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The Up Arrow is an item Mario must give to Merlon in order to power up his partners and teach them ultra-rank moves in Paper Mario: The Thousand-Year Door. It is found in a room hidden in the spike chamber of Hooktail Castle, where Mario found the Black Key to open the black chest. Mario must use Admiral Bobbery to blow the crack in the right wall and reveal the secret room.
The purpose of the arrow is apparently to remind Merlon where he put the Ultra Stone he was going to give to begin using. Merlon claimed that the message of the Up Arrow's existence came from star charts and was directed at Mario.