The Multiverse is a weird thing. Depending on the genre of the work, it can mean a dozen different things. But most sci-fi has a pretty clear definition of what a "Multiverse" is. For every decision someone makes, the universe diverges into several parallel dimensions, one for every possible choice. As such, there are a nearly infinite number of universes where every conceivable version of you (or the lack thereof) exists.
This is a problem for Omnicidal Maniacs. How can one possibly destroy all of reality if, somewhere, there is another reality where they fail? The answer is to find Earth Prime: If you find and destroy the original universe that all others diverged from, you can retroactively destroy all of them.
Not to be confused with dimensional travelers who simply call the universe they're from "Earth Prime" to avoid confusion.
Related to Expendable Alternate Universe. Compare Cosmic Keystone and No Ontological Inertia. Of course, Time Travel Paradoxes and Logic Bombs abound in this theory, so it definitely requires some Willing Suspension of Disbelief. Theoretically, the very act of doing that, would simply create ANOTHER infinite number of possibilities. Therefore, destroying everything should be impossible (of course, that won't stop the villain from trying).
Warning: May be some spoilers ahead.
- In Yu-Gi-Oh! ARC-V it is revealed that there are alternative dimensions in the Yu-Gi-Oh! franchise; also, the Arc-V worlds called Standard, Fusion, Synchro and Xyz dimensions were once a single world - Original dimension - which split into these 4 ramifications but disappeared in the process.
- Re:CREATORS: According to Meteora, the real world gives birth to the worlds of fiction. If it were destroyed, all worlds connected to it will be destroyed as well.
- The DCU:
- Pre-Crisis Wonder Woman Vol 1 storyline Judgment In Infinity establishes that the main universe Earth-One is the multiverse's keystone; and if it's destroyed, all alternate Earths will follow.
- As of 52, the multiverse hinges on "New Earth" - not just a specific universe, but a specific planet in that universe. This becomes a plot point shortly afterwards in the Sinestro Corps War, where Sinestro wants to conquer Earth for this very reason.
- Interestingly, DC's comic multiverse has generally not run on the constant temporal divergence model; Crisis on Infinite Earths established that the universes of the pre-Crisis multiverse diverged at the Big Bang, making that the point to attack, while 52 established that the new multiverse began as 52 identical Earths that got cosmologically edited.
- Note that "Earth Prime" refers to another universe altogether that's like ours, where superheroes don't exist outside of comics. In some stories, it is "our" Earth for all intents and purposes. And it was actually destroyed in Crisis on Infinite Earths. It came back as Earth-33 of the DC multiverse in The Multiversity.
- In Multiversity/New 52 continuity the main Earth is sometimes called Prime Earth (as well as New Earth and Earth-0), Earth-Prime having been renamed as above. It's described in The Multiversity Guidebook as "the foundation stone of the multiversal structure", and Doomsday Clock dubs it the Metaverse because of its unique status.
- Doomsday Clock: Dr. Manhattan observes in issue 10 that the main DC universe is set apart from the rest of the DC multiverse; when it's changed, the multiverse reacts, to the point of reconfiguring its very nature - infinite universes, a single universe, 52 universes, the Dark Multiverse, etc. Because of this, Manhattan dubs the DC universe the Metaverse. When the original Wally West escapes in DC Rebirth, Manhattan discovers the Metaverse is not a passive subject he can reshape as he sees fit - it's aware of how he changed it, and it's taking action against him.
- It's later posited that Superman is the axis around which the entire Metaverse/Multiverse turns. Every universe has some variation of Superman, even if it's simply as a comic book character (so yes, that includes our universe). And even if some timeline or universe currently lacks a Superman or Superman analogue, eventually one will show up or appear, even if it's a Dimensional Traveler from an entirely different universe.
- Following Dark Nights: Death Metal, the main DCU's Earth is no longer the center of the Multiverse; there are now two centers, they're currently beyond the instruments of the main DCU, they're opposites, and one of them is called the Elseworld.
