The Earth Prime Theory
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.
Related to All the Myriad Ways. 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! There are unmarked Spoilers ahead. Beware.
- Justice League: Crisis on Two Earths, has the Nietzsche Wannabe Owlman who wants to use the QED on Earth Prime, thus wiping out all life.
- As of 52, the DC Comics multiverse hinges on "New Earth" - not just a specific universe, but a specific planet in that universe (note that "Earth Prime" refers to another universe altogether that's like ours, where superheroes don't exist outside of comics). This becomes a plot point shortly afterward in Sinestro Corps War, where Sinestro wants to conquer Earth for this very reason.
- 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, via Xanatos Gambit.
- In The Fionavar Tapestry, Fionavar is a fantasy version of this; one world that all other ones spring from and depend on.
- This is what makes Amber so special - it's the primal reality which defines the cosmos.
- In Spider-Man:The Animated Series series finale, Spider-Man is sent to the reality of a powerless Spider-Man who is really an actor and hangs out with Stan Lee for a while.