In the climax of the Record of Lodoss WarOAV miniseries, only Parn, wielding both the Holy Sword of Falis and the Demon Sword Soul Crusher, can stop the resurrection of the Mad Goddess Kardis.
GaoGaiGar's "Broken" right hand represents destruction, his "Protect" left hand protection. Individually, they function as a Rocket Punch, Broken Magnum, and a shield, Protect Shade. Combined, they form the Finishing MoveHell and Heaven, which destroys a Zondar completely while cleanly removing its human core. Less directly, in the OVA, TenRyuJin is the combined form of KouRyu and AnRyu (Light Dragon and Dark Dragon), and Big BadPalparepa matches Hell and Heaven with his own God and Devil, though it's unclear if it really counts except in name. It's worth noting that in GGG, destruction isn't necessarily an "evil" power — GaoGaiGar's original form, Genesic, is a god of destruction, with the explanation that destruction is necessary for new beginnings, and allows for "the challenge of a new hero."
The final Big Bad of Slayers TRY is the fusion of Mazoku Lord Darkstar and its opposing god Volphied. It took the a spell that combined the powers of Ruby Eye Shabranigdo and Flare Dragon Celphied, channeled through the Dark Star weapons, in order to defeat him. The spell had the effect of a complete rebirth, allowing Filia to raise the Big Bad as a child.
Takamichi and Asuna uses a technique called Kanka which fuses Magic and Ki to dramatically increase the user's abilities, with a stance reminiscent of GaoGaiGar's "Hell and Heaven" above. (It is even called an "ultimate power".)
Not counting Negi himself, who starts to learn Dark Magic after getting his ass handed down by Fate. Negi's "Dark Magic", however, as described in the supplemental materials, rely upon a long stream of (cited and footnoted) psychobabble that essentially says that, unlike the diametrically opposed Ying/Yang power of Kanka, the darkness of Dark Magic is all-encompassing, and even includes the powers of light. (Including some references to Erebos and Aether from mythology for good measure.) By embracing the all-encompassing darkness, he can still control powers of light, since they do not oppose, but the danger of the spell comes from the way that he stretches his sense of self so thin that he can lose control of his own powers. In chapter 314, Evangeline refers to Negi's way of doing things (not necessarily his magic) as the "gray path".
In YuYu Hakusho, after being reborn as a half-demon, going to the Demon World, and setting up a tournament with unquestioned rule over all Demon World as the grand prize, Yusuke mixes his human Spirit Energy and his demonic energy together to enter a sort of super mode. The last time we saw anything like this, the character using it managed to beat almost the entire main cast at the same time, and the powerup for Yusuke is rather similar.
Yu-Gi-Oh! 5Ds has Rex Goodwin grafting the arm containing the fifth Signer mark onto his body once he becomes a Dark Signer. Thus, he becomes a Signer and a Dark Signer at once, planning to use the powers of their respective deities to destroy the world and remake it in his own image. Actually a very extreme attempt for him to Take a Third Option and end the Signer vs Dark Signer battle that has been going on for thousands of years with no end in sight.
In Fullmetal Alchemist, Kimblee's prefered attack is this. Each hand has tattooed on its palm a symbol; on his right there is a symbol that stands for the Sun, fire and gold, and on his left one that stands for the Moon, water and silver. When he joins both symbols, the result is this literally.
Natsu defeats Zancrow in this manner; by canceling his own magic, Natsu gains the ability to eat Zancrow's flames. Then he combines his red dragon slayer flames with black god-slayer flames for a truly brilliant finishing move.
In chapter 296, Sting and Rogue combine their Light and Shadow Dragon-Slayer magic in a Unison Raid during their two-on-one fight against Natsu. It's not enough.
The future version of Rogue Cheney is able wield both elements by himself. And he is more than enough to defeat Natsu at full power.
In Kenichi: The Mightiest Disciple, there are two types of martial artists; Outward focusing Dou types that fight aggressively and turn their anger into a source of power, and inward focusing Sei types, who are calm and calculating and derive their power from inner peace and concentration. And then there is 'Seidou goui', a technique that combines the two for an explosive boost in power but that causes permanent damage to the body if used for too long.
In Id the title character has to negate a Yin Yang bomb twice. The first time, instead of blowing up the continent the energy is used to transport him 10000 years back in time, the second time it conveniently moves him 10000 years into the future, netting in three weeks lived in the past. Later it turns out what he did during those three weeks in the past, has caused the creation of the world he's on and is the reason for almost the entire plot in the story.
In Dai no Daibouken this is the basis for Pop's most powerful magic attack, Medoroa: generate fire with one hand, ice with the other, and combine them to shoot a beam of annihilation that can destroy orhicalcum
Cannon God Exaxxion: The title mech (and the alien's space travel) is powered by a matter-antimatter reaction. In fact the main reason the aliens wanted the Exaxxion was for its massive reserve of antimatter.
