This is a very common Fantasy-genre trope. It's a way of explaining away various others, such as Joker Immunity, Failure Is the Only Option, etc.
Basically, Good and Evil are unionized, and have rules they must follow. These rules are usually towards overall self-preservation; no one side is allowed to "win" too much, and the Big Good and Big Bad restrict their fighting to a Divine Chessboard rather than going at it in person.
At times, this can get silly, with there being no reason behind why the absence of evil would be a bad thingnote Arguably, the definition of "evil" is "that which would better the world by its absence". Sometimes this is explained with the universe becoming boring, society stagnating or collapsing from within in the absence of something to struggle against or giving people a chance to show real nobility and virtue by risking their lives to defend each other. Other times, its enforced by depicting ultimate good as repressive (often Lawful Stupidnote more to the point, no longer good), or by declaring positive or neutral concepts such as free will or ambition as evil. This can confuse people who equate Good (the opposite of Evil) with good (the opposite of bad), as preserving the Balance of Good and Bad eventually means that you have to, for example, defend a murderous rapist's right to murder and rape, though if Rape Is a Special Kind of Evil someone could say that the rapist is wrecking the balance by throwing it too far in the evil direction. In those cases, "Good" and "Evil" become something more similar to physical forces like "Hot" and "Cold" and don't seem to have much to do with morality at all anymore.
Further, only Good seems to ever care about actually respecting the balance, while Evil almost never holds back. This can be especially problematic if Evil Only Has to Win Once to badly upset the balance. However, the penalty for this is usually permission for the hero to finally kill the villain. Occasionally it's reversed with a "Good" Well-Intentioned Extremist and "Bad" Anti-Villain or Villain Protagonist, invoking Light Is Not Good. In extreme cases, both the Good and Evil sides are bad and the protagonists form a third party trying to stop either side from taking over. The irony though is that the writers don't seem to recognize that "good" guys who don't do good things aren't good any more, but rather villains with good publicity. If the "good" guys are Well-Intentioned Extremists then it is a little more believable because they are doing good things but they are doing so by doing something wrong.
Good may be portrayed as being intrinsically weaker than the "unbalancing" forces of Evil, implying that maintaining the Balance is the best outcome it can even hope for. Which makes the struggle not functionally different from standard Good Vs Evil fare, just with the word "balance" mentioned a lot.
It may also be motivated by the belief, right or not (see Space Whale Aesop), that the universe always spawns a new, bigger evil every time the good guys win. Occasionally, this is used as justification as to why the Big Bad needs to be sealed away instead of killed, because killing him would create an inbalance resulting in a Bigger Bad stepping up to fill the void. By sealing away the Big Bad, they neutralize his threat (he can't cause death and destruction anymore) but he's still technically in the world (just imprisoned), so balance is maintained.
Sometimes, this refers directly to the heroes and villains in the story, instead of Good and Evil as a whole. In this case, the villain doesn't need to actually be DOING evil at the time, they're just evil, so good must balance them out. Likewise, the hero doesn't have to perform acts of heroism all the time; just being good provides balance for the villain's evil.
This can also be done by an ancient organization which have carried the duty for hundreds of years. They can be presented as neutral, or, in some cases, they can help the hero because the villain is breaking the balance.
Often relies on Good Is Boring. See also Good Is Dumb, Good Is Impotent and Evil Is Cool. The heroic and villainous versions of the Idiot Ball also usually play a part in allowing this trope to function. Lawful Stupid might also rear its head, as might Stupid Neutral if an outside force enforces the balance.
You can substitute Light and Darkness, Heaven and Hell, Order and Chaos, or any Yin and Yang for Good and Evil with the added bonus of making more sense linguistically—a balance between Order and Chaos probably is better than either extreme, not to mention the horrors that happen with too much Light or Darkness.
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Bleach does this to some extent. It turns out that souls in the after life and life need to balance out by being recycled. Thus when people die in the real world their souls move to the afterlife (and presumably vice versa.) This is revealed when a particular race starts destroying a bunch of Hollows completely, destroying their souls in the process and thus throwing the worlds out of balance. (Justifed to that race because Hollows are evil and tend to kill people randomly.)
Not really evil or random. While a great many hollows have been shown to have simply embraced their darker instincts, it's been shown that Hollows tend to attack humans and pluses out of a need to feed rather than out of simple randomness. That doesn't mean that they're all irredeemable (just look at Nel Tu, who's entire pre-turned into a child philosophy revolved around the idea that Hollows were able to overcome their instincts by becoming Arrancar, or even Ulquiorra, of all people, who was finally able to understand friendship before dying). The problem with the Quincy was that they were destroying the souls of both the Hollows and, by extension, those they fed on, using the methods they had developed. This threw out the balance not of Good and Evil, but of the very reincarnation cycle of their universe, which in turn would eventually have destroyed everything; hence the war between the Quincy and Soul Reapers. Since the Soul Reapers were unable to convince the Quincy to stop, they just killed them instead.
Magical Project S had re-establishing the Balance as its main goal, represented by a giant set of scales that adjusted themselves after every Sammy victory. Convenient! note Course, you could argue that since every Sammy victory was just her vanquishing the Evil introduced specifically to fight her in the first place by Pixy Misa, there should be no net gain in Good over Evil. But this is an Affectionate Parody of Magical Girl shows, so you really should relax.
And the scales only really started to tip the other way after she found a problem that was preexisting; Misao's sad life, and fixed it. Hmm...
Ah! My Goddess has the Doublet System, which prevents gods and demons from slaughtering each other by imposing a Mutually Assured Destruction scheme on both sides. Small conflicts are allowed, even encouraged, but outright celestial warfare is forbidden.
Hild, the leader of the demons, has tried to subvert the Doublet System and shift the balance in her favor by inducing Face Heel Turns among the gods. Thus far she's been unsuccessful, and given the nature of the series it'll probably remain that way.
It's later revealed that what Hild does care about is the balance, the reason she antagonizes the goddesses is that their presence in the Morisato home actually causes an imbalance that she needs to rectify. Of course, the reason she does this is because of her daughter, the goddess Urd.
