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The characters are attending a Masquerade Ball, a dinner party, or a similar event with a sizeable amount of people. Suddenly, all the lights go out, usually followed by chaos as the people scream and try to feel their way around in the dark.

The lights come back on, and someone has been murdered during all the commotion. The characters now have a mystery on their hands - in the dark, anyone could have done it.

In Real Life, this kind of murder would be very difficult to actually pull off. The murderer would have to be able to see in the dark, in order to avoid tripping over things or bumping into people. Then they would have to correctly identify the person they want to kill without seeing their face. (This sometimes leads to a plot twist where it turns out that somebody else in the room was actually the murderer's intended victim.) They'd also need to work out how to discreetly get the lights off in the first place - everyone is going to suspect that shady-looking fellow who was hanging around the light switch. Best not to think about it too hard.

Compare Darkness Equals Death, where the lights are already off.

Examples:

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Anime & Manga

Quite a few murders in Detective Conan rely on this trope, including one subversion where it turns out the act of turning the power back on kills the victim. ( Poor Hikaru Yasumoto, killing her boss without even knowing it!)
In Tantei Gakuen Q, there's a murder while a Phony Psychic has the lights turned off as part of a ritual.

Film

Mr. Boddy's murder in Clue. Mr. Boddy's actually the one who turns off the lights, expecting everyone in the room to attempt to murder someone else. It Makes Sense in Context.
- And he was actually faking his death because he realized someone was trying to kill him instead. Said guest succeeds later, but not in the dark.
In Devil, nearly every time the lights go out, someone gets murdered.
The Three Stooges often parodied this trope, when during brawls someone would turn out the lights, and when they were turned on again the fighting parties would be in a funny position, or the Stooges would be accidentally beating each other up.
Used to the point of a Running Gag in Dark and Stormy Night. At one point, during the brief blackout, Jack Tugdon is killed, his head taxidermied, and mounted on a wall. Towards the end of the film, the characters aren't even surprised anymore.

Literature

In the Miss Marple novel A Murder Is Announced, a personal ad appears in the town newspaper, announcing that a murder will take place in a certain house at a certain time, and inviting everyone to come over. The Genre Blind locals assume the ad is inviting them to a party game (sort of "Murder in the Dark" for adults) and attend. Naturally, the owner of the house claims to know nothing about the ad, and then at the right time the lights go off...
- Something of a subversion considering the person who is found dead is the person who was waving the gun around.
In Baynard Kendrick's novel "The Last Express" a killer stabs a man to death in a night club and no one notices because the room was bathed in red light for a performance and you can't see red blood under red light.
The Anno Dracula novella "Vampire Romance". Deals with some of the usual objections: All the suspects, being vampires, have (or at least may have) supernatural speed and the an ability to see in the dark; and the lights go out due to an existing intermittent wiring fault.

Live Action TV

Doctor Who has the Weeping Angels, lifeforms that actually function by this trope. They exist as statues when being observed, and only when no one is looking can they move. When the lights go out, they can move almost instantly from place to place.
- This also happened in the episode "The Unicorn and the Wasp."
Star Trek: The Original Series episode "Wolf in the Fold". While a seance is going on to find out who the murderer is, the flame that's the only light in the room goes out. The medium running the seance screams, and when the light comes up again, she's dead: stabbed in the back.
Morla: The lights were out. Anyone would've had time to kill the lady.
One episode of The X-Files had an enemy that would only attack in the dark.
The Foyle's War episode "The White Feather"
An episode of Family Matters has an imaginary parody of detective works with Urkel as the detective. Every time Urkel would accuse someone of the murders, thunder would make the lights go off briefly and that person would be found dead.
Parodied in Monty Python's Flying Circus. In one sketch the lights go off and a police inspector is killed. Another police inspector enters and decides to reconstruct the crime by having the lights turned off. While they're off, he is killed as well. The scenario repeats until there's a large pile of dead police officers in the room.
Used in the slasher-themed episode of Boy Meets World.
Used in Tracker. One of the fugitive aliens cut the lights in the Watchfire, then killed someone while the lights were off.
Happened once on Dallas. When the murderer was confessing to their crime, they said this was actually a coincidence. The victim had had a poison inside them long before the lights went off. This example also subverts Perfect Poison.

Music

The song Dangerous Dan McGrew. Barbershop quartets sometimes sing a variation, part of which goes:
And then suddenly there, all the lights went out, and a voice cried "Die you must!"
And a shot rang out and a woman screamed. Somebody bit the dust!
Then the lights flashed on and the Northwest Mounted Police came a crashing through
And they drew their guns, and they said "Which one is Dangerous Dan McGrew?"

Professional Wrestling

Very common in Pro wrestling, the most notable examples including The Undertaker. Lights go off, gong sounds, lights come on, and either 'Taker mysteriously appears in the middle of the ring or someone who was already in it is incapacitated by 'Taker.

Tabletop Games

If this isn't happening at least once per mission in Paranoia, you're not playing it right.

Visual Novels

Double subverted in the first case of Super Dangan Ronpa 2. The lights go off at a party, and when they come back up we see... Mikan fell down again. A few minutes later, a corpse turns up.

Western Animation

Invoked in-universe by Bart Simpson. After telling his class mates a ghost story, he reassures them that Dark Stanley would never dare attack a crowded, well-lit -- The lights go out, then back on, and Bart is "dead" on the ground with exposed brains.
In the first "Tales of Interest" episode of Futurama, Leela's last three victims are all killed this way.
Leela: Could you get the lights on your way out?
The South Park episode "Cartman's Mom Is Still A Dirty Slut" begins with Dr. Mephesto being shot in the chest. It becomes the focus of the entire episode.
In the Codename: Kids Next Door episode "C.L.U.E.S.", Numbuh Three's Posh Party Rainbow Monkey is stabbed in the back this way.
In the episode "Mystery Train" of Adventure Time, it's exaggerated and subverted. A lot of candy people die on the train one by one as the lights go off, leaving nothing but skeletons. Finn tries to solve the mystery, while it turns out that no one actually died; it was a birthday surprise set up by Jake, all the skeletons are fake, and the prime suspect, the Conductor, was Jake in disguise.
In the Family Guy episode "And Then There were Fewer", when everyone accuses James Woods of being the murderer, the lights go off and then back on to see that Woods has been stabbed.
- At the end of "Twelve and a Half Angry Men", Stewie notes there's still a maniac on the loose cutting peoples' power and killing them. When the lights go off, Stewie flatly says "We're dead".