Once you get your hand on the Lunar Whale, it's time to checkout our blacksmith at Kokkol's Forge. The old man should be ready forging one the best weapon in the game for you. 

Now take the airship and fly to the hollow Earth, Kokkol's Forge, then talk to the old man. He'll give the Excalibur, second best weapon for Cecil in the game. Equip it immediately because it'll make journey ahead so much easier.