Meanwile, Hagall and her crew don't care about the balance and just want to increase the demons "market share" This quickly causes the world to erupt into hilarious chaos.
The final arc of Dragon Ball GT is half this trope, half Phlebotinum Overload. Every wish made with the Dragon Balls produces an energy that is counter to the nature of the wish, i.e. good-natured wishes produce negative energy and vice versa. This energy was meant to dissipate between each use of the Dragon Balls, but due to the frequency of their use, there wasn't enough time for that to happen. This eventually led to the creation of seven dragons, born from the built-up energy. Because the majority of wishes were made by the heroes, the dragons were born from negative energy and therefore were evil, though one did have Noble Demon tendencies from the few wishes made by the villains.
Transformers Cybertron. Taking Unicron out of the equation resulted in an imbalance that caused a black hole that threatened to do in the whole universe (the multiverse in the comics.)
Umi Monogatari has this as a crucial part of the series. The girls are told to destroy the force of darkness, Sedna, with a Spear of Light. As it turns out, though, the islanders' unwillingness to accept and deal with the sorrow in their hearts created Sedna. In the end the darkness is accepted by them and becomes one with the light.
In most superhero universes, a kind of balance applies, in that while superheroes rarely lose, villains keep coming back (or are replaced by new ones.) One would think that villains would just ally to bring down the heroes with sheer numbers, but this rarely works because villains can't trust each other for long, while heroes can.
In the Marvel Universe, Adam Warlock got a hold of the Infinity Gauntlet, a MacGuffin that granted the user control over all of existence. His desire to become the "perfect" God caused him to subconsciously expunge both good and evil from himself completely. This backfired, as both became anthropomorphized and tried to take over the universe (after he had been stripped of the Gauntlet).
The evil side tried to take over the Universe. The good side merely tried to destroy it. In fact, she tried to destroy the whole multiverse.
Warlock also refused to kill Thanos, on the account that the Universe would be incomplete without him. This paid off later, when Thanos helped him deal with said good and evil sides, and eventually sacrificed himself to fix the Universe.
Galactus, the big guy without fashion sense who eats a few billion people a month, was also rationalised as "necessary for the cosmic balance" by the sentience of the Marvel Universe, for very trippy and flimsy reasons in John Byrne's "Trial of Galactus". The character has been rationalised as anything from a Mussolini-esque "storm to test the worth of civilisations, so those who don't survive deserve to die", to "the balancing force between life and death" (as if overpopulation didn't take care of itself), to keeping an even greater threat in check through his feedings, to "a force of nature Beyond Good & Evil... that nonetheless is fully able to frequently hold conversation with his food, or enlists its aid, is completely sentient, has a personality etc".
Galactus' characterization and food source tend to vary from writer to writer. Some some stories he can feed off of stars or other energy sources while in others he can only feed on those temporarily and sooner or later needs bio energy from planets. His characterization in some stories has him as a selfish, entitled, egomaniac that is little better than a petulant child. In others he is more of an aloof cosmic being who can hold conversations with humans and such, but when you think about it is truly portrayed as "alien" due to being billions of years old and power far beyond human comprehension.
Another explanation for Galactus (which may have been retconned away by now) explains that when the current universe ends, he will jumpstart the next Big Bang which will begin the next universe. So his existence is justified, though it's cold comfort to anyone whose planet he eats.
Yet another explanation was that the life-energies he consumed radiated from him as he used his powers - or even just existed - and would thus "seed" other worlds with the energies needed for life as he passed them. For every world he consumed, several would eventually develop life.
In the DC comics Universe, The Lords of Order and the Lords of Chaos fight an eternal war, with the winning side decided by repeating cycles. They use most of the magic-using heroes and villains as their pawns, and most people believed that Order was good and Chaos was evil, until both sides showed their True Colors. (Actually Chaos was overall pretty dangerous while Order was uncaring, making the DCU a Crapsack World.)
In Hellblazer John Constantine spends a lot of time fighting off demons, but he is nevertheless careful to prevent Heaven from striking any decisive blow against Hell. The idea seems to be that while Heaven and Hell are locked in eternal war, humankind have a certain amount of freedom from either side - but if one side ever won, the humans would be its slaves.
Subverted in a storyline in The Spectre: the godlike ghost had almost lost faith in humanity because of the manipulations of his Evil Counterpart Azmodus, and almost seemed ready to fall under his sway. Then Spectre's friend, a preacher, told him to find out the truth by himself by looking into the collective souls of humanity. Even his other allies feared that this would only confirm the accusations... but The Spectre found that there was more light (good) than darkness (evil) in the human race- MUCH more. Cue Heroic Resolve and demonic asskicking. A Crowning Moment Of Awesome for the series.
Speaking about the Spectre, he once tried taking out Darkseid. Well, it was successful, he he killed Darkseid dead. Only for the latter to rise in a skeletal form and begin reforming, stating that he couldn't be killed by the Spectre in this manner because he was a necessary evil in this universe.
Animal Man in the DCNU has a balance between the Red (animal life), the Green (plant life), and the Rot (death and decay). While the first story arc has the Rot and its avatar as the Big Bad, it's implied that any one of the three sides has the ability to grow beyond its means and become a threat.
In BetrayalHarry received three innate Animagus forms shortly after the final battle. One of them, a catlike creature called a Night Stalker, was assigned to act as a "balancer" between light and dark, which in practice meant that he was driven to subdue or kill people or beings who attempted to kill others. When Daphne Greengrass asked what would've happened had there been too much light in the world he replied that in that case, they would've been faced with an evil version of him which was driven to "bring the light down to a manageable level."
Both Yu Gi Oh Dark Messiah and the Dead FicTilting the Balance reference "the Balance", a force that holds four elements - Light, Darkness, Dreams, and Shadows - in check so that their combined strength holds back the all-destroying Chaos. When any of the four become too powerful, its opposite number acts to counter it via Chosen.
This is the main plot of the Russian film Night Watch. The eponymous group are the forces of good who stop the forces of evil (The Day Watch, to be the title of the sequel) from becoming too strong. The Day Watch does likewise, all because of a treaty between the two sides. The book series the film is based on makes it clearer; neither side directly acts on the Muggles, out of fear of giving power to the other side to maintain the balance. For instance, a Night Watch member has to get approval to cure a loved one's cancer, because doing so might give the Day Watch cause to go out and give someone AIDS.
