In Golden Sun: Dark Dawn for the Nintendo DS, there is an extra location you can access called Crossbone Isle, which is the location of the hardest boss in the game.
To access Crossbone Isle, you first need to beat the game. After the credits roll, save your game. Load your saved game, and sail south from Tonfon. Search around for a peninsula at the very edge of the world that has a whirlpool nearby. Sail into the whirlpool, and sail east once you’re through. Look for an island with a cave, and this should be the entrance to Crossbone Isle.