One possible ending to stories in which the hero or heroes are {{trapped|InAnotherWorld}} in an AlternateUniverse is to somehow force the universes to merge or collapse into one single time-line that exhibits (hopefully the best) features of both. A way to SaveBothWorlds, if you will.

The current world is headed to nothing short of [[TheEndOfTheWorldAsWeKnowIt disaster]], so the only choice is to put it back a different way.

For making a world from scratch, see WorldBuilding.

SisterTrope to SetRightWhatOnceWentWrong, which deals with changing events in the past in the hope that things turn out better.

As this is an {{Ending Trope|s}}, '''beware of spoilers!'''


[[folder:Anime & Manga]]
* ''Manga/{{Berserk}}'': The climax of the Falcon of the Millennium Empire arc results in this. When the Skull Knight tries to attack Griffith/Femto with his space-cutting attack, Griffith, having known he would try it, manipulates the attack so it instead strikes the face of the Apostle Ganishka; as Ganishka had descended into deepest parts of the Astral Plane twice, the attack caused a chain reaction that opens a fissure into the Astral Plane. The end result: the Astral Plane merges with the physical world and all mythical creatures and fell entities, including the rest of the God Hand, fully manifest in the physical world, turning what was already a CrapsackWorld into a literal HellOnEarth.
* In the manga ''Mugen Densetsu Takamagahara: Dream Saga'', awakening Amaterasu is meant to restore both worlds to their original, unpolluted glory. However, the ending offers the idea that Making a Better World isn't always ''better'': [[spoiler:the team learns that the original two worlds have to be destroyed and their populations killed. They resolve to SaveBothWorlds instead of make new ones.]]
* The objective of the villain in ''{{Anime/Noein}}'' is to converge the infinite possible universes into one world, which he thinks would be free from suffering. It also comes at the cost of all individuality ceasing to exist.
* In ''VisualNovel/ElevenEyes'', Kakeru comes to the realization late in the game that every one of the chosen ones was drawn from another, slightly different, reality since memories and information don't match up. Everyone worries that [[spoiler:they have to die because of Liselotte's soul in them and]] when the battle is finished, they won't be able to see each other again. [[spoiler:Kukuri heals the world back to normal.]] Everyone ends up staying together in the same world, a composite of all of theirs. They'd become such a tight group of TrueCompanions that it would be too cruel to split them apart.
* Used mildly in the Shara/Shura [[StoryArc arc]] of ''Manga/TsubasaReservoirChronicle''. In this case, the main characters don't even realize they are time traveling, but their actions do result in friendlier interactions in the original world. Not that there aren't consequences....
* Inverted in ''Anime/SuperDimensionCenturyOrguss.'' The protagonist's actions have caused a Merged Reality, it's wreaking havoc, and it becomes his and others' job to undo the damage and split all the realities and timelines back up.
* ''Anime/DualParallelTroubleAdventure'': After the BigBad is defeated, the hero Kazuki Yotsuga is forced to sacrifice his HumongousMecha in order to SaveBothWorlds -- except that Mitsuki Sanada [[WhatDoesThisButtonDo leans on the wrong lever]] just beforehand, causing all of the named characters find themselves in a Merged Reality, unlike either of the realities of the story, but preserving all of the unique memories and characters from both of its parents. (For example, both Mitsuki Sanada and her cousin-slash-{{alternate self}} Mitsuki Rara both coexist within the new reality and its new history.)