In reality, both the Day Watch and the Night Watch are one and the same; the power struggle between them is carefully monitored on the part of the Inquisition, which has all the real power. The Watches only exist to satisfy young Others' desire to satisfy their primordial alignment, the goal being to unite all Others, light or dark.
In the novels, it's shown that the war between Light and Dark is more of an Enforced Cold War than anything: all parties, including the Inquisitors, are afraid of the likely alternative. This results in combat by Gambit Roulette, seeing as how the heads of both the Night and Day Watches are incredibly old and cunning. Another point is that the Twilight, the source of magic in the series, is capable of taking its own steps to restore equilibrium, should one side hold too much of an advantage.
The Star Wars prequel trilogy has a Jedi prophecy about Anakin Skywalker bringing "Balance to the Force". In a subversion, balance means inbalance here, the lines from the movies which make the viewer think that the Jedi misunderstand the prophecy are set as true by Word of God. Anakin fulfills the prophecy at the end of the original trilogy, when he as a Sith finally turns on his treacherous master and proves the death of them both.
Still, one has to wonder what the Jedi Council was thinking. "We've had a thousand years of peace and prosperity. Now what we need is some balance!" And then they were surprised when Anakin was evil.
Except they didn't, the Council had noticed that the Force had gone out of balance some 200 years before hand, and by that point the Galactic Republic was mired in corruption.
Without Sith, they don't need a Chosen One who is a potential loose cannon. But, when Yoda makes states the Council's decision in the end, he also explains that Darth Maul was indeed a Sith, they are sure of that now. And that is why the Council changed their minds. And, since there will be a second Sith, they will actually need The Chosen One.
Luke accepts this view during Apocalypse After his tag-team battle with Darth Krayt, against Abeloth. Cooperation between the Jedi and the Sith is the only way to keep Abeloth from destroying the entire Galaxy since the Ones are Dead, so the warring between them in the times when Abeloth isn't an immediate threat is a necessary side-effect
In The Dark Crystal, balance is enforced de facto: The good urRu and the evil Skeksis are actually the separated good and evil halves of the same creatures, the UrSkeks. Whenever a good one dies, an evil one of equal stature instantly dies somewhere else in the world, and vice-versa. The UrSkeks' original plan was to rid themselves of moral imperfections using the Crystal, but instead they got split into two races, no doubt because of the balance between good and evil.
Mentioned in Little Nicky, although that movie dealt with the balance tipping to evil.
And in a neat twist, Satan was just as interested in maintaining the balance as God was. It was Satan's kids who were upsetting the apple cart.
A variant occurs in Avatar when Neytiri tells a praying Jake, "Our great mother does not take sides, Jake; she protects the balance of life." Fortunately for our heroes, she does choose to help the balance-respecting Navi against the unbalanced humans.
Between the forces of the forest (good) and the forces of rot (evil) in Epic.
The second Wishmaster movie. When Morgana tries to wish for there to be no evil in the world, the Djinn explains that he can't grant that wish as evil is one half of a perfect sphere, and therefore tied to creation itself.
The entire plot of the Book of Swords trilogy by Michael Moorcock was the balance between the good (or at least sane) Forces of Law and the depraved Forces of Chaos, both of whom were under the domination of the Cosmic Balance. That war ended when the hero Prince Corum, an incarnation of the Eternal Champion who had spent his existence fighting to preserve the balance, unleashed two gods from outside the rules of the Cosmic Balance who went on to kill off both sides, freeing their subject worlds to grow and evolve without a pack of selfish gods interfering.
In The Quest for Tanelorn (last of the Chronicles of Castle Brass trilogy), the Cosmic Balance or at least a representation of it is itself destroyed, with the narrative implying in places that it may be the belief in 'higher powers' in the first place that's holding mankind back from realizing its true potential.
Stephen King's novel Insomnia reveals that his Multiverse is ruled by four theoretically-equal forces: Life, Death, the Purpose and the Random. The Random is responsible for much suffering and chaos, and is the closest equivalent to Evil.
Earthquakes that kill a hundred thousand people can be part of the the Purpose, but the higher beings of the purpose are good and want to mantain balance. The highest being of the Random is the Crimson King, who was described as the embodiment of evil until he was nerfed in the Dark Tower. He wanted to destroy everthing and rule the hellscape that remained forever.
In Bring Me the Head of Prince Charming there is an organized competition between the forces of heaven and hell who will control fate for the next thousand years. However, this book is almost a parody.
Played with in A Night in the Lonesome October, which involved semi-regular contests between 'Openers' and 'Closers' over opening a portal to ancient gods of wonder and terror. If a contest was a draw, or won by Closers, the world remained the same, maintaining the current balance. Much of the month of the contest was filled with players carefully feeling out each other's allegiance and angling for advantage without sending the contest up into premature fireworks - maintaining another balance, if a pragmatic one.
The Chronicles of Amber series of stories. The Pattern and the Logrus contest with each other over control of the Shadows between Amber and the Courts of Chaos. In the novel Prince of Chaos the Primal Pattern says that it and the Logrus are fundamental principles by which the universe is organized. The Pattern represents order, reason and sanity, and the Logrus represent chaos, feeling and madness. However, neither seeks the other's extinction, because they know that if either existed without the other it would lead to a dead end.
This is the entire basis of Eve Forward's Villains by Necessity - similar to the Dragonlance example, Good has thoroughly dominated evil, obligating the protagonists to restore the balance to prevent catastrophe. Also similar to the Dragonlance example, the premise is rather undermined by the fact that most of the "Good" antagonists are Lawful StupidKnights Templar who have been brainwashing captured villains to make them good at the expense of their personalities, and their leader is the main protagonist's father by rapeand wants to have light consume the world, which just happens to destroy it in the process. And he is fully aware of this.
