The Legend of Chekhov

When the heroes of a story are told a myth, legend, or fairy tale, you can almost guarantee that the story is true (or at least the truth as filtered through generations of retelling and/or a primitive culture's viewpoint) and the heroes will have to deal with it at some point. This is used so often, in fact, that it's actually more notable when the heroes are told a story and it doesn't turn out to be some flavor of true.

This is largely a result of The Law of Conservation of Detail, which demands that taking time out from the main story to tell some other story must only be done when that side story is important to the main plot. The purpose in labeling something important as a myth rather than just explaining it outright is to build excitement, so that when the legend is later shown to be true, it brings a sense of wonder or discovery. It can also serve to foreshadow future events, while giving the author an excuse for giving only partial or deceptive information.

Compare to Prophecies Are Always Right, where it's a prediction that's virtually guaranteed to be right rather than a story from the past. These two tropes often live side by side, with the ancient legend packaged with an included prophecy.

Contrast Shrouded in Myth, where the legend turns out to be exaggerated if not outright false.

Note that this trope is about the characters within a story being told a myth, which turns out to be based on actual events within the story's universe. This is not about an author using real-world myths in a story (though the myth the heroes are being told may well be borrowed from a real-world source).

Anime and Manga

 * In Laputa: Castle in The Sky, from the very start, Pazu believes the mythical city of Laputa exists. He's not wrong.
 * Saiunkoku Monogatari begins with Shuurei telling her students the story of their country's founding, ending it by saying that according to legend, the eight immortal sages who helped the first emperor found Saiunkoku are still alive in secret among the people. This is absolutely true, and Shuurei goes on to become personally acquainted with several of them. A little later in the first arc, Shuurei begins to tell Ryuuki the story of the Rose Princess and how she married a mortal man. This story is not only true, it's.
 * In Fullmetal Alchemist, the ancient Xingese legend of the Western Sage is, while the Amestrian legend of the Eastern Sage is.
 * In the second episode of Baccano, we hear the Urban Legend of the Rail Tracer: a monster that slowly snatches up and devours the passengers of the train on which its tale is told. Then a 3-way war breaks out over train-hijacking rights and...something starts picking off the instigators, leaving their twisted and mutilated corpses behind. Subverted when the Rail Tracer is revealed to be

Comics

 * Bionicle:
 * In most cases, the Matoran consider most of the Turaga's stories as mere fairy tales. But most of them wind up becoming painfully true. From giant Manas to the hellish Karzahni.
 * In the backstory of the Matoran, which claimed that the Great Spirit brought the Matoran out of darkness to the island of Mata Nui. We later find out that it was actually the Turaga who rescued them (as Toa Metru) from their ruined city; they just credited the Spirit with giving them the strength and abilities to do so.

Film - Live-Action

 * The Mummy Trilogy spends plenty of time giving us the whole legend of the Mummy before revealing that it's all true. (Which comes as a surprise to nobody in the audience, since that's the title of the movie.)
 * In The Secret of Roan Inish the grandfather tells the legend of the selkie, which is important to the climax of the film.

Literature

 * The Stephen King novel Desperation tells the legend of why an old mine was abandoned (a trapped Chinese Laborer summoned a bad spirit). There really is a monster, though actually they just Dug Too Deep and set loose an Eldritch Abomination.
 * In Harry Potter, the legend of the eponymous Chamber, which Professor Binns tells to his History of Magic class and dismisses as a preposterous myth, turns out to be absolutely true, including the deadly guardian.
 * In Harry Potter, the 'Tale of the Three Brothers' is an exaggerated and mythified version of the truth.
 * In Earthsea the myth of human-dragon hybrids trapped in human form, mentioned at the beginning of Tehanu, is proven true at the book's end and forms the basis for the plot of the next novel, The Other Wind.
 * In The Eyes of Kid Midas, the teacher explains that the mountain they're camped out underneath is called the Eye of God, and some ways away down in the valley there's a tall, thin spire that's known as the Devil's Chair. According to legend, the peak of the Eye of God was the place where the world was created, and once a year, the very tip of the mountain's shadow falls exactly at the peak of the Devil's Chair. Though the teacher admits it's merely a legend (and the mountain's shadow actually never goes near Devil's Chair), Kevin climbs the mountain, and at the top he discovers a pair of sunglasses that allow him to harness the forces of creation. And lo and behold, when he puts them on, he sees the shadow move fall exactly where his teacher said it would.