In Card Captor Sakura, the two most powerful cards the Light and the Dark are always used together.
Comes full circle in the final movie. The Nothing consumes the nameless card to become the Hope.
In Magic: The Gathering there are "white" cards and "black" cards, which are powered by light/life/order and darkness/death, respectively. But printed guides heavily stress that they do not represent "good" and "evil", they can both be used at the same time. A combination of enemy colors in general can be seen to fit this trope, as can cards that are all 5 colors.
To wit: Unmake, one of the best removal cards in type at the time of writing, is cast with three mana of either black or white and is considered a black/white card; it had a predecessor in the Apocalypse expansion called Vindicate. There was also the Sword of Light and Shadow.
Multicolor-focused sets often have White-Black as an option. For instance, in Ravnica, the white-black Guild is The Orzhov Syndicate, who are in charge of business (or, to be more precise, a religion which is also a business), and are run by ghosts.
A rundown of enemy-color pairs: White/black is harsh, controlling, and often cruel. Blue/red is high weirdness and mad insight. Black/green is decay, life coming from death. Red/white is an army, equal parts discipline and fury. Green/blue is biology, either mutation or studying new life.
The ultimate Yin Yang Bomb is probably the ability on the Legacy Weapon artifact. One mana of each color to exile any single card. note For non-Mt G players, Exile was previously called "removed from the game", i.e. deader than dead, but this was changed after a number of cards were printed that could return things from the RFTG zone. Nowadays, similar to the Graveyard (where killed creatures and cards discarded from hand go), but referenced by fewer cards and much harder to get things out of. Which got a later variation in Door to Nothingness, which is two mana of each colour to kick a player out of the game entirely...but each individual Door will only work once.
Progenitus could also be considered as one, as it takes two of each color mana to summon but is also one of the most powerful creatures in the game.
More generally, gold-colored cards that require more than one type of mana to cast and creatures with abilities that require a color besides their own to use have been a staple of the game for quite a while, and are almost always designed to be stronger than equivalent single-color cards of the same cost, since they require you be able to produce multiple colors of mana and thus be more open to resource deprivation. Even before that, players themselves were encouraged to field more than one color to cover the weaknesses built into each one (including spells which can severely handicap a person whose cards are all of one color).
The Yu-Gi-Oh! card game plays with this, the main example being the now infamous "Chaos" archetype (which has been banned for years but radically shifted the metagame), consisting of three monsters summoned by removing from play a Dark-Attribute monster and a Light-Attribute monster. One of these, Chaos Sorcerer, provides the page picture. There are plenty of cards that act similarly, but the most prominent example is the appropriately named "Light and Darkness Dragon", a monster that has two attributes (Light and Dark, obviously) simultaneously. That and, well, just look at it◊
Elixir has one hand that heals and one hand that kills for his powers. More specifically, he has golden skin (healing ability) with just a black stain (hurting ability). The black stain can move from one spot to another, and doesn't hurt him. Sometimes, the black stain enlarges so much that the entirety of Elixir's body turns black, and may the Maker help whoever pissed him off enough to make that happen. It is true, however, that at least in one occasion he (involuntarily) used both his hands (one golden and one black) on the same person, with... unusual results.
The 2099 continuity had Xian, who also had the one hand heals/one hand destroys schtick.
Magik/Illyana Rasputina tried to be an example (we'll see if she's still trying since coming back to life), practicing both dark and light magic. Sometimes she'd combine the two, like when she attempted to heal someone (light magic) while sitting backwards in a pentagram (dark magic). She kept falling to the dark side, though.
During the Princes of Darkness arc of Justice Society of America (back when it was simply JSA), Dr. Fate goes through an arc wherein his powers have been stolen by the Big Bad, Mordru. He finally comes to the realization that he is both Chaos and Order, and so he can defeat Mordru, who is only a Lord of Chaos.
On some occasions, Hal Jordan has picked up Sinestro's yellow ring and used it in combat. The last time he tried during the Sinestro Corps War, Hal put on all the rings that his hands could hold and all-out attacked Sinestro with them. Sinestro just shrugged, took control of all that yellow light and sent back to him. Hal apparently can control fear a bit due to his stint as Parallax's host, but Sinestro has much more experience and control. It's not so much controlling fear as it's the wearer's ability to instill fear in others; hence why Batman was among the first to be offered a ring, his whole persona is based on instilling fear into his enemies. Hal has the same potential, both because many still fear what he will do if he ever wants to regain the power he held as Parallax and also because of the fear he spread while gaining said power.
He has also briefly wielded a green and blue together as well that; while they're not opposed to each other, they hold different objectives and their bearers have different methods.
Played straighter in Blackest Night, which sees Guy Gardner become both a Red Lantern and a Green Lantern at the same time. Since they were fighting superpowered zombies that could only be destroyed by both a Green Lantern and a non-green Lantern, this is very handy. What made that a Yin-Yang Bomb is that Green Lanterns Hal Jordan and Laira had both ended up as Red Lanterns, and had stopped being Green for the duration. Seeing how one is fueled by willpower and self-control, and the other is powered by losing self-control, operating both is probably a bit tricky.