In The Saga Of Recluce the Balance of Order and Chaos is a quantifiable phenomena. Order and Chaos are the basic components of the world, tightly bound together to create matter. Mages who use only Order or Chaos release an equal amount of the other component into the world. As such, the more an Order-mage creates, the more Chaos will be available for a Chaos-mage to wield. This poses the largest problem for Order-mages as Ordered creations are enduring while Chaos is fleeting; there is almost always a surplus of the latter. Chaos and Order are not Good and Evil, as illustrated by heroes throughout the series who wield one or the other, or even, in rare cases, both in equal measure. Absolute Order is death, as living things need to change, evolve and grow (a form of Chaos) but also need structure (a form of Order) or they will collapse. Usually only the central character is really conscious of it and strives to maintain the Balance, while the wizards on either side try to have *more* Order or Chaos, which usually backfires. Both Fairhaven and Recluce are destroyed, at different times, by Gray wizards. The Chaos Wizards wear white and their city is Fairhaven, while Order Mages wear black and Nylan is the 'Black City.'
Andre Norton's Witch World novel Horn Crown features good and evil prototypes of the gods. The evil ones declare, at the end, that they will win; the good ones merely observe that they know that the battle is unending. (The good ones are unquestionably right.)
In Good Omens, the forces of Good and Evil are in fact trying to destroy each other, but Aziraphale and Crowley (an angel and a demon, both very low-ranking) have been working opposite each other for so long that they've become friends. They work to maintain the balance because it allows both of them to report successes to their superiors without anything really changing over time, which coincidentally allows them to keep working together on Earth. It's suggested that this may be the underlying idea behind the conflict in the first place, so that neither side ever wins permanently, but God's plan is so ineffable that none of the lesser angels or demons know this for certain. It's implied that halfway between good and evil lie humans, who in turn are capable of the better acts than angels and worse acts than demons. For either side to win would destroy humanity, free will, and the whole world. The free will part of this is what makes Adam realize that doing whatever he wants with the world isn't the right way to go about it, because what's the point of having friends if you can make them do whatever you want?
The Night Watch trilogy features a balance between the Light and the Dark, enforced by the Night Watch and Day Watch respectively, though it seems to be more of a ceasefire than an actual cosmic law. There's an amusing scene in the first book where Anton, the protagonist and agent of the Night Watch, uses his magic to force an annoying street hawker to become more moral and is immediately called out on it by a pair of Day Watch agents. Anton tries, unsuccessfully, to persuade them that the hawker's quality of life will decrease as his morality improves, and thus he has not violated the balance.
As the series continues, it becomes more and more obvious that Light versus Dark isn't good versus evil so much as Utopia for everyone versus Utopia for few, among other things, Hitler was the Light Others' fault. And as it turns out, higher ranking and/or older Others don't see a great difference between the two, the dark siders just place themselves higher than everyone else.
Louise Cooper's Time Master Trilogy subverts this. The trilogy opens thousands of years after the Gods of Order have banished the Gods of Chaos from the universe. At the end, the Gods of Order don't care at all about their followers, and the Gods of Chaos decide to establish a balance rather than banish the Gods of Order in revenge.
An interesting case comes up in the Fighting Fantasy book Creature of Havoc: A trio of evil(ish) witches want the eponymous creature to defeat the Big Bad in order to prevent too much evil from entering the world and thus upsetting the balance.
This is actually justified in Animorphs. The conflict between the mostly good Ellimist and the utterly batshit and evil Crayak is less of a traditional balance and more of a Cold War. Both sides would be happy to wipe out the other given the chance, and actually tried in the Back Story. The consequences for both were catastrophic: their battle caused massive loss to the lives the Ellimist wanted to protect, and Crayak lost what little progress he had made in controlling the threads of reality. This led to the Ellimist and Crayak's current arrangement of fighting through proxies, with any final victory only taking place after eons of "playing."
Taken to an almost ridiculous extreme in the poorly known (only recently translated to english) series Amos Daragon, in which a person (the "mask wearer") is chosen to keep a balance on good and evil. Needless to say that, since all the evil gods are trying to ensure the extinction of mankind, this seems remarkably stupid - unless the author actually meant a "balance between light and dark" instead of actual good and evil, because I really don't think that its necessary to ensure that dragons and other reptilish monster go around killing off mankind just because some fairies and a goddess told you to "keep a balance"
Actually, the series is filled with Light Is Not Good and Darkisnot Evil. There are plenty of "Dark" creatures which don't want to massacre humans, yet get hunted down for no other reason than they exist. In fact, the last couple books involve stopping a crusade to wipe out the creatures of the night. The protagonist's job is to Stop the war between the gods, not kill all evil creatures.
This is built into the laws of reality in The Death Gate Cycle: good existing unchallenged long enough causes evil and vice versa, meaning that (by the standards of the cosmos as a whole) a rough equilibrium will always be maintained. The bad guys want to fix everything towards the evil end, as they feed off of mortal suffering.
In Changes, Harry crosses over into the Nevernever to find himself in an idyllic flower garden and immediately starts telling Bob that he suspects that it is a darker and more dangerous place than it appears to be. Bob chides Harry for his cynicism and tells him that every time the universe creates a place of darkness, it creates an equal place of light, so not everything can be bad. Then the centipede the size of a freight train that multiplies into more centipedes when it gets hacked up attacks.
In Small Favor, the Nickelheads' plot to kidnap Marcone as a ruse to kidnap The Archive is implied to have direct assistance from Ol' Scratch himself by him giving his minions Super-Hellfire to be able to possibly contain the Archive, so an angel is allowed to intervene by granting Dresden the ability to use Soulfire, the fires of creation itself.
And in Ghost Story, the revelation that Harry's choices towards the end of Changes were partially influenced by a lie, e.g. seven words whispered by a fallen angel means that the other side gets to whisper seven words of truth at a time of their choosing.
In general, supernatural forces of good take a hands-off approach, letting humans do anything but intervening to counter supernatural forces of evil in order to preserve free will. Mortals working for good are not subject to any such restrictions.
