Instructions
Proceed to the Great Crystal in Giruvegan.
Head North from where you enter. You are moving from Waystone VIII to Waystone IX. Touch Waystone IX when you reach it. You will appear at another waystone (Waystone X).
Head south one screen from Waystone X to Waystone XX. Continue two screens south (take either of the lower paths) to arrive at Sagitarius. Touch the gate stone there and return to Waystone XX by heading north again.
Go one screen east from Waystone XX to arrive at Waystone XII. Take Waystone XII to Waystone XIV.
Head southwest one screen. Then head northeast three screens. This is the location of the Excalibur chest.
Equip your character with the Diamond Armlet and open the chest. The Excalibur doesn't appear every time, however, and it may take multiple tries before you actually get the sword. If you don't get the Excalibur when you open the chest, return to Waystone XIV and back again to make the chest re-spawn. Repeat this process until you get the sword.