In Wario World, Wario must collect the four pieces of the Huge Treasure Chest Key[1] (also called huge treasure box key[2], giant key[3], or Giant Treasure Chest Key[4]) to unlock the huge treasure chest that the Black Jewel has locked itself in.
Wario receives a piece of the Huge Treasure Chest Key after defeating each of the major world bosses, DinoMighty, Dual Dragon, Red-Brief J, and Captain Skull.