Synchronization

Two people are linked together, or a person is linked to an object, such that whatever happens to one also happens to the other. Killing or harming one of them will lead to the death or equivalent harm of both. You wouldn't think this would be desirable, but it works the other way too; see Empathic Weapon.

If enemies are linked together, and are therefore forced to protect or aid each other to ensure their own safety, this becomes a form of Chained Heat.

Identical twins on TV often experience this, as with The Corsican Brothers of literature. It's also common with the Enemy Without.

Frequently goes hand-in-hand with a Psychic Link, because Your Mind Makes It Real. Compare My Significance Sense Is Tingling. Often exploited for very interesting effect.

If the link can ensure the death of a participant but doesn't otherwise transmit pain or other feelings, then it isn't a Synchonization but a case of Can't Live Without You. Soul Jar may work either way.

Anime and Manga

 * The mecha pilots in Neon Genesis Evangelion use neural links to control their Eva units. A meter shows the "synchronization ratio" which depends on several factors including plug depth (increases synch but puts the pilot under more strain as well as risking mental contamination by the Eva) and the pilot's psychological state (being depressed lowers the synch ratio). Low synchronization means you can't pilot the mech at all (it won't even move under 20%), and higher synchronization ratios give the pilot better and better control. It's made clear that higher synchronization ratios are dangerous in various ways, but it's not completely explained. Examples:
 * On one occasion a pilot was saved by cutting the link a split second before the mecha was beheaded.
 * When his synchronization ratio exceeded 400%, Shinji dissolved into his mech and had to be reconstituted.
 * In End of Evangelion, Asuka's ratio reaches 300% and the injuries to Unit 02 are reflected on her body. Seeing that she ran out of power just as she was speared in the head and that her opponents were, for all intents and purposes, cannibalistic harpies, it isn't necessary to elaborate the result.
 * It's said that the Eva units' synch rate depends on the pilot's need to "run to mommy". Which is why the units tend to go BERSERK and start acting on their own in moments of extreme turmoil. This is also why, in the original series, Asuka's synch rating actually goes down as she comes to grips with her mommy issues, to the point where she can't even pilot anymore.
 * This happens in many mecha series, in fact, such as:
 * Fafner in the Azure: Dead Agressor takes this to an extreme.
 * G Gundam
 * GEAR Fighter Dendoh
 * UFO Robo Grendizer.
 * Likewise Tamers and their partners in Digimon Tamers, once they reached a certain level.
 * In Digimon Adventure 02, Jogress Evolution (DNA Digivolution in the dub) was achieved (three times) when two of the human characters synched emotionally with each other, merging two Adult/Champion Digimon into a single Perfect/Ultimate. At least one of the three examples came along with a synchronized heartbeat from the two characters in question.
 * Fushigi Yuugi featured twin brothers Amiboshi and Suboshi, who felt everything that happened to the other, a la The Corsican Brothers. Also, for a while, Yui was synchronized to Miaka, so that (for instance) when the latter was drowning, the former couldn't breathe.
 * At one point in Vision of Escaflowne, Van became synchronized to Escaflowne. While this allowed him to win the battle in that episode, it also resulted in him taking serious injuries, and the only way to treat them was to get Escaflowne fixed (with poor Van screaming and BSODing in pain as it's done, since fixing Escaflowne is almost like fixing any machine, only more complex and taking the mental/physical link in consideration. And considering that Escaflowne turned black when it happened, synchronization could also count as a Super-Powered Evil Side.
 * In many Yu-Gi-Oh! battles, the duelists seem to be physically weakened by the carnage going on in the card game. In many cases there is some sort of supernatural explanation for this, but it sometimes happens even when no supernatural forces are at play. There is some sort of weak implication that the realism of the holographic systems used causes a stress response in dedicated players—in one early duel, Yugi's grandfather Sugoroku actually has a heart attack when he loses. Kaiba certainly seems to believe that using his own advanced holographic systems gives him an edge, as his opponents are overwhelmed by the realism of the simulation.
 * Yami and Yuugi could arguably be this as well, what with the whole two-souls-sharing-one-body business; they're usually in tune with each other's emotions, thoughts and, shown at least once, feel each other's pain. Not to mention that it has been implied that if one of them is killed, the other will die as well.
 * The Emilys in Soukou no Strain shared a psychic connection first seen when Sara encounters the doll and names it, and the girl takes the name as her own.
 * This is also implied (but not confirmed outright) to be the case in RahXephon—in one fight, the RahXephon is impaled through the shoulder, and Ayato is seen clutching his own shoulder afterwards.
 * It's more clearly the case for the people trapped into piloting the enemy Dolems. Ayato himself doesn't quite get this through his head until it's too late, though.
 * Mai and the demons in Kanon.
 * The Stands in JoJo's Bizarre Adventure are effectively astral projections, and damaging them results in corresponding damage to its user. Additionally, in Part 3 a Stand user named Steely Dan had the power to inflict a "Corsican Brothers"-type bond on another person (for example, getting tapped in the leg with a stick led to his victim almost breaking his and having his back scratched caused the sensation of being flayed alive). He used this on Jotaro's grandfather and threatened to kill him if Jotaro didn't play along. He then proceeds to abuse him until the Stand is removed, upon which Jotaro (having written down all the abuse suffered against his person) beats the crap out of him with his own Stand. For three whole pages.
