Through the Looking Glass
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Once the player has unlocked Luigi, they must switch to his character and locate the mirror room on the second floor of the Mushroom Castle near the Snowman's Land and Chief Chilly Challenge paintings. Luigi must snag one of the Power Flowers on display to become Vanish Luigi, then enter the mirror. He must then open the door in the back of the mirror, reflecting the one used to enter the mirror room. Luigi finds himself in a completely blank area with almost nothing more than the other side of the door and a Power Star, which he is then able to collect.
If Luigi's vanish ability runs out before he makes it out of the mirror, he may simply pick up a Power Flower in the mirror to become transparent again.