Excalibur is the legendary sword of King Arthur, attributed with magical powers and associated with the rightful sovereignty of Great Britain.
Merlin used his magic to set the sword in a stone. Written on the sword, in letters of gold, were these words: "Whoso pulleth out this sword of this stone is the rightwise born king of all England."
Arthur obtained the throne by pulling the sword from the stone. The act could not be performed except by "the true king," meaning the divinely appointed king.
Find the sword and you will find the cache.