[[folder: Comic Books]]
* The climaxes of ''ComicBook/CrisisOnInfiniteEarths'', ''ComicBook/ZeroHour'' , ''ComicBook/InfiniteCrisis'' and ''ComicBook/{{Flashpoint}}''. DC Comics seems to be in a habit of doing this every decade or so.
* Franchise/TheDCU character Superboy-Prime wants to bring back his world, the "perfect" world... and he's willing to blow up every ''other'' world in existence to do it. Moreover, he has no problem killing anyone who gets in his way (or anyone who doesn't), because he judges them to be "stupid" versions of people who'd exist on his world. Yes, to some extent, he is [[TakeThatAudience a superpowered avatar of fanboy rage]]. Fittingly, he [[HumiliationConga eventually gets reduced to]] [[spoiler: an internet {{troll}}.]]
* Happens with the ''Creator/{{DC|Comics}}'', ''{{Creator/Wildstorm}}'' and (sorta) ''Creator/{{Vertigo|Comics}}'' universes at the end of ''ComicBook/{{Flashpoint}}''. After the Flash is revealed to have accidentally broken the timeline, leading to events in Flashpoint, Pandora merges the three universes together to fix the problem. (why she can't simply undo the damage and has to merge in the other universes is not explained) This leads directly to the DC reboot and the ''ComicBook/{{New 52}}'', with some Wildstorm and Vertigo characters and concepts being added to the DCU.
* Marvel's ''ComicBook/EarthX'' trilogy: Mephisto is continually [[DealWithTheDevil tempting people]] to change history for this purpose, when in fact it creates alternate universes--so even if the new universe is better, the old one is still there and [[CrapsackWorld just as terrible as before]]. Meanwhile the Elders of the Universe are trying to put all these alternate universes back together to recreate the original--and [[UtopiaJustifiesTheMeans never mind that this involves the effective death of trillions of people native to the new universes]].
* A Creator/MarvelComics storyline had the ComicBook/FantasticFour aiding their [[HeelFaceTurn enemy-turned-ally]] Amazon Thundra to merge her LadyLand [[AlternateHistory Alternate Future]] of "[[MeaningfulName Femizonia]]" with the [[NoWomansLand misogynistic]] world of Machus to form a composite future of (theoretical) equality between the sexes. The men and women still fight each other constantly with deadly weapons, but Reed Richards just {{Handwave}}s that as the [[ProudWarriorRaceGuy Proud Warrior Race]] version of SlapSlapKiss. Naturally, Thundra finds this world boring and [[AlternateUniverse seeks out another version of Old Femizonia to settle in]].
* ''Comicbook/SecretWars2015'' is more or less Marvel's version of ''Crisis on Infinite Earths'', and it's been revealed that it will culminate in the merger of every single dimension in the Marvel Multiverse into... well first into Battleworld from which the ''Comicbook/AllNewAllDifferentMarvel'' will arise at the end of the event, the most prominent being Earth-616 (Mainstream Marvel) and Earth-1610 (Ultimate Marvel).
* The DC and Marvel multiverses briefly merge together into the ComicBook/AmalgamUniverse in ''DC vs Marvel''. Years later they are merged again (but not creating the Amalgam universe) in the ''ComicBook/JLAAvengers'' crossover.
* The end result of the ''ComicBook/SonicTheHedgehogMegaManWorldsCollide'' CrisisCrossover's ending towards Mobius in ComicBook/ArchieComicsSonicTheHedgehog. Thanks to Eggman's mucking, Mobius is now one part "Mobius Prime" and one part "Mobius Genesis", resulting in a blending of game and [=SatAM=] elements, among other {{cosmic retcon}}s. (Eggman's main base being the Metropolis Zone, Knothole being situated in the Wood Zone, Mobotropolis now in Westside Island...)
* ''ComicBook/SonicTheComic'': During the Shanazar arc, Sonic and Robotnik are shrunk down and sent to Shanazar, a sub-atomic universe with an ArabianNightsDays flair. Near the end of said arc, Robotnik creates a Dimension Blender device, intending to use it to enlarge Shanazar until it occupied the same space as Mobius, merging the two planets together, with Robotnik planning to retake control of Mobius in the ensuing chaos. However, Robotnik's plan backfires when Mobius and Shanazar fuse together without any ill effects; instead, new Zones are added to the planet and portals to various worlds and dimensions on Mobius, including various points in time in Earth's history, are opened.

[[folder: Fan Works]]
* Played with in ''FanFic/DigimonChildrenOfTime''. It's explained that the different versions of the Digital World as seen in ''[[Anime/DigimonAdventure Adventure]]''[=/=]''[[Anime/DigimonAdventure02 Adventure 02]]'', ''[[Anime/DigimonTamers Tamers]]'', ''[[Anime/DigimonFrontier Frontier]]'', and ''[[Anime/DigimonSavers Data Squad]]'' were all once one composite world, but were broken apart when the original [=DigiDestined=] defeated the darkness present during that time. After Tai, Davis, Takato, Takuya, and Marcus' respective groups saved each one, they have since merged back together into a single world.
* In ''[[FanFic/IcedFairysANewWorld A New World]]'', [[TheChessmaster Yukari Yakumo]] successfully achieves [[spoiler:the fusion of the worlds of technology and magic, with the added bonus of giving all youkai independence from human thought.]] She's been [[ThanatosGambit dead]] for three hundred years and was ''still'' able to trick ''everyone'' in both Gensokyo and Luna into executing her plan to the letter.
* ''Fanfic/OutOfTheCornerOfTheEye'' kicks off with this, as the [[Literature/CthulhuMythos Outer Gods/Great Old Ones]] discover the ''WesternAnimation/JackieChanAdventures'' universe and superimpose their own on top of it. This causes them, and everything related to them, to be retroactively inserted into the JCA timeline.
** The author uses the same concept in [[Fanfic/GuardiansWizardsAndKungFuFighters another story]]; as [[WesternAnimation/JackieChanAdventures Shendu]] is being resealed at the end of the Demon World story, [[ForWantOfANail he chooses]] to spitefully destroy the Book of Ages, triggering a TimeCrash. Jade desperately scribbles onto a remaining fragment of the Book that they all survive, which it interprets by fusing the remnants of their universe with the ''WesternAnimation/{{WITCH}}'' universe, retroactively making the JCA characters part of its history.
* ''Fanfic/EarthsAlienHistory'': [[spoiler: It turns out the whole timeline only exists because the Time Lords are combining universes in order to give humanity enough challenges that they'll evolve into the perfect warriors needed to help defeat the Daleks and win the Time War.]]