In a lower-lever version, the Sidhe Courts of Summer and Winter are all about balance. While Summer is considered the nicer of the two, Good Is Not Nice can always apply, and likewise with Winter being the darker, Dark Is Not Evil can work just as well. For half the year, one court grows in power as the other shrinks until the solstice. Most anytime one side aims to seek a gain, whether an item to retrieve or person to kill, the other side will respond in force to stop this gain. It is a small list of examples when both sides work in union with the other against a common foe. The potential danger for one side to gain a permanent and unmatched gain would result in either a new ice age if Winter won, or an expansion of growth if Summer won. This "growth" wouldn't just include humans or other animals and plants, but things like Ebola and other diseases.
The High House series by James Stoddard uses this, but substitutes good and evil with order and chaos (most likely because the author doesn't seem to have any problem with the presence of good overwhelming the presence of evil). Too much order is portrayed as tyranny and a lack of free will; too much chaos is portrayed as anarchy and a lack of personal safety. Arguably, this makes the books the perfect fantasy series for libertarians.
In Discworld, as in the England of our world, there are morris dancers who perform their ritualistic dance in spring with bells and colourful costumes to welcome in the summer. However, in order to balance this, there are also Dark morris dancers who use silent bells and black costumes, dancing in the autumn to welcome in the winter. In Wintersmith we see what happens when this equilibrium is disturbed.
Part of Rock's shtick in Warrior Cats is that there must be a balance between light and darkness, because without one the other would not exist.
Exists in The Girl Who Would Be King, with good embodied in the Bravermans and evil in the LeFevers. The families have been fighting for centuries, neither capable of ever actually winning.
This, combined with balance between Order and Chaos is the main motivation of Grundy from A Fable of Tonight series. Grundy brings order to the world with too much chaos and chaos to the world with too much order. However, because he is utterly evil, the only means of doing so he understands involve a lot of pain and suffering, because of which he is opposed by a force he calls the Adversary, who represents good and intervenies when his actions shift the balance too much for the other side. Despite being antagonistic forces, Grundy belives both he and his Adversary understand that one cannot ever kill another because that would upset the balance too much. He also belives that cutting him out of any world means that something else will take his place to perserve the balance.
A more sensible variant than usual occurs in The Quest of the Unaligned, in that Good and Evil are inextricably entangled in the four forms of elemental magic, and only the most powerful magical artifact in the world can separate them, and then only for one person at the time. So getting rid of the Balance Between Good and Evil is not so much undesirable as impossible.
This is not to be confused with the Balance among the four elemental magics. That Balance behaves in the conventional way, where it is possible to tilt the Balance towards any one of the four, but doing so is disastrous. For example, the current king and queen have become infatuated with wind magic, causing the Balance to tilt and the country to be plagued with droughts, earthquakes, tornadoes, and cyclones.
In Buffy the Vampire Slayer, Willow's resurrection of Buffy at the beginning of season six allegedly caused an imbalance that created an opportunity for the First Evil to become the Big Bad for season seven (now that there's an army of Slayers, the balance must have snapped like an elastic band). Whistler from "Becoming" Parts 1 and 2 identifies himself as an agent of the Balance, although some fans argue that he was actually an agent of Good who identified himself as Balance because he worked to counteract other "demons" who were apparently predominantly evil.
In Angel, it's implied that there is a balance imposed by one side. Evil demons, gods, and so on, are constantly trying to take over and/or destroy the world, and the Powers That Be only ever do just enough to prevent it. Given how much they could do, it's clear they're only trying to stop evil from taking over and not actually defeat it permanently.
Somewhat tangential, but in the similarly-disrupted evil balance, the same sort of mutilation was enacted for such trivial niceties as saying "Gesundheit" when someone sneezed.
Essentially, the point being made was that Good and Evil cannot tolerate the others' existence, and therefore in a world dominated by one, any act (no matter how minor) that runs contrary to the ideals of either is punished harshly. It is the mix of the two that provides tolerance and temperance.
Comes up in Paige's introductory episode. Apparently, there is a 48-hour window after a witch receives her powers during which she can be swayed to good or evil. When Piper demands to know who would make up such a stupid rule, Leo looks upward and Cole looks down, indicating that this is another of those balancing things The Powers That Be on both sides have agreed on.
In the Star Trek: The Original Series episode The Enemy Within, Captain Kirk gets split into his good side and his bad side. In the end, it's shown why Good Kirk needs Bad Kirk: Bad Kirk has the confidence to make decisions. One could say that Bad Kirk is only bad morally, but that he might still have some good (but not necessarily moral) qualities (such as confidence), except the confusion comes from the fact that Bad Kirk also has the quality of Cowardliness (the reason why he needs Good Kirk), which isn't necessarily an immoral quality at all, but which he is described as having because it's "bad". As a result, it seems arbitrary which qualities are chosen to be "good" or "bad".
One episode of 7Days has, as a villain, an inmate who has converted his asylum into a cult dedicated to preserving the balance with astrology. He thinks that this are tipped in favor of Good, so naturally he has guys committing random crimes to even things out.
Invoked in the Smallville episode Quest. A deluded religious fanatic thought that if he managed to kill Clark Kent, the most powerful force of good in the world, then all the evil in the world would just disappear.
LOST invokes the trope with a balance scale holding black and white rocks. After arranging for Jacob's death, the Man in Black throws the white rock into the ocean.
In the original run of Doctor Who, the Black and White Guardians are embodiments of this principle.
Zoroastrianism is among the best example of this trope. Unlike Abrahamic religions in which God is omnipotent and Devil is much weaker, in Zoroastrianism there are two equal Gods of good and evil. Manichean Christianity had the same idea.
That being said, it's quite clear that in the end Ahura Mazda will triumph over Angra Mainyu, so Zoroastrianism isn't of the "balance for balance's sake" school of thought. note To elaborate, traditional Zoroastrian thought holds that good and evil are, more or less, balanced in the present age of "mixing", in which good and evil forces are present in the world in roughly equal amounts. However, Zoroastrians hope and work towards an ultimate triumph of good over evil and the permanent banishment of Angra Mainyu from the universe. Angra Mainyu loses not because he's less powerful than Ahura Mazda per se, but because the nature of evil is inherently ignorant and self-defeating.
