According to  Geoffrey of Monmouth eXcalibur was forged on the Isle of Avalon by Otherworldly craftsmen.  

After the failure in battle of the Sword in the Stone. Merlin took Arthur to the waterside, where the Lady of the Lake affirmed his rightful kingship, with her gift of the magical sword eXcalibur.

'Then she rode into a valley where many great stones were, and when she saw she must be overtaken, she shaped herself  by enchantment unto a great marble stone'  

Sir Thomas Malory, Le Morte D'Arthur. Book IV, Ch. XIV

After the Battle of Camlann the wounded Arthur commanded Bedivere to return eXcalibur to the Lady of the Lake

'A hand and arm arose from the water and caught Excalibur by the hilt'