In stories involving multiple Alternate Universes, one possible plotline has the universes merge or collapse into one single timeline that exhibits (hopefully the best) features of all of them. This can be a way to Save Both Worlds when used for the ending of a work, for if all of these worlds are headed to nothing short of disaster, the only choice may be to put them back a different way. This isn't always the reason, though. Sometimes it's the villain, rather than the heroes, trying to pull this off for some sort of personal goal; in this case, it's probably not an Ending Trope, since the protagonists naturally want to prevent or reverse this. Other times all the danger has already passed and the day has been saved, with the merging of realities serving as a bonus reward to the protagonists who were about to angst that saving the day also meant being unable to see their new friends ever again.
In any case, expect all the questions that you'd probably have following such an event, from the Culture Shock of different worlds now having to co-exist to the logistics of "how" they can even coexist (did the universe double in size or...), to be glossed over or only mentioned in passing outside of stories where the characters want to prevent this trope.
For making a world from scratch, see World Building.
As this can be an Ending Trope, beware of spoilers!
- 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 Crapsack World into a literal Hell on Earth.
- 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: the team learns that the original two worlds have to be destroyed and their populations killed. They resolve to Save Both Worlds instead of make new ones.
- The objective of the villain in 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 11eyes, 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 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. 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 True Companions that it would be too cruel to split them apart.
- Used mildly in the Shara/Shura arc of Tsubasa -RESERVoir CHRoNiCLE-. 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 Super Dimension Century Orguss. 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.
- Dual! Parallel Trouble Adventure: After the Big Bad is defeated, the hero Kazuki Yotsuga is forced to sacrifice his Humongous Mecha in order to Save Both Worlds — except that Mitsuki Sanada 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.)
- The climaxes of Crisis on Infinite Earths, Zero Hour! , Infinite Crisis, Flashpoint... DC Comics seems to be in a habit of doing this every decade or so.
- For a particular case in point, The DCU 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 a superpowered avatar of fanboy rage. Fittingly, he eventually gets reduced to an internet troll.
- It happens with the DC, WildStorm and (sorta) Vertigo universes at the end of 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 New 52, with some Wildstorm and Vertigo characters and concepts being added to the DCU.
- Marvel's Earth X trilogy: Mephisto is continually 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 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 never mind that this involves the effective death of trillions of people native to the new universes.
- A Marvel Comics storyline had the Fantastic Four aiding their enemy-turned-ally Amazon Thundra to merge her Lady Land Alternate Future of "Femizonia" with the 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 Handwaves that as the Proud Warrior Race version of Slap-Slap-Kiss. Naturally, Thundra finds this world boring and seeks out another version of Old Femizonia to settle in.
- Secret Wars (2015) is more or less Marvel's version of Crisis on Infinite Earths, and culminates in the merger of every single dimension in the Marvel Multiverse into... well first into Battleworld from which the All-New, All-Different Marvel arises 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 Amalgam Universe in DC vs Marvel. Years later they are merged again (but not creating the Amalgam universe) in the JLA/Avengers crossover.
- The end result of the Sonic the Hedgehog/Mega Man: Worlds Collide Crisis Crossover's ending towards Mobius in Archie Comics' Sonic the Hedgehog. 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 retcons. (Eggman's main base being the Metropolis Zone, Knothole being situated in the Wood Zone, Mobotropolis now in Westside Island...)
- Sonic the Comic: During the Shanazar arc, Sonic and Robotnik are shrunk down and sent to Shanazar, a sub-atomic universe with an "Arabian Nights" Days 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.
- The third Batman/Teenage Mutant Ninja Turtles crossover centers on Krang acquiring DC tech and using it to merge the Turtles' universe with the DC Universe to create a reality he could control at will. In the merged universe, Batman was raised alongside the Turtles by Splinter, Casey Jones is a GCPD officer, and both Batman and the Turtles' Rogues Gallery have been melded together (the Foot Clan with The Joker's gang, for example).
- Played with in Digimon: Children of Time. It's explained that the different versions of the Digital World as seen in Adventure/Adventure 02, Tamers, Frontier, and 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 A New World, Yukari Yakumo successfully achieves the fusion of the worlds of technology and magic, with the added bonus of giving all youkai independence from human thought. She's been dead for three hundred years and was still able to trick everyone in both Gensokyo and Luna into executing her plan to the letter.
- Out of the Corner of the Eye kicks off with this, as the Outer Gods/Great Old Ones discover the Jackie Chan Adventures 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 another story; as Shendu is being resealed at the end of the Demon World story, he chooses to spitefully destroy the Book of Ages, triggering a Time Crash. 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 W.I.T.C.H. universe, retroactively making the JCA characters part of its history.
- Earth's Alien History: 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.
- In Super Mario Bros., the asteroid that apparently killed the dinosaurs actually split our reality into two. King Koopa wants to remerge them because the other universe has a major water shortage and he wants our seas.
- Averted in 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 Reset Button by accident and put everything back the way it was (minus one host body for Visser 4).
- Although there was a sort of Merged Reality when Elfangor and Visser Three both activated the Time Matrix in The Andalite Chronicles.
- Piers Anthony's Apprentice Adept 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.
- Diana Wynne Jones' Chrestomanci: Witch Week 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 "if the world isn't destroyed first", which is what will happen if their world (which was not supposed to exist) isn't merged with the other one.)
