The Iron Giant is a hidden superboss in the 3-D remakes of Final Fantasy III. It is hidden in the new dungeon under the sea. To battle it, the player must receive via Mognet all the letters from the Four Old Men and King Alus. For the mobile versions, it is required to receive all the letters without the Mognet. It is the most powerful boss in the game. For defeating the Iron Giant, the player receives the Metal Card key item as proof of their victory.