Before doing anything, make sure that you take the time to equip your familiars with weapons that can do fire damage, such as the Burning Blades for your sword users. It’ll help make one of the next boss battles much easier.
If you look at your World Map, you’ll see a star constantly circling Autumnia to the northeast; this is your next destination, and you can only encounter it by ship, so go ahead and use your ‘Travel’ spell to visit Hamelin. From there, head back to the southern coast and board your ship, then sail northwest, following along until you run into a ghost ship, the Black Porpoise. You’ll be thrown into a quick, not-too-tough boss battle, followed IMMEDIATELY by another (rather tough) boss, so be sure to be ready!
Boss Fight: Jetsam
Boss Fight: Cap'n Crossbones - Scourge of the Seven Seas
After both battles, you’ll receive the [SUN STONE] . Only one more stone to go!