Before you can wield Excalibur, you must collect 11 mysterious tablets from all over England. Some are carried by Order Zealots, while most are found in caverns. After collecting all 11, head to Myrrdin's Cave in Hamtunscire. Deep in this cave you will find Excalibur surrounded by a bunch of pillars. Interact with each one to place the tablets and gain access to Excalibur.