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The blackout is an event in Luigi's Mansion. After Luigi defeats Boolossus and opens the door to Area 4, several bolts of lightning strike the mansion. This causes the power to go out, darkening every room, including ones Luigi has already completed, and allowing ghosts to wander freely.
During the blackout, set groups of ghosts appear in rooms, even replacing ones Luigi has not caught, including Portrait Ghosts. This can make the mansion difficult to navigate. Gold Mice reappear in the darkened rooms if Luigi had not captured them before, so the blackout can be used to his advantage. Ghosts in a room respawn if Luigi captures all of them and reenters the room during the blackout. The only rooms without ghosts are the Foyer, the Bottom of the Well and the Breaker Room, though they still go dark. Also, some previously accessible rooms lock up during the blackout. They reopen after Luigi turns the lights back on. The player cannot save during the blackout as none of the Toads appear and no Boos can be caught.
In order to turn the power back on, Luigi must find and capture Uncle Grimmly in the Wardrobe Room, causing a chest containing the key to the Breaker Room to appear. He can get a hint about this by talking to Professor E. Gadd in the Telephone Room. Luigi can use the key to enter the Breaker Room and pull a switch that restores the mansion. Rooms that Luigi had not previously completed stay dark except the Breaker Room, which lights up.
Until the blackout situation is resolved and Uncle Grimmly captured, Luigi cannot go back to the lab via the Mansion's entrance in the 3DS remake of the game as the door is locked by a "mysterious force".
This list shows the different ghosts that appear during the blackout and the rooms in which they are found. They are listed in roughly the ordered in which they are encountered.