[[folder: Literature]]
* Jack Williamson's Legion of Time is one of the earliest novels in which this trope appears as a (somewhat illogical) happy surprise ending. The alternative futures of Utopian Jonbar and Dystopian Gyronchi fight for existence. [[spoiler: The hero struggles to bring about Jonbar's future, but somehow his efforts cause a fusion of the two futures, with the best features of each - including a fusion of the two women in them he loves, good girl Lethonee and bad girl Sorainya.]]
* Creator/PiersAnthony's ''Literature/ApprenticeAdept'' series eventually merges the two worlds into one; as the characters are explicitly paired across both worlds (mirror-universe twin kinda thing), each pair merges into one being, and each pair has to time-share their body. Substantially easier for the heroes than for the villains, since heroes are used to putting the needs of others on par with or ahead of their own.
* Creator/DianaWynneJones's book ''Literature/WitchWeek'' ends this way, with the merging character explaining to the cast that they will all melt quietly into the people they really are in the other world. Since they're probably going to be burned as witches otherwise, this sounds quite appealing. ('Probably' in context means '[[TheEndOfTheWorldAsWeKnowIt if the world isn't destroyed first]],' which is what will happen if their world [[spoiler:(which was ''not'' supposed to exist)]] isn't merged with the other one.)
* Creator/OrsonScottCard's ''Literature/PastwatchTheRedemptionOfChristopherColumbus''. The plot is rather complicated, but suffice it to say it emerges that [[spoiler: our own timeline is the product of interference from ''another'' timeline, in which the Tlaxcalans of Central America conquered Europe and possibly the world. (The author does an excellent job of making it sound plausible). The people of ''that'' timeline viewed the Tlaxcalan invasion as the greatest catastrophe in human history, and so they altered their own timeline, by motivating Christopher Columbus to sail west (with a holographic God). Of course, in the story our own timeline appears to be headed for human extinction, so the characters make their own alteration, preventing both the Central American and European civilisations from wiping each other out. They are apparently more successful than the original alterers.]]
** This is because [[spoiler:the protagonists have realized that the reason the original "interventionists" failed was because they made a single alteration and didn't see it through. Instead of sending a holo-recording, they decide to send three people with a plan for each. One's job is to destroy Columbus's ships and die in the process, preventing him from returning to Europe. Another, a native Mayan, has to unify the Central American peoples and teach them a milder version of Christianity. The third, an African female, has to teach Columbus the error of his ways and, along with him, unify and educate the peoples of the Caribbean islands in the same manner as her partner. The end result is that, when the ocean-going ships from the new American confederation arrive to Europe, they do so as equals. This prevents the world dominated by either the Europeans or the Native Americans. No direct explanation is given for the lack of an ecological catastrophe in this version of history]].
* Averted in ''Literature/{{Animorphs}}''. When the kids travel back in time to stop Visser 4 from messing with history, they consider using the Time Matrix to change history for the better. However, they hit the ResetButton by ''accident'' and put everything back the way it was (minus one host body for Visser 4).
** Although there was a sort of MergedReality when Elfangor and Visser Three both activated the Time Matrix in The Andalite Chronicles.
* A more malicious example nearly happens in the ''Literature/RodAlbrightAlienAdventures''; [[BigBadDuumvirate BKR and Smorkus Flinders]] planned to merge Dimension X with Dimension Q, the dimension Earth is in, thus creating a composite universe where everyone would be subject to Dimension X's Reality Quakes and potentially be driven insane. It's outright stated that BKR is doing so purely ForTheEvulz, despite the fact that he will be affected just like everyone else.