Played still straighter in War of the Green Lanterns, in which Guy Gardner manages to wield a red ring and a violet ring together, the two colors at the farthest extremes of the Emotional Spectrum. They represent rage and love, making this the most strongly opposing ring pairing.
At one point, a green and yellow ring chose Mother Mercy at the same time, but she quickly decided to reject the yellow ring.
Pride High has Lightspot, a faculty member who controls both light and darkness. The darkness power allows him to teleport by traveling through the shadow dimension, which has been hinted to be a very evil place.
From the short-lived Razorline imprint of Marvel Comics, we got Saint Sinner, who wound up possessed by both a demon named Runesmith and an angel named Regina. He could "evolve" things with one hand and devolve them with another.
In Black Moon Chronicles, the thief Pile-ou-Face is dual-wielding two intelligent swords, one good and one evil. A prequel comic shows how he came to possess them, after walking on an evil warrior and a paladin fighting with said swords, and scoring a Mutual Kill (to their weapons' frustration). Both swords then urged Pile-ou-Face to pick up one and destroy the other. Instead he grabbed both, ignoring their recriminations.
Mace Windu's Vaapad form, detailed in Shatterpoint, definitely counts; he refined the lesser lightsaber form of Juyo into a devastatingly effective technique by allowing himself to enjoy the fight, which channels the Jedi's aggression but also skirts dangerously with the Dark Side. He was the only Jedi to master Vapaad without succumbing to the Dark Side (although in fairness Depa mastered Vapaad and then fell to the Dark Side from something else).
Knights of the Old Republic features a good-aligned Force user with a criminal past who's living in a self-imposed exile from the Jedi Order and draws equally on the Light and Dark sides to accomplish his goals.
In the recent novel tie-in to The Old Republic, Revan, when Revan recovers all of his memories and thus his full knowledge of the Dark Side, his most powerful Force technique is to simply gather up as much of both as he can hold... and smash them together. it's enough to knock back an enemy who is fighting with the Dark Side energy of an entire world's population
It takes a while to get there, but this trope becomes the entire point of the end of the trilogy. The prophecied Hero of the Ages destined to combine the powers of Ruin and Preservation could be male or female based on the ancient grammar. It turns out he's a eunuch instead.
In addition, this trope is the secret of the immortality of the Big Bad of the first book. The first book reveals two magic systems: Allomancy, which works by ingesting various metals and consuming them to get temporary power-ups, and Feruchemy, which works by saving up a person's own attributes like strength or health in metals and consuming them later in an Equivalent Exchange. Both are Inherent Gifts. Allomancers are generally more dominant in society and more oriented towards combat, but we're told that Feruchemy has its own strengths and weaknesses and can do things that Allomancy can't. In the end, we learn that if someone is both an Allomancer and Feruchemist, they can store up their own youth and health in metal and consume it over time with much greater efficiency, making them effectively immortal.
Defied in The Bible. As 2 Corinthians 6: 14 puts it, "For what do righteousness and wickedness have in common? Or what fellowship can light have with darkness?". It's an analogy referring to physical light and darkness, not figurative. It's saying that righteousness and wickedness are as alike as light and darkness (meaning mutually exclusive), not that righteousness = light and evil = dark.
In The Dresden Files, once Harry gains mastery over ice magic, coupled with his existing mastery of fire magic, he sometimes uses both, one after the other, to cause his opponents to explode into pieces.
The Chronicles of Thomas Covenant: Thomas Covenant becomes one of these in the second trilogy finale White Gold Wielder when he attempts to burn the venom out of himself by walking in to the banefire but instead fuses with it to make himself an alloy of wild magic and venom, restoring his mastery over the power.
Most of the truly important mages of either faction in the Saga of Recluce are gray mages, capable of using both order and chaos magic. These tend to be the most powerful mages in the world in whatever time they happen to live.
The Wheel of Time features Mutually-Exclusive Magic of the sort where men and women get their powers from different sources, produce even identical magical effects in completely different ways (most explicitly mentioned when certain people learn how to Travel), and even use opposite techniques to just access magic...but the most powerful magical feats, like the cleansing of saidin or the Bowl of the Winds, require the use of female and male magic together. The symbol for the union of male and female magic is itself a simplified yin yang. In the climax of the series, Rand uses both the One Power (good) and the True Power (evil) to seal the Dark One's prison much more effectively than Lews Therin did.
In book 7, Castle Death, the owner of said castle, Lord Zahda, uses a Lorestone and a Doomstone.
Lone Wolf himself can wield a holy weapon like the Sommerswerd and infernal weapons like Helshezag. (Theoritically not at the same time, but that doesn't stop some players to have houseruled Dual Wielding.)
