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The Wario Key is an item in the game Super Mario 64 DS. It was used by Chief Chilly to imprison Wario inside Princess Peach's Castle after he captured him. Upon entering the room in the castle with the giant mirror, Luigi can grab a Power Flower, granting him the "vanish" ability. The intangible Luigi can then pass through the mirror and jump into the painting of Wario. In the painting world, Luigi can claim the Wario Key after defeating Chief Chilly.
The Wario Key is a gold key about Luigi's size. It has a giant "W" in the center with Wario's Cap on top. After Luigi unlocks the door that Wario was locked behind, Wario becomes a playable character and Wario's Caps became available in courses.