Acquired the airship.
Story-based, cannot be missed.
After defeating the Lich, you must travel to the far East side of the map to Crescent Lake, and there obtain the canoe, which will give you a way to cross shallow water. Use the canoe to travel through the series of waterways West of Crescent Lake until you reach the Ice Cavern. In there you can obtain the Levistone. Once you have the Levistone, head to the small desert area just South of the cave, a cutscene will play, and you will have the airship.