[[folder: Live Action Television]]
* On ''{{Series/Sliders}}'', the BigBad of the last season tried to do this, experimenting by first merging people from different universes together.
* This is what essentially would have happened if Buffy hadn't stopped Glory in ''Series/BuffyTheVampireSlayer''. It would have been only temporary, though.
* In ''{{Series/Fringe}}'', due to the activation of [[spoiler: [[CosmicKeystone the Vacuum]] by Peter, a bridge is created between both the Prime and Alternate universe, preventing them from further collapse. This allows both universe's team to have the time to try and solve the problem.]]


[[folder: Video Games]]
* Inverted in the computer game ''VideoGame/TheLongestJourney'': the idea is to ''prevent'' the two worlds from combining, at least under the current uncontrolled conditions.
** Similarly, the combining of the two worlds in the ''Film/SuperMarioBrothers'' [[TheMovie Movie]] was a plot of the antagonists to rule them both.
* ''VideoGame/TalesOfSymphonia'' ends up with this being the goal of the main characters, although for much of the second half of the game they are under the mistaken impression that they need to do the exact opposite in order to SaveBothWorlds. [[spoiler:It just so happens that the resulting combination ends up being the world map of ''VideoGame/TalesOfPhantasia''.]]
** It doesn't quite go as planned, as shown by the [[VideoGame/TalesofSymphoniaDawnoftheNewWorld sequel/spinoff]] that came after. [[spoiler: Only when Ratatosk (in the Good Ending) decides to rewrite the laws of nature so that life does not need mana to survive does everything ultimately work out.]]
** In ''VideoGame/TalesOfEternia'', the two main worlds begin colliding. The heroes only manage to fix it just in time by [[spoiler:exploding the barrier between Inferia and Celestia, blasting the planets away from each other and altering the very nature of their universe. In the final scene, one of them is planning to upgrade their spaceship so they could go between worlds and visit their friends.]]
** ''VideoGame/TalesOfInnocence'' revolves around multiple factions looking to get their hands on a MacGuffin for different purposes. The main characters' is to unite Heaven and Earth into one paradise.
* ''VideoGame/FinalFantasyV'': this is exactly what OmnicidalManiac Exdeath wants to happen and the heroes are trying to ''prevent''. As it turns out, the [[MacGuffin power]] he's after was sealed by splitting the world in two hundreds of years ago, and he can only release it from the Dimensional Rift by reuniting the worlds (which entails the destruction of the [[CosmicKeystone Crystals]], so with or without Exdeath actually claiming his prize and wreaking havoc with it, everybody's pretty much screwed). [[spoiler: Exdeath succeeds in reuniting the worlds, so the heroes settle for protecting the merged world.]]
-->'''Exdeath:''' I will return the world to its original form!\\
'''Bartz:''' To a world of evil!\\
'''Exdeath:''' Are you even listening to me?
* In ''VideoGame/MortalKombatVsDCUniverse'', the BigBad is Dark Kahn, a merged version of Shao Kahn and Darkseid, who starts merging the realities of Mortal Kombat and the Franchise/DCUniverse, while feeding off the rage of the characters. He's also playing the heroes and villains of each world against each other, which think that the other is an invading army. Eventually, though teamwork, Superman and Raiden manage to defeat Dark Kahn and split the merged universe back into two and Dark Kahn himself back into Darkseid and Shao Kahn... who end up in each other's world and therefore powerless. Darkseid gets confined into the Netherrealm, while Shao Kahn gets banished to the Phantom Zone.
* Implied in ''VideoGame/TheLegendOfZeldaMajorasMask''; in the ending, you see all the people you helped in the sidequests, no matter how many times you exploited the GroundhogDayLoop or even whether or not you helped them on your final loop.
** Averted, however, in ''VideoGame/TheLegendOfZeldaOcarinaOfTime'' - a ContinuitySnarl has led to a WordOfGod-supported explanation that the events of that game created multiple timelines that have not been known to merge again.
* In ''VideoGame/EternalDarkness'' the true ending is obtained by playing the game three times, choosing a different Ancient as Pious' patron each time. In each path a different Ancient is slain. The true ending reveals that the entire game was Mantorok's plot to eliminate the three Ancients since he could no longer keep them in check thanks to Pious turning his own magic against him. The Tome of Eternal Darkness guided the player characters throughout history to kill a different Ancient in each timeline. Then Mantorok merged the three timelines so that all three of the Ancients were killed, leaving him [[spoiler:and his own counterpart Ancient]] as the last of the Ancients.
* Happens in the browser-based game ''VideoGame/NoOneHasToDie''. Four normal endings, each with only one character left alive, each of them walk into a [[spoiler:time machine]], and meet up in a MergedReality for the final ending.
* ''VideoGame/JudgeDreddDreddVsDeath'': When Judge Death retreats to his home dimension after his body is destroyed by Judge Dredd, he reveals to Dredd that his new plan is to use the Psi-Judges' energy to merge [[DeathWorld Deadworld]] with Dredd's universe, killing every living thing in Mega City One.
* ''VideoGame/SuperRobotWarsZ'', using the same plot device from Orguss (listed above) in concert with the Black History from ''Anime/TurnAGundam''.
* In ''[[VideoGame/SonicRushSeries Sonic Rush]]'', Eggman's theft of the [[CosmicKeystone Sol Emeralds]] from Blaze's dimension is causing it and Sonic's dimension to merge together. Eggman and his doppelganger Eggman Nega plan to build a transdimensional Eggmanland in the wake of the collision, but the heroes are able to stop them before both worlds are completely ruined.
* With multiple uses of time travel in the whole ''[[VideoGame/CommandAndConquerRedAlertSeries Red Alert]]'' series this trope is normally averted (timelines are, canonically, overridden in succession). However, a merging of timelines is deployed for the ending of the Allied campaign in the ''Yuri's Revenge'' expansion of ''VideoGame/CommandAndConquerRedAlert2'': after Yuri's initial plot at simultaneous worldwide psychic domination while the Allies were distracted celebrating the defeat of the Soviets hits a snag in San Francisco due to an ultimately-fortuitous Harrier crash on Alcatraz, the Allies' task forc jump back in time to warn their past selves - they end up back at the initial Soviet invasion of San Francisco during the original game. Among other things, this means the assassination of General Carville by a Crazy Ivan just prior to the battle in the Black Forest[[note]]10th mission in the Allied campaign of the original [=RA2=][[/note]] gets retconned out. During the denouement of the Allied campaign everybody's screens start shaking like earthquakes as Einstein explains the timelines are merging since two sets of events at the same time cannot coexist - thankfully, the best events of both timelines for the Allies are kept: as before the Soviets are still defeated, but now Carville is still alive and Yuri is now in a TailorMadePrison to render his psychic abilities useless.
* ''VideoGame/MarvelVsCapcomInfinite'': Ultron and Sigma use the Space and Reality [[MacGuffin Infinity Stones]] to not only [[FusionDance merge themselves together]], but also merge the Franchise/MarvelUniverse and the various Franchise/{{Capcom}} universes together into one. At the end of the game, when Ultron Sigma is defeated, it's discovered that the Reality Stone was cracked during the struggle, meaning the merger cannot be reversed.
* ''VideoGame/DistortedTravesty'': At first, our heroes Jerry and Jeremy are running around trying to find a way back home after being teleported into a videogame world. This goes out the window when they come across their own hometown within the game world, which is when they realize that the virtual and physical worlds have been permanently melded together. Separating them again is presented as utterly impossible.

