Stranger Behind the Mask

"You tricked and fooled your readers for years. You've tortured us all with surprise endings that make no sense. You've introduced characters in the last five pages who were never in the book before!"

—Lionel Twain, Murder By Death

So you've got this mystery, such as the identity of the villain. There are a bunch of theories. Then The Reveal.

Wait, who's that?

You have just met the Stranger Behind The Mask, where The Reveal proves to be something or someone we've never heard of before, and had no way of expecting. This can often result in an Anticlimax, and is almost always a Shocking Swerve. Both Ronald Knox and S.S. Van Dine attempted to create rules for Detective Fiction, one of which was created in order to either prevent or avert this trope from occurring. Knox, indeed, made it his first commandment: "The criminal must be someone mentioned in the early part of the story..."

Typically a Writer Cop Out. If what is revealed also isn't particularly consistent with the story, it's an Ass Pull. Compare with Deus Ex Machina, where the unpredictable event is a solution to an otherwise unsolvable problem, and with The Dog Was the Mastermind, where it is revealed that it was a minor character that nobody would have suspected, but had been previously introduced. Often relies on Contrived Coincidence to keep the audience interested. Though often seen as unsatisfying, this is often a case of Truth in Television.

When Played With, this can turn from Bad Writing into a very skillful twisting of the story.

By the nature of this trope, all examples will inherently be mild spoilers.

Examples of Stranger Behind the Mask include:

Comic Books

There was an early Spider-Man storyline, in which a costumed crime-lord was built up as a huge mystery, before he was revealed to be someone neither Peter nor the reader had ever heard of. Steve Ditko wanted to repeat the gag with the Green Goblin, but Stan Lee thought it only worked once.
- Despite lots of foreshadowing that he may be Harry Osborn (among others) when the fifth Green Goblin was unmasked, he turned out be... nobody. Literally, it was some kind of Artificial Human created by Norman Osborn.

Film

Murder By Death. Lionel Twain cites this as one of his guests' myriad crimes against their readers during his "The Reason You Suck" Speech at the end of the movie. Then he takes off a mask to reveal himself as Yetta, the supposedly deaf-mute cook.
Phone Booth: Near the end, it's revealed that the mysterious sniper was the pizza delivery guy Stu humiliated in the beginning of the film. But then this is reversed a few minutes later, when it turns out he was actually some guy who had never before appeared in the film.
In the original Friday the 13th, the killer turns out to be Pamela Voorhees, who first appears seconds before The Reveal, and whose only foreshadowing was a random throwaway line about a boy (her son, Jason) who drowned in the lake decades ago uttered around the beginning of the film.
- Actress Betsy Palmer (Pamela Voorhees) even went to the director, Sean S. Cunningham, and asked to be put in the coffee shop scene at the beginning in order to give the audience some kind of foreshadowing, knowing full well her appearance at the end would result in the audience feeling cheated. Her request was denied since it was deemed inconsequential to the story. She relates this story on several DVD special features.
Speed Racer plays with this one. Speed is convinced that the masked Racer X is actually his brother Rex, but when Racer X does unmask himself, he's just some guy we've never seen before. At the end of the film, it's revealed (to the audience, not to Speed or his family) that he really is Rex, just with Magic Plastic Surgery to hide his identity.
The Italian Horror film Deep Red initially presents an extremely minor character as the killer, but at least the killer was a character. Then a few minutes later it's revealed that the killer is actually the absurdly minor character's mother.
Donnie Darko—but it actually works. Rather than being a plot-related reveal it deepens the surreality of the film.
The House on Sorority Row leads us to believe the killer is the Not Quite Dead Mrs Slater. Turns out it was her before-unmentioned son Eric born mentally unstable and physically deformed and saw the girls from the attic window. It's not fully explained in the film and you'd have to look up a full synopsis to get the proper details.
Ben Willis in both I Know What You Did Last Summer and Ill Always Know What You Did Last Summer.

Literature

Invoked in Murder on the Orient Express and its many adaptations. There is a murder. It takes place on the Orient Express. Hercule Poirot, who happens to be on board, investigates and discovers that Everybody Did It. However, it turns out that the victim was very much the asshole variety, having committed a heinous crime he would never otherwise be brought to justice for. In the end, Poirot chooses to tell the authorities that a random stranger sneaked on board, murdered the man, and then escaped - the only other remotely plausible explanation at this point.
Gaston Leroux's The Mystery Of The Yellow Room is a partial example. The killer is a character that was introduced early on, but is also revealed to be someone else the reader never heard about, and this revelation comes from off-page knowledge of the protagonist's.