- Doctor Who Expanded Universe: An unintentional example in the Missing Adventures book Millennial Rites: a rich industrialist begins preparing some sort of massive summon. Former companion Anne Travers is deathly afraid he'll try to summon the Great Intelligence, so at the pivotal point she casts a specific counterspell designed to banish it. However, it turns out the Intelligence wasn't the being summoned, and instead the crashing spells create the Great Kingdom, a mad version of Central London vibrating between the rules of the current Universe, the one that preceded it and the one following it.
- 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. 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.
- Orson Scott Card's Pastwatch: The Redemption of Christopher Columbus. The plot is rather complicated, but suffice it to say it emerges that 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 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.
- A more malicious example nearly happens in the Rod Albright Alien Adventures; 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 For the Evulz, despite the fact that he will be affected just like everyone else.
- This is what essentially would have happened if Buffy hadn't stopped Glory in Buffy the Vampire Slayer. It would have been only temporary, though.
- Doctor Who: At the end of "Father's Day", after Rose saved her father from his death and accidentally dooms the timeline, eventually forcing him to make a Heroic Sacrifice, the timeline has been restored, but the exact circumstances of Pete's death have been altered: it's no longer a hit-and-run, the location was different, and grown-up Rose was there with him when he died.
- In Fringe, due to the activation of 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.
- In Kamen Rider Build, this ends up being the heroes' ultimate plan to save the world. They combine their Earth with the Earth from a parallel universe (implied being the main Kamen Rider universe) where the series' events never happened, using the Big Bad as fuel to ensure he doesn't exist in the resulting world. Most of the characters end up with parallel counterparts in the new world, who don't remember anything that happened. Sento and Ryuga, however, because of how fundamentally they were altered, end up as anomalies who do remember the show's events and actually coexist with their parallel counterparts.
- On Sliders, the Big Bad of the last season tried to do this, experimenting by first merging people from different universes together.
- Inverted in the computer game The Longest Journey: the idea is to prevent the two worlds from combining, at least under the current uncontrolled conditions.
- Tales of Symphonia 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 Save Both Worlds. It just so happens that the resulting combination ends up being the world map of Tales of Phantasia.
- It doesn't quite go as planned, as shown by the sequel/spinoff that came after. 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 Tales of Eternia, the two main worlds begin colliding. The heroes only manage to fix it just in time by 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.
- Tales of Innocence 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.
- Final Fantasy V: this is exactly what Omnicidal Maniac Exdeath wants to happen and the heroes are trying to prevent. As it turns out, the 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 Crystals, so with or without Exdeath actually claiming his prize and wreaking havoc with it, everybody's pretty much screwed). 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 Mortal Kombat vs. DC Universe, the Big Bad is Dark Kahn, a merged version of Shao Kahn and Darkseid, who starts merging the realities of Mortal Kombat and the DC Universe, 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.
- The Legend of Zelda:
- Implied in The Legend of Zelda: Majora's Mask. The game's ending has you see you see all the people you interacted with in the sidequests, with the game showing them in their best case scenarios no matter how many times you exploited the "Groundhog Day" Loop or even whether or not you helped them on your final loop.
- Averted, however, in The Legend of Zelda: Ocarina of Time - a Continuity Snarl has led to a Word of God-supported explanation that the events of that game created multiple timelines that have not been known to merge again. (Some sources make it look like the timelines eventually merge into The Legend of Zelda: Breath of the Wild, but the actual explanation given is that Breath of the Wild could be on any (one) timeline and the devs decided against choosing a specific one.)
- In Eternal Darkness 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 and his own counterpart Ancient as the last of the Ancients.
- Happens in the browser-based game no-one has to die.. Four normal endings, each with only one character left alive, each of them walk into a time machine, and meet up in a Merged Reality for the final ending.
- Judge Dredd: Dredd vs. Death: 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 Deadworld with Dredd's universe, killing every living thing in Mega City One.
- Super Robot Wars Z, using the same plot device from Orguss (listed above) in concert with the Black History from ∀ Gundam.
- In Sonic Rush, Eggman's theft of the 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. It's also noted that this is only happening because the two sets of Emeralds aren't being controlled properly, and once Blaze learns how to properly utilize the Sol Emeralds' powers she's able to traverse the dimensional barrier between the two without risking the dimensions.
- With multiple uses of time travel in the whole 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 Command & Conquer: Red Alert 2: 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 Forestnote 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 Tailor-Made Prison to render his psychic abilities useless.
- Marvel vs. Capcom: Infinite: Ultron and Sigma use the Space and Reality Infinity Stones to not only merge themselves together, but also merge the Marvel Universe and the various 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.
- In Jump Force, the machinations of the game's villains are causing the "Real" world to merge with the worlds of "Jump".
- Distorted Travesty: 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.
- In The Adventures of Dr. McNinja, this turns out to be the main motive of the Big Bad, King Radical. His homeworld, the Radical Lands, were drained of Life Energy by Sparklelord and gradually dying. He escaped to an Alternate Universe, 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 Blue and Orange Morality 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 Supernormal Step, 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 The End of the World as We Know It, a witch separated everything magical away from the main universe into a pocket dimension of its own, resulting in The Magic Goes Away for the main universe. In chapter 22, the Big Bad 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 The Magic Comes Back makes the resulting chaos even greater.