One Myth to Rule Them All

"I hate the good wizard from faerie tales. They all turn out to be him."

Closely related to All Myths Are True, but rather than every myth and legend being fact, they are mostly hokum—but with one common grain of truth in all of them. If the story is about vampires, then Rasputin was a vampire, Hitler was trying to breed an army of super suckheads, and Dracula was a wimp. If it's about wizards, Moses was an uber-powerful mage and there's evidence Jesus was, too (if he's not suspiciously absent). If the story is about aliens, everything can be explained by alien superpowers and Sufficiently Advanced Technology.

Often results in a Take That at history, Historical In Jokes, and Sadly Mythtaken moments. See also A Mythology Is True for a similar concept, as well as Beethoven Was an Alien Spy (he might have been, in this universe). Ancient Astronauts are often involved when the myth is aliens. Is also related to Doing In the Wizard—it wasn't a wizard, it was aliens! (or what have you)

Done insensitively or with too much cultural bias on the author's part, however, this trope can be full of Unfortunate Implications, as it can potentially imply "My gods are real and everyone else is a godless heathen monster."

Compare to Conspiracy Kitchen Sink, where All Myths Are True but not necessarily related.

Examples of One Myth to Rule Them All include:

Anime and Manga

To LOVE-Ru has all sorts of things, from ghosts to witches, explained by aliens. Normally its wacky alien powers, but sometimes its strange technology. A notable exception is the Cute Ghost Girl.
A subversion happens in Haruhi Suzumiya. Depending on whom you ask, aliens/time-travellers/esper powers can explain everything, with their own Techno Babble.
In Shaman King, it was stated that both Jesus and Buddha were former Shaman Kings.
In Mushishi it is explained that most supernatural phenomena like ghosts are actually mushi; mushi are strange, mystical life-forms that defy human concepts of "life" and "death" and usually have bizarre properties. Sorcerers are really just guys who've mastered how to manipulate mushi, and "curses" or "blessings" are usually the result of a mushi either being exploited or malfunctioning when it interacts with humans.
In Transformers Cybertron, it was revealed that most of Earth's legends (Loch Ness Monster, Bigfoot, vampires) were an ancient race of Transformers. Or something like that.
It's implied in +Anima that creatures from legends like werewolves, mermaids or angels are really people with Anima, whose power is to turn into Petting Zoo Person. Husky, fish +Anima, is at least two times taken for a mermaid and Cooro, crow +Amina, constantly freaks people out, because they think he is black angel.

Comic Books

Jack Kirby's Eternals, immortal beings created by space gods Celestials, have been the source behind multiple myths. Thena has been inspiration behind goddess Athena, Ajak has huge influence on Aztec and Mayan mythology, Icarus was the inspiration behind Ikaris, and Gilgamesh not only existed but was an amnesiac Eternal. Similiarly, their Evil Counterpart Deviants were inspiration of most legends and tales concerning demons. Atlantis and Lemuria existed and were Deviants' territory, which was destroyed by Celestials. And then Marvel put them all into the Marvel Universe with its Crossover Cosmology, so now they exist alongside real gods, and Thena fought with Athena a few years ago.
- The Celestials themselves are responsible for a good bit of Marvel Universe wackiness, such as the X-gene that forms the basis of X-Men.

Film

In The Matrix, "Every story you've ever heard about vampires, werewolves, or aliens is the system assimilating some program that's doing something they're not supposed to be doing."

Literature

The Harry Potter series has a number of historical events caused by wizards, and at one point it is mentioned that real witches enjoyed being burned at the stake, because of a spell that rendered the flames harmlessly ticklish.
In Percy Jackson and The Olympians, everything is caused by gods, their children, or some other Greek monster. Well, anything note-worthy. For example, Houdini, the Beatles and Blackbeard were all demi-gods, and that's only the start of it.
- The Kane Chronicles, which take place in the same universe, adds Egyptian wizards to the mix, channeling their gods to repeat archetypal stories throughout history.

Live Action TV

In Stargate SG-1, most of Terran myths are actually accounts of aliens visiting Earth.
As the page quote indicates, a lot of Doctor Who stories seem to cast aliens as the inspiration for Earth myths, with the Doctor as their foil.
All myths in Lost Girl are actually different subspecies of Fae. Yes, everything from almost-human oracles to completely-not-human-at-all Emotion Eater spiders.

Tabletop Games

Each gameline in Old World of Darkness generally tends to claim that all history was orchestrated by the supernatural group on which it focuses (though vampires have the upper hand thanks to the overwhelming amount of supplemental material compared to other races). How their intents have managed to avoid contradicting each other is Handwaved.

From Vampire: The Masquerade fluff:

"Year 33: The Malkavians claim that their greatest practical joke happen during this year, when they perform a bit of graverobbing in Jerusalem."

Video Games

The Assassin's Creed series explains that anything in human history, ever, happened because either a Templar or an Assassin did it.
- The former also rewrote the history books, completely wiping out all evidence of the first civilization.

Web Original

The Global Guardians PBEM Universe is a superhero setting. So... all those historical and mythical warriors we hear about in the stories? You know, Hercules, and Ajax, and Achilles, and Pontiac and Roland and King Arthur? Yeah... they all had superpowers.

Real Life

This is Demonization, in a nutshell: "Your gods are all just our demons."