Contrary to popular belief, not present in Daoism or anything else that uses the yin-yang symbol. Yin and Yang represent harmony through complementary opposites like dark/light, female/male, earth/sky, etc. In such philosophies, morality is seen as a social construct rather than a cosmic absolute (like in Zoroastrianism). The closest thing to 'evil' in the western sense is anything that disrupts harmony, i.e., not part of yin or yang.
Ancient Egyptian religion focused heavily on a balance between creation and destruction, worship being done primarily to keep the destructive forces (often translated as demons) from getting too out of control. It still fits this trope since the biggest baddest destroyer, Apep, was evil and its primary opponent, Ra, was good.
This trope is a core element of the ur-Role-Playing GameDungeons & Dragons. In fact, one part of the Druid's job description in the early editions of the game is "Keeper of the Balance", a reference to both the balance between Good and Evil, and the one between Law (Order) and Chaos. To facilitate this, Druids had to be Neutral in alignment in the original basic set.
In the later editions, they only need to be partially neutral; the alignments there are the nine combinations of [good, neutral, evil] and [law, neutral, chaos]. In the basic game, neutral was one of the three alignments, along with law and chaos, which were originally just euphemisms for good and evil, respectively.
It's also stated on occasion that the gods won't interfere directly much, because doing so will inspire opposing gods to step in and add their own influence.
The Dragonlance campaign setting's backstory shows us why this whole balance business is necessary. In brief, after a hero wielding the eponymous weapon defeated and banished the evil goddess Takhisis, things improved quickly. But with no real bad guys to fight, the Church of Paladine began persecuting anyone who wasn't good enough. Then, their Pope-analog decided that to purge evil from the world completely, he needed to become a god. The Dragonlance campaign setting takes place After the End.
In Dragonlance, if there's no Evil, some of the Good becomes Evil to compensate. And if there's too much Evil, then the Evil turns on itself and kills itself off. It's a kind of self-maintaining balance.
In the D&D 4th edition core setting, the balance between good and evil is maintained by the primordials, a group of god-like eldritch abominations who are at war with the gods. The war's mostly cooled off at the moment, but could flare again at any moment. Because of this, the good gods and evil gods are loath to battle each other; the evil gods know that if they were to slay their good counterparts and take over the cosmos, they'd rule for about five minutes before the primordials swarmed them. Likewise, the good gods know the villian they slay today could be the ally that would have saved them from a primordial tomorrow. Not all the gods play by these rules, and some are even likely to come down on the side of the primordials when the fight starts up again, but for the most part the looming threat keeps the peace between them.
The classic Greyhawk setting has the Circle of Eight, an order composed primarily of wizards and lead by Mordenkainen to maintain (Mordenkainen's vision of) the balance. Keeping Evil around is required to keep Good working together and on the ball, lest they start with the infighting as mortals are wont to do. And no, not all good guys are on the same side on Oerth by default - they need someone on the outside pushing them together.
Planescape has the Rilmani, the living embodiments of True Neutral much like Devils are the embodiments of Lawful Evil or Aasimon (angels) are Made of Good. The Rilmani are concerned with balance because "Good" and "Evil" (and Law and Chaos) aren't just points of view, but very real metaphysical forces that compose the multiverse. Too great an imbalance has serious cosmological implications. It would be like tinkering with the fundamental forces in particle physics.
The characters in Legend of the Five Rings tend to frame themselves as good and the forces of Fu Leng as evil, and The Powers That Be seem to want a balance between the two, however this seems to be a case of Unreliable Narrator or Values Dissonance. The Samurai clearly represent the extreme of Order, while the Oni represent Chaos. Interestingly, a being called the Lying Darkness wants to destroy all reality, and it has been overtly stated that this is NOT good or evil. Based on this it's not unreasonable to assume that their definitions of Good and Evil are actually just Order and Chaos from the POV of Knights Templar.
Definitely Values Dissonance. In brief, D&D assigned most of the samurai the alignment of "Lawful Evil" in their conversion of the game system. Long version: the samurai recognize superstitions encouraging them to kill any peasants that cross their path or even look at them, and The Powers That Be enforce a Celestial Order that requires the samurai to make most of society into property, or at best half-human tools, while not valuing life at all - their own, or anyone else's. In a world where "compassion" means "killing your enemy as swiftly and brutally as possible so that he need not live with the shame of defeat, which would most likely just cause him to commit ritual suicide," one can bet that the good guys are by no means good. The formerly Chaotic Shadowlands faction is now an organized, strictly hierarchical Clan, so all of that's a bit muddled... To be fair to the poor Rokugani, that description of their culture is incredibly simplistic and very biased. Which makes sense when you're trying to force them into the more rigidly-defined and somewhat Eurocentric morality slots present in D&D alignment systems, but which is ridiculously ethnocentric and borders on Values Dissonance from a real-world perspective, or even Fantastic Racism.
A specific example: On the side of Good are the Oracles of the Four Elements, and the Evil side has the Dark Oracles. Exempt from this is the Oracle of the Void, who's considered neutral...until the then-current Oracle married the Emperor, at which point The Powers That Be considered her to have chosen Good, and thus chose someone to become the Dark Oracle of the Void — and she was not happy about it...
In the mythology of Werewolf: The Apocalypse, the major source of conflict both in the game and throughout all of reality was the corruption of the force of balance known as the Wyrm. The Wyld (chaos) and the Weaver (order) would simultaneously work to conceive new things and give them form, respectively, and the Wyrm (entropy) would destroy those things whose time had passed in order to provide raw materials (and space) for new things to be created. Eventually, sick of having everything it made torn apart and destroyed, the Weaver entangled the Wyrm in its web, driving the Wyrm insane and turning it into the force of evil and corruption instead of balance and natural decay.
Apropos of that, the Kitsune (werefoxes) had actually appeared in a book before the release of Hengeyokai, in which they took on a considerably different role; they were cast as the agents of the Balance Wyrm, and were purported to take that responsibility very seriously. Aside from the short write-up in the back of an unrelated sourcebook, though, they never appeared in other material in that form before being retconned into the tricksters of the Beast Court (where the were-snakes filled the roles of Balance Wyrm's agents).
La Pucelle Tactics has a goddess of light (Poitreene), goddess of dark (the fallen angel Calamity), and another supernatural being in charge of keeping things in balance (Hermes).