Common theme in the Robin HoodRobin of Sherwood series. Robin and his sword Albion are specifically said by Herne the Hunter to have the "power of light and darkness".
One episode of Charmed had Phoebe and Paige traveling to a Mirror Universe and fighting their Evil Twins. The battle between the two Paiges causes explosions due to their opposing powers colliding. Later, they team up with their evil twins and form "The Power of Four" by combining their powers to devastating effect.
In the Star Trek universe, most starships derive the vast energy needed to travel faster than light and to unleash devastating weaponry by harnessing matter and antimatter. The former method requires that streams of both matter and antimatter be annihilated in a controlled manner by passing them through dilithium crystals. The latter method packs matter and antimatter into projectile torpedoes and forces them together to make devastating explosions in the multi-megaton range.
Henry Mills in Once Upon a Time seems to be heading for this. The kid's got one hell of a Tangled Family Tree; his paternal grandfather is The Dark One, Rumplestilskin, his paternal great-grandfather is Peter Pan, who is a terrifying sociopath in this timeline, his maternal grandparents are Snow White and Prince Charming. His adopted mother is Snow White's stepmother, and her mother was even worse. His birth mother is explictly called a prophesied "Savior". The kid has just hit his teens, but survived the same sleeping curse that hit grandma Snow, used it to deliver an artifact to Sleeping Beauty via dreams, used the heart-ripper spell his adopted mama is fond of (though he used it on himself), and has shown to have some of the best and worst traits of all of the above kinfolk. Some fans just joke he ought to be handed a keyblade and be done with it.
Myths & Religion
In the religion of Voodoo, a bokor is someone who "serves the loa with both hands", meaning they combine light and dark magic.
The Shadow Sun Ninja Prestige Class in Dungeons & Dragons. The requirement of the class is a good alignement, despite most of the powers being rather dark in nature, or mixing light/darkness (and hot/cold) for both healing and hurting. This is probably a necessity to not give in too much to the dark side, a definite risk with the peak abilities of the class.
Major NPC Vaslov Jakoba in Ironclaw can cast both black and white magic, a source of much consternation in the Church. Mechanically, there's nothing preventing player characters from doing the same.
In Super Robot Wars K, El Dora V gains a finishing move which is a visual shoutout to GaoGaiGar's Hell and Heaven. Its name? "El Inferno y Cielo". Regrettably, El Dora Soul does not retain it.
In Terraria, you can explore the blighted, dark lands of The Corruption and the joyful, light-aligned Hallow and find respectively Dark and Light Souls, which are used to make or upgrade dark or light-themed equipment. Fuse them with two particular (and rather rare) items and you obtain the Dao of Pow, an Epic Flail which is essentially a huge spiked Yin Yang on a chain capable of massive damage and confusing any enemy it touches.
A variation of this is one of six elements in Skies of Arcadia: the powers that are bestowed by the Silver Moon are that of life and death. Silver spells either revive fallen comrades or cause instant death. This also applies to the Silvites, the old peoples who once lived during the Old World under the Silver Moon: instead of having just one crystal, all of them hold a fragment of the Silver Moon Crystal in their bodies, and removing them, as Ramirez tries to demonstrate on Fina in-game, kills the host.
Final Fantasy IV. The main prophecy speaks of "light and darkness cast aloft". Cecil is pure light at the end of the game, but it does take both him and his brother Golbez, still in the dark side, to deal with the Big Bad in the end.
Palom and Porom's Twincast works off this concept, as do a couple of Bands in the sequel, Final Fantasy IV: The After Years: Rosa and Rydia's "Holy Burst", Cecil and Golbez's "Ultima Spark", and Palom, Porom, Rosa, Leonora, and Rydia's "Infinity".
Before this, Final Fantasy III had the Light Warriors unable to defeat the Big Bad without invoking the power of the Dark Warriors, who had previously combatted the unnatural growth of Light (The world now being in an unnatural state of Dark.)
The DS version of the game replaced the Mystic Knight class with the functionally identical Dark Knight class. Allowing a Light Warrior to fight with Dark power.
An interesting superboss example comes into play in Final Fantasy XII, with the Hell Wyrm boss. Despite it being called the King of Hell in the bestiary, it's strongest attack is Holy elemental. This is also considering this boss has access to darker abilities, such as Darkga.
The Title Character of Bayonetta is born of both a Lumen Sage who turns out to be Father Balder and a Umbra Witch
Most main characters in the Persona (or Shin Megami Tensei) series can wield both Light and Dark magic. Curiously, they are both insta-kill magics instead of healing (which most often shows up on Wind or Water Personas).
Shin Megami Tensei: Devil Children: Book of Light and Book of Dark (DemiKids in the states) was all about this. The game was One Game for the Price of Two, one "dark" version and one "light" version, which followed one story from two viewpoints, Jin and Akira, also light and dark, respectively. The final boss was in two parts, also one light and one dark. Akira fought the light half, Jin fought the dark half, and the storyline had it so that both were fighting at the same time.
