Excalibur was the name of a mythical sword in Arthurian legends. Little was known about it.
At some point in the history of the sword, the prophetic demon Whistler acting on behalf of the Powers That Be ushered the young King Arthur to retrieve the sword from the stone, as in the myth of the Sword and the Stone.
The witch Vulcana explicitly referred to the Slayer weapon Mʔ as a counterpart to Excalibur in her incantation which repaired the broken weapon.