Played with in Secret of Evermore, where, after defeating the Big Bad, the hero learns that he's destroyed the balance between good and evil and that he needs to get himself and the others that came from his home universe out of the world of Evermore before it collapses.
The Keepers of Thief give "the balance" as the reason they try not to interfere. This turns out to be more than just talk: after a major blow is dealt to a force of Chaos in the first game, a force of Order gets uppity and becomes the major villain in the second.
In Jade Empire, the bad guys stole a goddess' power to end a major drought. But because of the need for balance, while the Empire has water in abundance, another land goes thirsty.
One of your followers is a good demon named Chai Ka who exists in the mortal realm by inhabiting the body of a little girl. He's been charged by the heavens with aiding you in your quest. An evil demon named Ya Zhen also inhabits her body, put there intentionally by the Celestial Bureaucracy, and the reason always given is the need to "preserve the balance" (although, at least one more cynical observer claims that nothing gives the heavens more pleasure than watching mortals deal with just these sorts of problems).
In The Longest Journey, the worlds of Stark and Arcadia are the order and chaos halves of what was once one world. They are kept from remerging destructively by the Guardian of the Balance; said guardian's absence triggers the events of the game. In addition, one of the villains had his chaotic essence separated from him, turning him into a cold-hearted and purely rational being while the swirling chaos was released to terrorise Arcadia. It also turns out that he's the next Guardian , whom the villains caught and separated in an attempt to control him, and before you can have him take up his proper job you have to reintegrate both sides. Once you do that, he becomes a surprisingly nice guy.
Primal features a balance between Order and Chaos, in which Order mostly functions as Good and Chaos functions as Evil; the game's plot involves the balance being tipped towards Chaos, requiring the protagonist to even things back out.
Supposedly in Castlevania, Dracula's very existence fills up the "evil" side of the Balance. In the Sorrow duology, a cult of "good" believes that his previous final defeat was going to mess everything up. Whether this is true isn't known; but in the good ending one character speculates that if a Dark King is needed, then the Universe will provide; no one person has to be evil.
The necromancers of the Diablo franchise base their actions on the notion of a Balance. The novels attempt to (not always successfully) play with this. In one book, the villain is a necromancer who points out that all the necromancers do is fight evil, which doesn't adhere to the concept of a Balance. The hero of the book, another necromancer, continues to fight him, but doesn't bother to explain why he's wrong. In another book, the Balance is interestingly defined not as Good and Evil, but Good and the absence of Evil. It's explained that light and dark are not necessarily good and evil, and while the balance tipping to evil would mean torment, the absence of evil would lead to stagnation. One additional point to consider that the Balance is sometimes portrayed (though not described as) not as being between good and evil, but making sure that neither the angels or the demons of the series gain too much of a foothold in the world, as both are jerks. It just so happens that at the time of the second game, the "evil" forces are much more overt in screwing with mortal reality, and only one angel is bothering to do something.
Apparently the books make it more confusing than the game. In-game materials and the guidebook make it clear that necromancers are True Neutral or even Chaotic Neutral. It's just that when the world is completely full of Always Chaotic Evil demons, and there are not one but three Satan Expys running loose, True or Chaotic Neutral is right there side by side with Lawful Good.
This a running theme with Neutral alignments in everything, and mentioned in more than one flavor text description of the Neutral alignments in D&D. In general, even characters utterly devoted toward neutrality and balance will almost always side with Good in the end, especially if the forces of Evil are strong enough. Rarely is there an instance of the opposite occurring, unless the good guys are going the Well-Intentioned Extremist route or the Neutral characters lean more towards Evil themselves. It's often stated that Neutral characters begrudgingly admit that Good-aligned characters make slightly more manageable neighbors, as opposed to their much more unpleasant cousins.
Darksiders: The Charred Council is charged with doing this and are granted significant freedom on how to do so.
Master Xehanort of Kingdom Hearts: Birth by Sleep tried to do something about the balance between light and darkness, as light was waay too powerful for universal safety. In doing so, he jumped off the slippery slope, and ended up causing mass suffering to millions, as the darkness grew in strength. Success!
The main problem with Xehanort is that the universe was already balanced, with light in the Realm of Light and darkness in the Realm of Darkness. Xehanort tried to balance the two in the Realm of Light alone which would of course throw everything out of whack.
Also in the same game, The Fairy Godmother, in a conversation with Aqua seems to be a believer in this, in that one creates the other (Cinderella's "light" creating Lady Tremaine and the stepsisters' "darkness") and you can't have one without the other.
After all, one of the first lines in the first game is "the closer you stand to the light, the larger your shadow becomes." Essentially saying that nothing can exist in the world of light without also adding to the darkness.
In the late 80's rpg Dungeon Master, the backstory is that a magical experiment gone awry separated The Grey Wizard into Lord Chaos and Lord Librasulus (obviously an order counterpart). The protagonist, the Wizard's now ethereal apprentice, has to guide a group of heroes to obtain the Firestaff (which caused the incident) for Lord Librasulus. However, on the way you find several scrolls about how dangerous the staff is in the wrong hands, how neither order nor chaos is truly balanced, and so on. If you take the staff back to Lord Librasulus, he rewards you... with death. Instead you have to complete the original experiment, then use the staff to trap Lord Chaos and fuse the two sides together into a balanced whole again.
In RuneScape, there are three major gods. One of law, one of chaos, and one of balance. This is a partial subversion in that law and chaos aren't necessarily good, and a different partial subversion in that neither is balance.
In MARDEK, this (in the variant of Balance Between Light and Dark) is the argument of Clavis and Qualna make against Rohoph, who is more on the side of bringing good to his homeworld. It is also a recurring theme in that chapter, as balance in general seems to be crucial to the peace between reptoids and Sun Temple priest, the Elemental Crystals to Belfan (the planet you're on), and the governments of both Belfan and Anshar (where Rohoph and Qualna are from). The irony is that though the Governance de Magi is villainous, Rohoph's Knight Templar tendencies are not much better. Indeed, Qualna comes off as an Only Sane Man, killed by a light-obsessed fanatic.
