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, there are 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.
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 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.
- 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.
- The Sonic the Hedgehog comic book 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.
- 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.
- 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 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.
- 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.
- 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 main goal is to make the only choice that matters (in his own twisted mind), as there won't be an alternate version of him making the opposite choice (his problem with the multiverse). When Batman foils his plan by sending him and the bomb to another reality, where Earth is a frozen wasteland, he intentionally gives Owlman a chance to stop the countdown. Realizing that, no matter what he does, another Owlman will do something else, he lets himself die.