Furthersome, Naoto from Persona 4 was practically a pure incarnation of this. Specially considering how in the Persona series it's hard to hold both Light and Dark attacks simultaneously.
The strongest personification of this trope in the Persona series is Armageddon, the strongest attack in the games doing 9999 non type unavoidable damage to everything or an automatic victory. It is made by combining the opposing aspects of Satan and Lucifer.
In the MMO Shin Megami Tensei Imagine, they are no longer light and dark, but Death and Expel. In an odd variation, player characters cannot use death or expel themselves. The closest, however, is that they can use both white magic (healing/buffing skills) and dark magic (damage based/debuffing skills)in unison, and each one is a separate class, so you could have a person who can both debuff and buff, or in closer symbols for light and dark, revive all party members at full health and then blow up the enemies with a blast of destructive energy (or combinations thereof). Any character who masters two skills that correspond with each other could be considered a non-alignment yin-yang bomb, if you master counter/dodge and a melee based skill you have the distinct possibility of becoming unhittable in one-on-one combat, however only in one-on-one.
During the last hit of the Eternal SessionCombination AttackLimit Break, Sora and Riku charge their Keyblades with Light and Darkness, respectively, and crash them into each other for a superpowerful attack combining the two elements. Some of Riku's sleights in Re:Chain of Memories work the same way, just with Mickey channeling the light power instead of Sora.
Xemnas uses a yin-yang bomb style of attacking in the final battle, throwing out beams of darkness and light. Xemnas even trades out his trademark black robe for a robe of black and white swirls.
Roxas' element is Light, but as a Nobody he's technically a being of nothingness. Its his two Keyblades that are a representation of the trope, the light-aligned Oathkeeper and dark-aligned Oblivion. Sora's Final Form also somewhat qualifies — it symbolizes him manifesting Roxas' power, and his mostly black outfit takes on elements of Roxas' silver and white clothes, resulting in an outfit covered in white and black swirls. Artwork for Final Form also shows it with the Oathkeeper and Oblivion like Roxas.
The Oathkeeper and the Oblivion are obvious if you know Japanese. The Oathkeeper's teeth are the kanji for "light", while the Oblivion's teeth are the kanji for "darkness". Also related is that both of Sora's best friends are each symbolized by these Keyblades; Kairi is light and is represented by the Oathkeeper (the keyblade has her lucky charm as a keychain and is obtained after he got a clue of where she was in KHII) and Riku is darkness and is represented by the Oblivion (he has used the keyblade himself and in both KH and KHII it is obtained after an encounter with him). This is supported by the fact that Riku uses the power of darkness to fight and Kairi is a Princess of Light.
If you live outside Japan and haven't looked it up, you'd never know, but in Kingdom Hearts II, Sora gets the Two Become One keyblade after defeating Roxas. The keyblade itself is covered in silver, white, and black sections, symbolizing a combination of Sora's light and darkness (and his combining with Roxas). Its special effect is called "Light and Darkness", which causes him to, when he Drives, become either Final Form, as described above, or Anti Form, a shadowy Heartless-like Sora, no matter which (usually low-cost) Drive form you actually chose. In this case, Final Form represents light, and Anti Form represents Darkness.
In Birth By Sleep, Master Xehanort preaches the virtues of this path to Terra, telling him that Eraqus's way of shutting out and killing the darkness is just as wrong as succumbing to darkness entirely, and that the right way to protect the worlds is to wield light and darkness in equal measure. He's full of crap. The possible truth of his statements notwithstanding, he's only telling Terra this so that he can get him to give into darkness. His written intention for wanting to start the Keyblade War is to forge a new world with the two powers in balance, but thus far, he's only shown alignment with Darkness, and obsession with its power.
Also in Birth By Sleep, the χ-Blade. is a Yin Yang Sword, forged by combining a heart of pure darkness with a heart of pure light.
The Shadow Breaker command in Kingdom Hearts 3D consists of two whirling slashes, the first dark element, and the second light.
Judas in Tales of Destiny 2 wields a dark dagger and a light dagger. He also is proficient at both light and dark-type spells, which really doesn't make too much sense, as his elemental affinity is technically Earth.
Shing (light) and Kunzite (dark) in Tales of Hearts have a Combination Attack called Senanreppajin - in kanji, "Flash Dark Tearing Break Sword". It's activated by synchronizing two of the best multihit elemental sword moves they have.
In the PS3 version of Tales of Vesperia, Yuri and Flynn's combination mystic arte, Bushin Soutenha, has them team up and fire a beam of intertwined darkness and light at their victims.
Richard from Tales of Graces utilizes light and dark magic as his primary elements.
Elise from Tales of Xillia combines dark based offensive magic with light based healing. She and Milla can use the aptly named Chaotic Fuser link attack, which forms a sphere of light and darkness around the victim that eventually explodes.