In the Myth series, the balance between good and evil is maintained by alternating which side rules the world as part of a great cycle. As of the events in Myth II: Soulblighter, the good guys seem to have finally broken the cycle.
This is what kicks off the plot in Shining Soul as the defeat of the Big Bad in the last game has left Light stronger than darkness; thus causing a sudden resurgence in evil.
In Immortal Souls, the supernatural Isis Corporation serves as a mediator between the hordes and gangs of shadow creatures that attack the humans and the holy warriors that protect the humans, doing whatever is needed to keep the two forces evenly matched. They attack the former only as needed to keep them from doing major damage and overrunning things, and the latter only in self-defense and when needed to remove any major advantage they happen to gain. The normal humans themselves are treated as collateral damage.
Soul Edge was originally too evil to simply exist by itself, so Algol created Soul Calibur to act as its balance from a fragment of the cursed swordnote Although this was a reactionary move to the evil sword corrupting his son and forcing Algol to slay him. Unfortunately, creating a holy sword from a cursed sword causes the holy sword to think that "Balance Between Good and Evil" really means "freeze everyone so no one can hurt one another ever again". It is kind of a "balance" but in functional terms all you get is a world of carnage or a world of nothing, neither of which help the people caught in the middle.
Defied in Ultima IV, where the main goal of the game is to prove that good can still exist even in the absence of evil.
In Dominic Deegan, early on, the viewers are made to believe that Order equates to good and Chaos to evil (the main character even believes he is supposed to be the "Champion" of Law for awhile). We find out later, though, that neither is more inherently good than the other and both can be used for good or evil deeds; the main character is tasked with protecting the balance between them. It helps that there's no cosmic axis of Good v. Evil to complement that Law v. Chaos one; instead it's Creation v. Destruction.
Blue Unity: Imagine that the two halves of the universe rest on a giant seesaw.
First Folio: OK.
Yellow Unity: We hope that has made everything clear to you.
Subverted in Darths & Droids. It was prophesied that the chosen one would bring balance to the force. That is not a good thing. 
This is a reference the the fan theory that Anakin brought balance to the Force by equalizing the number of Sith and Jedi, thus bringing it into numerical balance, as opposed to the "order instead of chaos" balance George Lucas says the prophecy refers to.
We also get glimpse of what happens when there is one too few in Marlowe's arc, as that also triggers an Apocalypse level world ending events, everyone was lucky they fixed it before things went into Brain Bleach territory.
In Homestuck, cherubs serve this function, which promotes the development of civilizations that can play SBURB and thus allow the universe to reproduce itself.
In Goblins, it's mentioned that evil is currently winning across the multiverse.
Parodied in College Saga. The antagonists in the movie are Evil Vegetarians who plot to kill off all meat-eaters. At the end, the hero says "I have found my healthy balance" and bites into a hamburger with salad.
In the Whateley Universe, Bladedancer has been given the mythical sword Destiny's Wave and is expected to be the "Handmaid of Balance". Even if that means she has to kill whoever or whatever is causing the imbalance between order and chaos.
Actually, it's not verified as 'order and chaos'. About all that's known is that previous Handmaidens have killed good men and bad. (A king who was making a Utopia, someone creating an army of demons.) Chou has, at least once, been assigned to kill an innocent person.
In the Merry stories, meanwhile, it's explicitly stated that the forces of Heaven and Hell are in fact careful to maintain a stalemate instead of engaging in an all-out war that might allow other interested parties to take advantage of any resulting moments of weakness. Or at least that's Satan's side of the story (God seems to be mostly ignoring Merry despite her technically being one of His knights, quite possibly because her soul already belongs to Sara/Kellith), but he does come across as reasonably honest in their conversations, if still not exactly nice.
Ferret mythology in Tasakeru always ends badly if someone killsDeath because of this trope.
In Jackie Chan Adventures, stronger enemies appeared after the first season because the heroes destroyed (instead of "sealing away") the villain of the first season. This kept happening until that first villain was revived and consequently sealed.
In The Venture Bros., evil is unionized, in the form of the Guild of Calamitous Intent; they have arrangements with the various law-enforcement agencies in order to secure non-interference, with the price of accepting certain restrictions on their activities, thus creating a sort of ersatz balance. When one side violates the agreements, retaliation by the other side is swift and brutal.
Aladdin: The Series had Al help out the weak, fearful Good half of a pair of supernatural brothers in the Far East (they're named Yin and Yang). Once the good brother actually stands up to evil, competent brother, they... merge into a white dragon that starts helping everybody and fixing everything.
The Fairly Oddparents: It has recently been confirmed that there is one evil anti-fairy for every good fairy that exists. Why it took a full season from Poof's birth for Foop to be born is unknown.
Yin Yang Yo played this trope straight and surprisingly well: early on season 1, Yin and Yang got rid of their worst traits, Yin's bossiness and Yang's agression. Master Yo then said that everybody needs their good and bad traits to be complete, much to everybody's (viewers included) confusion. However, this action aslo led to the birth of Yuck, and Yin and Yang found that their bad traits helped them into being better warriors (Yin's bossiness helped her to come up with plans and strategies, and Yang's aggression gave him the motivation to fight).
Game theory, used in fields as diverse as economics and evolutionary biology, is concerned with quantitative evaluation of competing strategies for success. Except under extreme, usually-temporary circumstances, most game theory analyses of alternative strategies (cooperation vs. conflict, high vs. low investment, fair dealing vs. deception, etc) find that it's the moderate and/or mixed strategies which "win" in the long term, providing the most gain for the least cost. Balance prevails, either due to moderate strategies' success, or rival strategies' persisting side by side at stable frequencies.
There is a fair amount of selection bias here, however. Situations where a single simple strategy is obviously superior (hence no balance between different ones) are so boring, from a game-theoretic viewpoint, that nobody wastes time doing a formal analysis that would count in this statistic.
Also many situations in game theory are not zero sum (gain on one side means loss on the other) like the Balance Between Good and Evil is, so a different set of strategies is required. In zero sum games, cooperation is a bad idea, since any one player can only gain at the cost of any or all the other players. In such a case, cooperation can only lead to complete equality, with no one ever acting to gain or lose anything.