[[folder: Webcomics]]
* In ''Webcomic/TheAdventuresOfDrMcNinja'', this turns out to be the main motive of the BigBad, King Radical. [[spoiler:His homeworld, the Radical Lands, were drained of LifeEnergy by Sparklelord and gradually dying. He escaped to an AlternateUniverse, the Dr.'s world, which is a cross between our mundane world and the awesomeness of the Radical Lands. In order to save his own world, he manipulated a time portal to start merging the worlds together in order to save his people. Why is this wrong? Because the process would make every Radical Lander take the place of a normal person from Doc's world, effectively forcing them out of existence, which would ultimately kill billions. But by the BlueAndOrangeMorality of King Radical, there is nothing wrong with this because native radical residents would be spared, and boring people don't deserve to live anyway]].
* In ''Webcomic/SupernormalStep'', [[spoiler:in ancient times, Fiona's mundane world and the Supernormalverse were one and the same. To prevent magic from going out of control and causing TheEndOfTheWorldAsWeKnowIt, a witch separated everything magical away from the main universe into a pocket dimension of its own, resulting in TheMagicGoesAway for the main universe. In chapter 22, the BigBad manages to overload the spell that separated the two worlds, causing the pocket dimension to collapse back into the main universe. The result is a massive, worldwide catastrophe as buildings from the pocket dimension end up in the same spots as buildings in the main universe. The fact that this also causes TheMagicComesBack makes the resulting chaos even greater.]]