Jak from Jak and Daxter gains both Dark Eco and Light Eco powers over the course of the story. Though not referring to him directly, he is even told that "the two types, light and dark, when combined form great energies."
Dante, with Ebony and Ivory. He uses both demonic and human weapons. In the third game, Vergil hates humans, and exclusively uses demonic weapons, and Lady hates demons, and uses ordinary human guns. Dante's usage of both types of weapons proves to be more powerful.
In the reboot, it appears that Dante and Vergil are now half-demon/half-angels, playing this trope even straighter.
Grandia II. Good and evil were necessary to win the game, as you had a holy priestess with you and an evil demon, both in the same person, and you, being the hero, were kind of the neutral mix of both. May also apply to the Big Badwho is both the pope of the same holy religion as the priestess, and embodies the same devil entity as the demon chick — cranked up to 11, obviously
Wisp of Knights in the Nightmare has the power to switch the "Battle Phase" between Law and Chaos at will, which causes its Knights to execute different attack patterns. Also, the amount of Power Crystal (required to Break Out) dropped by enemies depletes over time, but returns to full when the Phase changes, forcing Wisp to switch between the Phases in order to keep up the assault.
In Legaia II: Duel Saga, the White Mage turns out to be a powerful sorceress, and the first special technique she masters is "Big Bang," which combines Earth, Wind, Water, and Fire, two sets of diametrically opposed elements, to create a powerful Light-elemental explosion. Her attack-line is actually "Power to Create! Power to Destroy!"
If you explore during Michaela's battle, you'll find the White Ring, Black Ring and Evil Ring. Though it may not sound like it, the Black Ring is the Yin-Yang Bomb. It's cursed just like the Evil Ring, but it's the only ring that can only be equipped to the hero, and in battle it perfectly complements the White Ring. The Evil Ring is only a Disc One Nuke.
Shining Tears has a system where you pair off in battle with a Light- or Dark- aspected partner, and your alignment is shifted to the opposite thanks to your Yin/Yang rings.
One of the weapons for a character was the "good" sword corrupted by the "evil" sword, while the other time, an immortal man brings the two swords together. Bad things happen as a result.
And in Soul Calibur IV, Talim's final weapons (she uses a pair of "elbow blades") are Soul Edge and Soul Calibur: one in each hand. Kilik's ultimate weapon is Embrace of Souls, a fusion of Soul Calibur and Soul Edge that leaves both swords inert (though still powerful), in staff form.
The item crash between a light glyph and a darkness glyph (except for Dominus) results in a literal, and very powerful, Yin Yang Bomb. It's also a Game Breaker. 4 hits of that attack and within 10 seconds you've just beaten the final boss.
Albus also uses his "Balance Cannon Shot", firing a light projectile and a dark projectile which whirl outward in a helix. It hurts.
The Xel'naga temple in StarCraft: Brood War requires both the Uraj crystal, powered by High Templar energies, and the Khalis crystal, infused with Dark Templar power, in order to activate and blow everything within a wide radius straight to Hell on the armageddon express.
This wasn't the first time, either. Tassadar channeled High and Dark Templar powers simultaneously to kill the Overmind. This is also very significant, as the last guy to combine both High Templar and Dark Templar energies was Adun. As in "En Taro Adun." There's a reason they now say "En Taro Tassadar."
The Twilight Archon in StarCraft 2 was planned to be this, a combination of a Dark Templar with a High (Light) Templar. Then the unit was replaced by the normal Archon, the only difference is that now it forms as a combination of any two Templar units regardless of their alignment. So it can be a yin-yang bomb, but doesn't have to be. The concept of the Twilight Archon being this trope was later adapted to the lore behind the Void Ray, which uses both forms of energy to power its Wave Motion Gun.
The game inverts this trope. If a character chooses moves with opposing elements in the same turn, the elemental damage cancels out, and the combined result is weaker than an equivalent turn made from moves with non-opposing elements.
The prequel plays this straight by revamping the combat system. Several EX Combos use elements opposed to each other, including Guillo's tremendously powerful light and darkness combos.
Django is of both Solarian and Lunarian blood, which lets him wield light while being extremely resistant to darkness. Nevertheless, he's bitten by a vampire his father in the second game and becomes "half" vampire. He gains enough control that he can shift between normal and vampire mode at will. This symbolism is broken in the third game where his Karma Meter decreases the more he uses his vampiric self.
Done straight in the final boss of the first game and the second to final boss of the second game where Both Django and Sabata use their respective Light and Dark guns to power the pile driver and use the Wild Bunch attack, and in the second game where Sabata uses his black hole attack to pin down a strong and agile boss so Django can use his Gun Del Sol and deal a lot of damage.
Also done straight in Boktai DS (Lunar Knights), where the entire point is for Django and Sabata to work together.
Jolee Bindo is a Grey Jedi, and uses Light and Dark powers interchangeably.
