of the Lake Nimue took it from the castle and gave it to Arthur.
Nimue! What are you doing? Put that sword back where it belongs! Humans can't have it!
Excalibur's fate after the anglo-saxon conquest of Britain is shrouded in mystery. Some say that anglo-saxon kings had it, others that Arthur destroyed it. There are even people who believe that the sword was returned to the Fey.
It is said that in the English wilderness there is a stone in which Excalibur was put after Arthur's death. No one has been able to find it yet. Legend say that only a descendant of King Arthur can take it out of the stone
Excalibur has been enchanted to enhance the wielder's strength. In its hilt there is a quartz which was magically imbuded with the whole sword fighting knowledge of the Fey. The wielder has access to it.
The sword has also a specific aura. Fey are immune to its effects, but Humans are not. As the result, for Humans the sword seems to give the wielder authority over other people.