To get Excalibur
Buy the "Magic Finger" item at the Treno Auction early on. Once you reach Dargelo, give this item to an eldery man you meet there -- and he will give you Excalibur in return.

To get Excalibur II
This one is way more difficult. You have to get all the way to Hades in under 12 hours. After beating Hades, check the farthest right pillar for the sword.