Live Action TV

The "333" sub plot in CSI New York lasted for several episodes (with Mac constantly getting a silent phone call at 3:33 every day, getting his luggage back with a padlock set to 333, etc). When the episode which finally had him uncover the truth it turned out the one behind it all was a childhood friend who was motivated to call Mac out for freezing up when his brother was being beaten to death after he was touted as a hero in the papers after the previous season's finale. He hadn't appeared once (not even earlier in the episode when the flashback of the event took place) until The Reveal.
In an episode of Bones, after much investigation, and with only a couple of minutes of program left, it was suddenly discovered that the Victim of the Week was killed by a random burgler who the victim had walked in on during the burglary, whom we hadn't seen before.
Starting around season five, 24 set up a huge conspiracy with who was behind the events that carried over for that day, and partly leaked over to season six as well. Come the second half (and especially the last third) of season seven, the conspiracy is played out once again, and assumed to be reaching its endgame, come the season seven finale. Finally, the viewers watch rogue agent Tony Almeida get to The Man Behind the Man, and made some rather nasty decisions to reach him. So when we see the guy, it's... Alan Wilson, someone the viewers never spotted at any point or have any connection to, whatsoever. What made this twist even more jarring is that during this very season, the writers introduced Jonas Hodges, a much more engaging and charismatic villain who could've been a worthy choice to be the conspiracy leader. But instead, we have this.
Power Rangers Zeo did this with the identity of the Gold Ranger. After former Ranger Billy kept turning up missing around battles, after Tommy's brother was introduced to the main cast, after even Skull had a couple of moments where he disappeared unexpectedly when the Gold Ranger was around, it turned out to be... some alien from another planet that had never even been mentioned prior to that point. And he's losing his powers, so we get a not-quite-so-out-of-nowhere-but-still-unexpected case of this when Tommy elects Jason, the original Red Ranger, who hasn't been on the show for TWO WHOLE SEASONS by this point to take the powers (admittedly, Billy had a Hand Wave excuse.)
On the Angel episode "Harm's Way," Harmony wakes up after a one-night stand to find the guy dead, and though she doesn't quite remember what happened, she eventually realizes that she was set up for the murder. It turns out the real killer was...some random other vampire chick named Tamika working at Wolfram and Hart, who was upset that Harmony beat her out for her job through nepotism. (Arguably more The Dog Was the Mastermind, since Tamika had appeared very briefly earlier.)
In "Study in Pink", the first episode of Sherlock, the serial murderer turns out to be a random taxi driver, whose reason for killing the people he did had nothing to do with the victims themselves.
After three seasons' worth of build-up, Jack, the mysterious serial killer on Profiler, turns out to be... some random guy we've never seen before.
Psych: The Serial Killer Yin turned out to be his partner Yang's father, a character who had never appeared or even been mentioned on the show outside of his Yin persona prior to The Reveal.

Radio

Invoked In-Universe in an episode of The Burns and Allen Show. Gracie has been listening to a detective show and George comes into the room near the end. The killer is announced to be...Ebenezer Macgonogal! Gracie said she would never have guessed. Cue George asking, "Who is Ebenezer Macgonogal?" Gracie: "I don't know. That's the first time his name was mentioned."

Video Games

The identity of the Shadow Broker in Mass Effect 2 is widely discussed, both by the fans and in-universe. The announcement of the DLC "Lair of the Shadow Broker" drove speculation to a fever pitch. In the end, it was revealed that the Shadow Broker was a yahg, a species never before seen or mentioned in the Mass Effect universe.
Umineko no Naku Koro ni: Discussed and Invoked by Battler, in order to avoid having to accuse one of his family members of murder.

Web Comics

In Homestuck, Vriska's appearing suddenly in the fifth act and proceeding to dominate the plot afterwards is stated by Word of God to be an experimental attempt to make the entire plot dependent on a character who had not been previously encountered.
- And then there's Lord English. For the longest time, we only saw his coat and eyes, leading us to wonder who he is. When he is finally shown, he is... clearly nobody we've seen before, unless they've changed a lot.
  - In this case however, the readers were the only ones who supposed that Lord English was someone that the readers had seen before—the comic itself indicated no such thing. Ultimately, the reason only his coat and eyes were ever shown was both to build up suspense and to hide certain facts about his appearance; namely that he is quite obviously possessing Doc Scratch's now hideously mutated body and that he bears a resemblance to Demonic Dummy Lil' Cal, who was used to make Scratch and English. Note that the Cal connection was Fore Shadowed in advance.

Western Animation

Scooby Doo Where Are You episode "Spooky Space Kook". The villain at the end (the guy wearing the costume) was someone the audience had never seen before.
In one episode of A Pup Named Scooby-Doo, the villain is someone who was not mentioned earlier in the episode, to the utter confusion of the main characters... but then it's immediately subverted in that the character actually had appeared earlier in the episode disguised as an old woman; it was merely the fact that he wasn't really an old woman that hadn't been revealed.
Played with in an episode What's New, Scooby-Doo? The episode's Monster of the Week is actually a scientist who faked her own abduction in the episode's Action Prologue. As a result, the audience (and side characters) had seen her, but the heroes never met her, which frustrates Velma enough to declare the case void.
Technically happened in the Teen Titans cartoon. Viewers and the Tians never saw Slade's face, not even at the end. Viewers unfamiliar with the comics complained that we never found out who he was until pointed at the comics—turns out he's Deathsroke the terminator operating under his first name due to cartoon censors. So we knew his name the whole time.
- To be fair, it seemed like he had a secret ID.
In Spider-Man: The Animated Series, Jason Macendale was introduced in the very same episode that the Hobgoblin was unmasked, making it painfully obvious and quite anticlimactic when he was revealed.