So you've got this mystery, such as the identity of the villain. There are many theories on this, and audience is kept busy speculating to the very last moment. And well, would you look at that!? The heroes has captured the villain and are about to remove his mask. It is finally time for the 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 reasonable 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.
The driving motivation of Kill la Kill's main character is to find the woman who killed her father. The woman appears about midway trough the series, and it's no one that had been seen before.
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. Spidey lampshades it by thinking "Sometimes, the culprit isn't always the butler."
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.
It's easy to forget this, but Venom was actually done like this. During Venom's introductory story arc Spidey was being stalked by this maniac in the black symbiote suit he'd discarded who seemed to know his identity and monologued angrily to himself about how Spider-Man had ruined his life. He was seen unmasked early in the story, but the readers were unable to identify him, leaving them puzzled about who this mystery man actually is. Then when he finally captures Spider-Man and unmasks himself before him... he's a completely original character, whose backstory was Retconed into an existing Spider-Man story (the infamous Sin-Eater arc). Even worse, Peter knows who Brock is (although not to the extent that they knew each other in Spider-Man 3), making this a Stranger Behind The Mask for the readers only, verging on Remember the New Guy.
Happened again during The Clone Saga, thanks to an editorial screwup. In an attempt to clean up the out of control storyline, Marvel retconned everything into being the work of a mystery man named Gaunt. He was intended to be Norman Osborne, the only spidey-villain with the credibility to pull off such a wide-ranging plot, but one writer didn't get the memo and dropped hints that Gaunt was serving a more powerful villain. They did an Author's Saving Throw by making Osborne this more powerful villain, and Gaunt was eventually unmasked as... Mendel Stromm, a D-list villain called "The Robot Master" whom had had all of two previous appearances: the first in 1966 and the second in 1986, a full ten years before the clone saga.
In-universe example: when, after years of being his enemy, the Vulture finally unmasks Spider-Man, he has no idea who Peter Parker is and is quite disappointed.
The third volume of The Ultimates began with the assassination of the Scarlet Witch. It is unknown who did it, and the mystery stays for some issues. Magneto, in vengeance for the betrayal? Some other mutant terrorist of the Brotherhood? The conservative Captain America, horrified by her open incest? Black Panther, who may be a villain? Hawkeye, one of the few who could make such a sure shot? None of them: it was Ultron. A known character of Marvel, but who had never appeared before in the Ultimate universe, except as a series of Faceless Goons.
For the first several issues of DC's famous Crisis on Infinite Earths, the villain was kept shrouded in darkness, his identity a mystery, leaving readers to guess at who it might be. At least one letter-writer guessed that it was Darkseid, probably because a similar technique had been used to hide his identity as the villain of "The Great Darkness Saga" in Legion of Super-Heroes. Then, when the villain's identity was finally revealed, it was the Anti-Monitor, a character who had never been seen or mentioned before, and who had been created solely to serve as the antagonist of Crisis.
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.
In the original Friday the 13th (1980), 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.
The villains of at least two Kitty Norville short stories. The werewolf she helps isn't the killer; it's just some random psycho they hunt down together. The guy who created the zombie borders on this; he's not a recurring character and didn't do it for any plot more complicated than being a possessive, sexist asshole.Tropes Are Not Bad; those stories and the series in general are more focused on her personal growth and place in the world than on the Monster of the Week anyway.
Live Action TV
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 burglar who the victim had walked in on during the burglary, whom we hadn't seen before.
The Gormagon arc had a similar ending - he turns out to be just some old guy who'd never appeared in any of the previous episodes.
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 he 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.)
Kamen Rider Wizard had a Mysterious Protector known as the White Wizard. When his identity was revealed, he turned out to be...a guy who first appeared at the end of the previous episode. It's then subverted when we learn the guy has another mask...the Big Bad Wiseman.
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.
Done in the series finale of Monk. The Big Bad of the series, the man responsible for the murder of Monk's wife, ends up being a fairly boring character introduced earlier within the same episode, and in a waythat makes itobvious he'd end up being the murderer a few minutes later. Thankfully this reveal only happens halfway through the finale; the bigger case ends up being trying to determine his motive after he kills himself, which is a much more satisfying mystery, followed by a good ending focused less on the detective work and more on the characters themselves and the conclusions to their development in the story.
Criminal Minds often did this in its early seasons, and did it well, being that the mystery was figuring out what kind of person the killer was before using it to actually reveal who the killer was.
How I Met Your Mother has, after 8 seasons, finally revealed The Mother's face. She is no one we have seen before, and not particularly noteworthy looking.
Played with in a George and Gracie radio 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."
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.
Used to great effect, as while the protagonist is dumbfounded by the revelation, Liara (who actually was the one pursuing the Shadow Broker) uses some obscure knowledge about his species to make some accurate (and embarrassing) guesses about him in order to rile her opponent. All from less than a paragraph's worth of knowledge of yahg biology!
The revelation of the AI behind the AI in Mass Effect 3 was received this way by the series' fans initially. After two DLCs' worth of Orwellian Retcons adding foreshadowing of its very existence (even leaving out the identity or nature), most of the fandom is still not convinced this was a smart thing to do for the authors.
At the end of the first Nancy Drew game, Secrets Can Kill, Jake's murderer ultimately turns out to be a suspect who was never seen or mentioned in the game until the third act. The Remastered version fixed this by introducing the suspect halfway through the game, and then including another culprit—this time, Nancy's contact Detective Beech.
In Castlevania: Lords of Shadow - Mirror of Fate, you'll spend much of the game wondering about the Lost Soul, a mysterious masked entity who inhabits Dracula's castle and acts like he knows the heroes. It's never revealed in the story. However, beating the game unlocks the Lost Soul's bestiary entry — which explains that he's a personification of Fate and not a character at all!
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 Foreshadowed in advance.
Parodied in Yu-Gi-Oh! The Abridged Series when Paradox finally removes his mask. The three protagonists all act like they are stupefied to recognize someone they knew... before admitting they have no idea who this is. Paradox then points out they couldn't know anyway, since he is from the future, prompting Yugi to ask him why he was wearing a mask to begin with if they couldn't have recognized him anyway.
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.
Subverted in "A Clue for Scooby-Doo" where the gang captures a ghostly diver and unmask him, but have no idea who he is. But then Shaggy remembers seeing his picture earlier and recognizes that it's actually the seemingly deceased diver all along pretending to be his own ghost.
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 (who declared she had deduced who it was before triumphantly removing the mask and finding out she was wrong) so much that she tried to declare the case void.
Technically happened in the Teen Titans cartoon. Viewers and the Titans 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 Deathstroke the Terminator operating under his first name due to cartoon censors. So we knew his name the whole time, and it only seemed like he had a secret identity.
The identity of the anti-bending terrorist Amon in Legend Of Korra turned out to be Tarrlok's brother Noatak, a character who had never been mentioned before.
The fact that the mask-reveal was actually to make the audience think that Amon was telling the truth about his identity makes this a tricky example. The character was introduced before the mask reveal, it was just that his visual identity was unknown. That, along with all the information we were given about Noatak before we were even informed of his existence makes the reveal less of a "Who's that?" than a "Oh, it wasn't him?"