In the underground, go to the land of monsters and get the rat's tail in a chest near the entrance. Then, in the overworld, use the HoverCraft to get to the silver mine close to Silvara. Give the guy the rat's tail and he will give you Adamant. Return to the underground and find the weapon master in the lower-right hand corner. Speak to the guy on the second floor and he'll transform your old Legend Sword into the super-powerful Excalbur. NOTE: Only Cecil can use the Excalbur.