- Marvel Universe:
- In the All-New, All-Different Marvel era, Earth-616 was reclassified as Prime Earth, due to it being the first Earth recreated by the Beyonder-powered Reed Richards. This still doesn't give it any more cosmic significance though, it only means this is where the world displaced characters are most likely to end up (some people even kept referring to it by its old number).
- Not that the Marvel Universe was averse to the theory before that era either. The real world (our world) in that case would be Earth-1218, the problem is it's unreachable from the rest of the multiverse so nobody can tell what happened if it got destroyed. (An editor did claim it was destroyed in an Incursion and subsequently recreated, but how could that be tested?) To date when anybody claimed to find Earth-1218 in any Marvel media it always turned out to be just a Close-Enough Timeline.
- However, according to the Maker (Reed Richards) in The Ultimates 2, 616 is unique because it is the "hub" the rest of the Multiverse revolves around. As a result the laws of reality are more flexible: science and magic are interchangeable, and higher levels of existence can be accessed more easily.
- Gwenpool supposedly comes from the real world, or at least from a world where all the rest of Marvel continuity is the stuff of comic books. Notably, when she believes she's been sent home, she discovers that she really hasn't, and it's actually just a comic book version of her real home. She then returns to the main Marvel Universe.
- Sonic the Hedgehog (Archie Comics) has Mobius Prime, which all other parallel worlds (called Zones) are based off. Interestingly, despite the franchise starting with video games, Mobius Prime is the comic's Zone and not the one of the games. It's unknown how far this has stuck after the Continuity Reboot, as the entire multiverse has collapsed in on itself.
- The Pony POV Series chapter "Applejack's Dream" establishes the existence of a "Heart World" from which the Pony POV Series world and all other Alternate Timelines diverged from and which anyone still connected to is affected by, implied to be the main series timeline. However, in an interesting twist, it is possible for a world to "break off" and continue on its own without any connection to the Heart World. The plot of that chapter involved preparing for the possibility that would happen so as to prevent Applejack from going insane and turning into Nightmare Mirror in the event that it did. This later bites Nightmare Eclipse in the flank when her plan has basically made her the Heart World Nightmare Eclipse, and tied all her other selves to her — in other words, defeating her defeats all versions of her.
- Deconstructed in 5 Years Later; Eon takes the idea of the classic Ben 10 series being the prime universe like anyone would: by being insulted by the implication that his timeline is nothing more than an Expendable Alternate Universe.
- A Diplomatic Visit: In Diplomacy Through Schooling (the third story of the Diplomacy-verse), Luna explains that their multiverse began from one of these, as the Creators and the Powers born from Them helped to shape a world — the Prime, the first world in Existence, with Others coming from ripples of its movement and the decisions of those who lived there, dividing endlessly into other worlds within their own separate universes. Consequently, all the different series in this multiverse take place in universes spun off from the original.
- Present in The Hitchhiker's Guide to the Galaxy. Vogon Jeltz, tasked with demolishing the Earth, is deemed unsuccessful by his superiors because he only demolished one Earth, whereas in fact there are millions of others still existing in alternate universes. At the end of Mostly Harmless, he finally succeeds in eliminating every single one.
- In Guy Gavriel Kay's The Fionavar Tapestry, Fionavar is a fantasy version of this; one world that all other ones spring from and depend on.
- In The Chronicles of Amber, this is what makes Amber so special - it's the primal reality which defines the cosmos.
- William Shatner's Quest for Tomorrow series introduces this concept in the last novel prior to the reboot. In fact, the main characters' goal is to reboot their reality by altering the result of a coin flip in the prime reality. Unfortunately, Shatner abandons the series two books after the reboot.
- Stephen King's The Dark Tower books establish there are two - one where most of the action takes place, in which the Tower manifests as an actual tower; and the other, Keystone Earth, is explicitly our world, in which Stephen King is writing the adventures of the characters in the other universe. Should the Tower in either of these worlds be destroyed, all reality will topple.
- After Season 2 of The Flash (2014) establishes the existence of The Multiverse, the season finale has Zoom confirm that Earth-1 (the main setting of the Arrowverse) is the center of it all, being the doorway to all the other Earths.