Western Animation

 * Scooby Doo plays this trope for laughs. Typically, something spooky is going on, but is actually engineered by someone wanting to scare people away. Half the time, it's an Invoked Trope, as the fake monster is based on an old local legend (or, occasionally, the villain makes up a story and claims it's an old legend). Once in a while, there's some evidence of actual spookiness, which the heroes may or may not see.
 * Jackie Chan Adventures plays this for laughs in an episode about Stonehenge. Apparently, it has some sort of great magical powers, but nobody really knows what it does. Jackie Chan, upon being told this, sarcastically remarks "Yeah, and some people think it's used to contact aliens." The bad guys figure it must be some kind of weapon, and Jackie Chan goes into action to stop them from activating it. Amazingly, the bad guys actually succeed at pulling off their Evil Plan to activate Stonehenge, revealing to everyone present that Stonehenge does... absolutely nothing. Everyone goes home, and then, in the last scene of the episode, a UFO lands at the now-deserted Stonehenge.
 * Kim Possible. Mystical Monkey Power is all about this trope.
 * The first incarnation of He-Man and the Masters of the Universe does this constantly. Anything that's supposedly just a myth, turns out to be real. EVERY. SINGLE. TIME.
 * The Real Ghostbusters and its Sequel Series Extreme Ghostbusters follow this trope pretty faithfully:
 * It's subverted in one episode of The Real Ghostbusters, where the Ghostbusters must deal with a creature from Irish folklore. According to legend, the creature can only be stopped by a four-leaf clover. All the characters go out searching for one, except Egon, who, playing the role of Agent Scully, insists that the creature can be captured using the same "scientific" methods they always use. In the end, the four-leaf clover fails (it was a fake taken from a parade float), and Egon saves the day by capturing the creature "scientifically", exactly as he said he would.
 * Ben 10 devoted an episode to the Navajo legend of the Yenaldooshi, as told by one of Max's former teammates. The monster in question is indeed real, although it turns out the legend got a lot of things wrong and it was actually an alien.
 * Futurama: the in-universe legends of mutants living under the city and El Chupinebra turn out to be completely true - as does the more common myth of crocodiles in the sewers.
 * In Thundercats 2011, the Catfolk-populated magical kingdom of Thundera, stuck in Medieval Stasis, considers technology to be mythical. Stories of "ships that could fly" are fairy tales told to cubs. The populace has become similarly skeptical of the existence of the Book of Omens and Mumm-Ra. In the space of one night, protagonist Lion-O and the Thunderians see their kingdom ruined when old enemies the Lizards invade, bringing with them futuristic technological superweapons, given them by Sorcerous Overlord Mumm-Ra. Lion-O and his Thundercats are then sent on a race to find the very real Book of Omens before Mumm-Ra can get his hands on it.
 * The first episode of My Little Pony Friendship Is Magic has a Storybook Opening of the tale of "The Mare in the Moon". Despite being referred to as an "old ponytale" by others, Twilight Sparkle believes the legend has a basis in reality and is concerned by prediction of Nightmare Moon's escape from her imprisonment on the moon, which is scheduled to happen within days. Of course it's all absolutely, literally true, and the return happens exactly as predicted.

Webcomic

 * Digger spends more than a few strips explaining the hyena mythology surrounding their progenitor-gods "He-Is" and "She-Is", and the "badness" that came from their interactions with the demonic Sweetgrass Voice. Naturally, it's eventually up to Digger to Set Right What Once Went Wrong.
 * In The Adventures of Dr. McNinja, if anyone mentions anything unusual or absurd-sounding -- like giant lumberjacks, dinosaur-riding bandito paleontologists, zombie ninjas, or the ghosts of dead NASA astronauts -- you can bet it'll not only exist but have a direct impact on the plot of the current arc.