An even better example is Kylefrickin' Katarn, who can very well be considered a Gray Jedi. One of the first things he taught to his pupils was that the powers they use are neither good nor evil, it's how they use them that is important. The one apprentice who took that lesson to heart has beaten an ancient Sith Lord in single combat.
Lucas implies that fueling your Force powers with emotions (i.e. giving into the Dark Side) amplifies said emotions, making you give into them a bit more every time instead of just using them as a power source. That's what makes the Dark Side so dangerous — you WILL go mad with power, the faster the more confident you are in your 'control' over them. As for "Light" and "Dark" Froce powers; thy're more a staple of philosophy (protect vs. attack) that's more likely to employ them (and thus rely more on the Light or Dark) than specificaly a tool of either Side. It just happens that lashing out with emotion makes for a stronger attacks, hence the association.
You can use the Player Character as one in Knights of the Old Republic, though it is rather difficult to throw around light-side buffs and neutral or dark-side attack powers unless you are a Consular with maxxed Wisdom and so many force points it isn't even funny. In aversion of this trope, though, trying to rely on both makes you weaker than focusing on either, because of Side penalties, as well as powerful Side-specific items. Without them, it would be played straight.
This is a feature of an optional character in Chrono Cross, Pip/Tumalu. You can evolve him as light/light, dark/dark, or light/dark (or dark/light, works the same way). The third is often considered the best standalone. Also, to get the Good Ending, you have to pull this trope off with all three Element colour conflicts (white/black, red/blue, yellow/green; these represent light and space/gravity and hell, fire and lava/water and ice, and earth and lightning/nature and wind.
In Seiken Densetsu 3, you get two class changes per character. Similar to the Chrono Cross example above, each time you class change, you can pick either the Light or the Dark advance from your current class, leading to Light-Light, Light-Dark, Dark-Light, or Dark-Dark.
A long part of Albion's gameplay requires the player to retrieve two scrolls that together, hold the knowledge to combine magic and technology, which are polar opposites of each other. The end result is a magical seed that can absorb nuclear energy and grow into an entire jungle in just a few minutes, spurting vines strong enough to crush an otherwise indestructible supercomputer, and overgrow the mining facility it controlled.
Reimu's Yin Yang Orbs are likely the most literal example of this trope one will find.
Yukari's power over boundaries seems to be all about this. "Mesh of Light and Darkness" "Curse of Dreams and Reality", "Balance of Motion and Stillness" etc etc. So basically her power is a myriad different Yin Yang Bombs.
Dante from Dante's Inferno uses the holy cross and the infernal deathscythe.
An unintentional example is used to explain the change of setting between the third and fourth games. Gelu wields a Flaming Sword, the Armageddon's Blade, in battle against Kilgor, who wields his own magical weapon, the Sword of Frost. When the two swords strike one another, they unleash a destructive magical shockwave that almost destroys the planet.
In IV's expansion The Gathering Storm, one of the protagonists uses order and chaos magic together, and his campaign involves seeking a Yin-Yang-related artifact that will power both. Another protagonist is able to use a combination of White Magic and necromancy.
Balance-specced druids switch back and forth between solar- and lunar-based spells as one of their main mechanics. Their signature move, Starsurge, is a powerful blast formed by fusing the power of the moon and sun.
Priests have holy spells to heal and buff allies or smite enemies, but also rather evil-sounding shadow spells that can inflict extreme pain on enemies, or liquefy their very mind. They can switch between Shadow and Holy magic at will, but they technically can't use both at the same time. The proportion of "Yin" and "Yang" used so by a priest depends on the amounts of talents he or she has learned in the appropriate spell school, with a Shadow/Holy hybrid spec being the closest thing to a true Yin-Yang Bomb. (As of Wrath of the Lich King it's also possible for a multispeccing priest to have a Holy build (usually for healing) and a Shadow build (for damage) to switch between as appropriate, becoming a Yin Bomb or a Yang Bomb as the circumstances call for.)
There is also the Mage, capable of casting both Fire and Ice spells.
The origin of humanity, and the mortal world of Sanctuary in general, in Diablo.
Chrono Trigger has the Antipode dual-tech chain, created by combining Marle's ice magic and Lucca's fire magic.
MARDEK: Chapter 3 has Zach's strongest attack, "Immoral Injustice," which deals large amounts of damage to both light and dark elemental foes. The animation looks like a yin-yang. His strongest weapon is a double-bladed sword called "Yin and Yang."
In Arc Rise Fantasia, both Dynos and Cecile have an Excel Act each (Doubly Dark and Light and Dark!, respectively) that merges a mass of light and darkness together to form a huge combined beam. Apparently it's a "forbidden art".
Magicka gives you lots of opposites. While combining them yourself is impossible, you and another sorcerer can combine opposites (for example, Life and Arcane), which will generate a devastating Antipodal Blast.
