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The Flashlight is an item Luigi uses in Luigi's Mansion. When Luigi first enters the mansion, he is only armed with the Flashlight. When Luigi progresses in the game, however, he can use it to stun ghosts.
In the Nintendo Land minigame Luigi's Ghost Mansion, which is based on Luigi's Mansion, each of the four Mii players looking for the Gold Ghost Mii is armed with a Flashlight, which is used to attack the ghost. It can be turned on and off; however, if players leave it on for too long, its batteries will run low until they run out altogether. New batteries that appear in the mansion can be collected to restore the Flashlight's battery life.
The Flashlight is an inventory item in the Nintendo Adventure Book Flown the Koopa. If they have it, Mario and Luigi use it to explore the basement of Magikoopa's fortress, but the batteries die, leaving them vulnerable to a pack of Mega Moles.