Videogames

 * In pretty much all the Final Fantasy games, if you talk to any random NPC who tells you about some legend, the legend is bound to be true. Hidden magical weapons? Yup. Super-powered monsters? Yup. Maze-like hidden cities? Yup.
 * In Kingdom Hearts II, there are The Seven Mysteries of Twilight Town, urban legends which invariably turn out to be for real when Roxas investigates (and serve as clues to the nature of the world he's been living in). But when Roxas's friend comes along to do the write-up, he assumes each was just a misunderstanding of something mundane.
 * Bungie's Marathon series contains an interesting example. The second game has a single terminal midway through the game that references a S'pht creation myth where the god Yrro flung a chaotic being into the star that Lh'owon orbits. This terminal is never mentioned by any character for the remainder of the game. The myth then forms the entire plot of the third game, Marathon Infinity -- sure enough, the Jjaro (or Yrro) trapped an Eldritch Abomination inside the system's star. Which the Pfhor destroyed in the finale of the second game.
 * In Pokémon, any myth or legend you hear about from an NPC is almost guaranteed to be a hint as to where to find a particular Legendary pokemon.
 * A set of daily quests in World of Warcraft has you investigate myths about three maidens who will grant powerful swords if you do each a favor. Naturally, all three of them turn out to be true.
 * In Borderlands, the person who sends you to kill Crawmerax the Invincible says that most people think he is a myth and doesn't really exist. Naturally, when you get to the designated spot, he shows up. Subverted in that the quest-giver is quite surprised that you actually managed to find and kill Crawmerax since
 * Funny subversion in Lufia Curse of the Sinistrals remake. At one point you're sent to find the legendary sword, which the legends claim to have been forged by the gods and able to cut mountains in half. Just one line after that you're informed that all of that is just legend, and the sword itself is probably not even magical, but that you should get it just to boost the morale of the people. It turns our that the myth really wasn't true... but the sword is still good enough to be useful by the time you get to it.
 * In Star Wars: Knights of the Old Republic, you can learn about the creations myths of the Selkath and the Tusken Raiders. It turns out that both are true and are references to the Rakata, the creators of the Star Maps.
 * In Dwarf Fortress Adventurer Mode, if someone talks to you about some dragon who razed his hometown long time ago or a forest where the dead are said to rise and stalk the living, you can be absolutely sure he's telling the unvarnished truth. The only exception to this trope are centaurs, chimeras, and griffons, who sometimes appear on engravings but don't exist in the game world... yet. What's more, the stories will be told with impeccable detail. A thousand years on, everybody in the world still remembers which particular tooth was knocked out of the mouth of a random peasant by a marauding Bronze Colossus.
 * In The Legend of Zelda the Wind Waker, the intro cut scene with the story of the hero of legend and the kingdom of Hyrule turns out to be entirely relevant to the storyline, and last part of the game mainly takes place in the now flooded kingdom. Similarly, all those rumours about the 'triumph forks' turn out to be about the Triforce of Courage that Link finds in the endgame.
 * The Elder Scrolls III Morrowind plays with this trope, in that the particulars of a certain historical event relevant to the main plot of the game are recounted differently by different parties.
 * In the Game Boy game Final Fantasy Legend II (SaGa 2 in Japan), one of the worlds your characters explore has one myth that turns out to be true, although there's also another myth that turns out to be false.
 * In Paper Mario the Thousand Year Door, you can find a quirky storyteller in Rogueport who tells a tale about a horrible evil monster and the four heroes who fought it before being themselves sealed away. The monster is totally real -- there's a demon sleeping underneath Rogueport right now -- and Mario actually encounters each of the heroes in the form of talking cursed treasure chests. They're pretty nice.
 * Some of the local legends recounted to the protagonists of Chrono Cross are correct but... slightly skewed.