In Dragon Age: Origins, you run across quite a few Darkspawn Emissaries and Blood Mages with skill in healing in addition to their darker powers, which makes them much more obnoxious to fight since they've got the Creation-school healing to let them keep throwing out the Entropy-school hexes for longer.
In Demons Souls, the greatswords Soulbrandt, which becomes stronger if its wielder's soul is demonic, and Demonbrandt, which becomes stronger if its wielder's soul is untainted, can be forged together into the Northern Regalia, a greatsword that is at its strongest if its wielder's soul is purely demonic or untainted.
Where the Hierarchy have Humongous MechaBase on Legs and the Novus have lightning fast mobility with the use of flow networks, the Masari has this as their gimmick. They can actively switch between Light and Dark at any time, albeit with a small cooldown upon switching. Light is geared for offence while Dark is geared towards defence. Managing between the two modes is key in winning battles when playing as them. A good strategy is to use Dark mode when base building and scouting, then switch to Light mode when attacking enemy bases or defending against Hierarchy Walkers.
This is played much more straight with the Balance branch, which utilizes both Light and Dark to create a very stealthy army. Special mention goes to the hero unit unlocked from this branch, Prince Zessus, a Half-Human Hybrid who can balance, combine and control the powers of Light and Dark to create immense energy no other Masari can ever harness without even losing control. He's able to do this by balancing the nobility of a Masari (inherited from his mother, Queen Altea) and the hot-bloodedness of a human (inherited from his father, a human). He does struggle to balance between the two at times however. As much as he is an overall nice guy and a noble person, he is pretty brash and tends to be a Leeroy Jenkins due to sheer hot-bloodedness.
In the Shadow Hearts series, both Will, magic that governs the soul, and Malice, magic that governs the body and primal instincts, are required to perform a successful resurrection with the Emigre Manuscript's rituals. Most of the resurrections attempted in the series failed because the users used too much or not enough of one or the other.
Taigong Wang from Warriors Orochi produce Yin Yang marks after using his C2, C3, or C5 attacks. Activating his special attacks will make them explode.
"Shadow" enemies in AdventureQuest use Darkness and Light attacks and take reduced damage from both. During the quest chain that features them players can obtain rewards that let them wield similar powers.
In Find Mii 2/Streetpass Quest 2, you can have Miis of the same or similar color shirts team up, like a team of Blue-Blue miis or a team of Blue-Light Blue miis, though miis with the same color shirt are a better team. The exception is a mii wearing a white or black shirt, they can team up with other miis of the same color shirt like White-White or Black-Black, or team up with each other despite wearing opposite colors to form a Black-White team. And while they don't have a Combo Magic command like they would if the team had the same color shirts, their Combo Attack command is the only thing that can break Shadowlight shields, whereas most other shields only require a single mii wearing the required shirt.
In the Sonic the Hedgehog fan flash movie Nazo Unleashed this trope is invoked against the eponymous Nazo in his Perfect form. After both Super Sonic and Dark Super Shadow get soundly curbstomped one after the other, Sonic (light) and Shadow (...well, shadow) use Chaos Control to merge into Hyper Shadic, who proves strong enough to challenge the "darkness" that is Nazo.
At the end of the "Storm of Souls' arc, Dominic Deegan has to fight Celesto Morgan, the Champion of Chaos and Darkness. Celesto assumes that Dominic is the appropriate counter for him — the Champion of Law and Light. But no, he is the Champion of Balance, imbued equally by Light (thanks to his brother, Gregory, a powerful White Mage), Law (through Klo Tark, a master of mind-magic), and Dark (through the Anti-Hero Necromancer, Rillian). When Celesto strikes him with an attack concentrating the pure Chaos of the Storm of Souls, Dominic absorbs it. However, it's soon revealed he needs to take it a step further by gaining the physical elements — which Leafette's Heroic Sacrifice provides him.
In Mario Brothers, the lava unnaturally erupting from Koopa's castle was the result of Koopa's dark magic and Toadstool's own magic mixing when they both fell into the lava.
Equinox's schtick in Batman: The Brave and the Bold, going as far as wearing a Yin Yang symbol on his chest. His costume was half black and half white, and he tried to maintain the balance between Chaos and Order (naturally going insane in the process).
Most of the universe is composed of Matter. Antimatter is its exact opposite. Mixing the two will invariably result in annihilation, a technical term meaning all their mass is converted to energy (particles). Keep in mind that the energy of an object is the mass multiplied by the square of the speed of light (E=mc^2). You get a very big boom. Fortunately, most of the anti-matter in the universe seems to have exploded already. For a long time, this was one of the great mysteries of physics. Theoretically, the amounts of matter and antimatter in the universe should have been equal, resulting in complete annihilation of the universe.
Thermal shock: super-heat a piece of glass or ceramic, and then quickly introduce it to extreme cold, and watch what happens. Ideally you should be wearing eye protection when you do this.