- After the Crisis, the new multiverse creates Earth Prime that appears to be an amalgamation of Earth-1, Earth-38 (Supergirl's world), and Black Lightning's Earth. Some characters from other Earths have also been moved to Earth Prime like Jay Garrick (native to Earth-3), and exiles from now-destroyed realities occasionally trickle in, although they are unable to coexist with their doubles for long.
- Loki (2021) plays with this idea. The movies and shows prior to Loki are a part of the "Sacred Timeline", with which all alternate timelines and universes spawn from. The "Sacred Timeline" is not a natural consequence of the multiverse, but instead enforced by the Time Variance Authority by pruning anyone and anything that does something that they weren't supposed to, whether it's be late for work or run away with the Tessaract as the protagonist did. The Time Keepers enforce the Sacred Timeline to prevent a Multiversal War from occurring. After Sylvie, a variant of Loki, kills the head of the TVA, the agency is unable to stop the multiverse from branching out, rendering the concept of a "Sacred Timeline" moot.
- In Sliders, Earth Prime is our Earth, which the protagonists originate from. At least until the Kromaggs conquered it. There is also Kromagg Prime, the Homeworld of the Kromaggs, from which they were exiled by that world's humans. It's also Quinn's real home.
- Supernatural: There are many alternate universes with many alternate Sams and Deans, but the two main characters inhabit the "real", Prime Universe. This is because God is easily distracted, so he often creates more universes to explore every possible story. In the final season, after it is revealed that God Is Evil, he starts dismantling the rest of the multiverse, then starts planning a "hard reset" of the Prime Universe to recreate it in In Their Own Image... despite, well, having already done so. He just hates Sam and Dean that much.
- Dungeons & Dragons: Certain sourcebooks like Tasha's Cauldron of Everything and Fizban's Treasury of Dragons mention of a primordial version of the Material Plane known as the First World, where many of the peoples and monsters of the D&D multiverse originated. After an unknown cataclysm all but annihilated the First World, countless parallel worlds came into existence along the Material Plane with the progeny of the First World's inhabitants appearing in every one of them. By this theory, worlds like Oerth, Toril, Krynn, Eberron, Exandria, and many others are reflectionsor in some cases, distortionsof the First World.
- In Magic: The Gathering, the plane Dominaria is the Nexus of the Multiverse, situated at the center of the Multiverse. Events on Dominaria reverberate throughout the Multiverse; most notably during the Time Spiral arc where its imminent destruction would have destroyed the rest of the Multiverse with it, and even fixing it turned all planeswalkers from Physical Gods into merely gifted mages. Unusually for this trope, it's not Earth at all, but was a Standard Fantasy Setting until the plot turned it post apocalyptic. Nor was it always the Nexus; there are planes, such as Equilor, which are older than Dominaria, and the role of multiversal nexus seems to vary depending on the "orbit" of the planes. As of Strixhaven, the current multiversal nexus is unknown.
- In the movie Turtles Forever, when the Utrom Shredder realizes that there are literally hundreds of Teenage Mutant Ninja Turtles across the multiverse, he decides to destroy them all by defeating the versions from Turtle Prime; that is, those from the first issue of the original Mirage comic. This was used again in the Teenage Mutant Ninja Turtles (2012) episode "Trans-Dimensional Turtles" with the 87 Krang's plan to destroy reality to get rid of the Turtles.
- Justice League: Crisis on Two Earths has Owlman, who wants to use the QED on Earth Prime, thus wiping out all life. His reason for this is that he learned that any choice made by anyone creates a separate parallel universe for each possible outcome and decided that the only truly meaningful choice that anyone could possibly make is to destroy everything (it doesn't matter that another Owlman will choose not to destroy everything since he would also be destroyed). When Batman foils his plan by sending him and the bomb to another universe where Earth is a frozen wasteland devoid of all life, Owlman has plenty of time to stop the countdown and save himself. Realizing that no matter what he chooses to do, another Owlman will make the opposite choice, he snarks, "It doesn't matter," and lets himself die.