Fuchin Falls Cave is a dungeon-style location in Golden Sun, which is connected to Fuchin Temple on the east end of the continent Angara. It is blocked by a man by order of the temple's head monk, Master Nyunpa, and it is a test of Psynergy powers, which Nyunpa teaches as "Ki" spiritual power. Nyunpa will grant Adepts with Psynergy access into the cave. The cave itself includes puzzles involving rolling logs, and among its treasures are the Arctic Blade and the utility Psynergy item, the Orb of Force.
This is actually an optional dungeon that can be skipped, and the Orb of Force is not required to reach the game's end. The Orb of Force is required to get to some major secrets both in this game and in Golden Sun: The Lost Age, however.