 * It is only because of this synchronization that Jotaro is able to defeat Big Bad Dio Brando;
 * An Otome's Master feels everything his or her Otome does in Mai-Otome. This was done because in addition to being a prettified bodyguard, an Otome is a Weapon of Mass Destruction. A Master whose life is on the line as well is theoretically less likely to engage in war. Likewise, a Master/Slave Unit in the same series.
 * The Slaves that members of Schwarz use follow this principle. If one is damaged, the user is injured, and if one is destroyed, the user dies. The reverse is also true, as a sniper kills a Slave user, and causes the Slave to disappear.
 * Magic Knight Rayearth
 * The Knights are synchronized with the Rune Gods they've established a contract with. If the Rune God they're riding is injured, it will wound the Knight herself—to the point that breaking the Rune God's weapon will destroy the Knight's own Escudo sword. This synchronization is particularly evident in the Alternate Continuity OAV, where Alcyone's hand was torn apart (and remained a bloody mess afterwards) when Hikaru punched through her Rune God's claws.
 * In the trial for escudo, while Hikaru and Umi face copies of their loved ones (Hikaru's puppy, Umi's parents), Fuu fights a fake version of herself, and each gets the other's wounds.
 * The whole plot of Loveless revolved around the empathic connection between "Fighters" and "Sacrifices", in which the first are capable of commanding spells while the second suffer the effects of attacks in combat. The connection between both parties is vital to ensure victory and survival.
 * Subverted in a rather cruel fashion in My-HiME:
 * One later battle.
 * Gun X Sword has Van, who his life is synched to Dann of Thursday, his Armor. He grows ill if he spends too long without riding it, and if it's seriously damaged he suffers as well. If it's destroyed, he dies. Oddly enough, although no surgery is in his backstory, Ray Lundgren has a similar occurrence --.
 * In Fullmetal Alchemist, when Ed gets severely injured, Al blanks out. It's not explained, but one can guess that it's because the seal that binds Al's soul to the armor is made from Ed's blood.
 * In the Manga/Brotherhood, it is confirmed that they are linked, and that Al's body is actually feeding off Ed's. (Which is part of the reason why he's so... short. *cue Berserk Button rage*)
 * In Shikabane Hime, the death of a contracted monk usually leads to the death of his Shikabane Hime. If she goes on living, she is considered just another Shikabane and must be executed.
 * In Higurashi no Naku Koro ni, Rika is implied to share this sort of connection with Hanyuu, though it is played for laughs rather than drama, with Rika threatening to eat spicy food whenever Hanyuu fails her. One would assume that either the connection is one-way, or Rika has a much higher pain threshold than Hanyuu.
 * In Bleach, Captain Konmamura's bankai is a giant warrior that mirrors his movements, and any damage inflicted on the bankai will in turn be inflicted on Konmamura's body. Since no one else's bankai has this problem, it makes his power in particular a double-edged sword. His typical strategy is to finish his opponent with one blow.
 * Basil Hawkins, a minor character in One Piece, makes use of voodoo dolls that somehow transfer any damage done to him to whatever unlucky sap he managed to make a voodoo doll of.
 * Yuka and Mika, twin girls in Hot Tails, share not only each other's pain, but also pleasure. Since Hot Tails is a Hentai manga, you can guess how this is applied.
 * Naruto
 * Hidan can turn himself into a living voodoo doll. Any damage he suffers would then happen to his victim. Since he is immortal, he can easily kill someone with this ability. It is, in fact, his favorite way to kill people. His tactics almost all revolve around setting up this technique and then "killing" himself.
 * Yamanaka Ino could do something similar with her clan's Valentine jutsu, which allows a shinobi to invade the mind of another, at the expense of leaving the practitioner's body comatose and suffering damage equal to the possessed body. Her teammate Nara Shikamaru's family has a similar jutsu; one stretches one's shadow out to lock on an opponent, making the practitioner and the victim move identically. The latter has shown itself to be less impractical, and has been used in combat a few times over the series. It helps that Shikamaru is a Teen Genius and very respectable strategist.
 * They later show advanced versions of this technique, which don't have this disadvantage, but are harder to perform.
 * A limited variant of this applies to Shadow Clones. If said clone is gathering nature energy and overloads, thus turning into stone, the creator will suffer the same fate.
 * In Black God, a Tera guardian and a human can form a pact to sync up their "Tera", granting the guardian great strength, though this can be physically tiring to the human partner.
 * Played for laughs in Sonic the Hedgehog The Movie, in which Sonic and Hyper Metal Sonic achieve synchronization while fighting each other. Cue a scene where Sonic suddenly grabs his head in pain while grinning mischievously. Hyper Metal Sonic just got a peek up Sara's skirt and got kicked in the head for it.
 * In Eureka Seven, Coralians are linked to the Scab Coral. If the Scab is being attacked, all Coralians will feel the pain at the same time, hinting that Eureka will die if the Scab Coral is dead. It becomes an important reason for the protagonists to stop Dewey's plan to annihilate Scab Coral. This synchronization caused Renton, while in the movie ending,
 * In Ronin Warriors (Yoroiden Samurai Troopers), the first Dynasty Warrior that Ryo fights causes this effect between himself and Mia (Nasuti) and Yulie (Jun), so if he gets injured, they suffer all of the pain. Ryo eventually defeats him by using his armor's power. (Though how this didn't kill them is not explained.)
 * Piccolo and Kami in Dragon Ball. If either one dies, the dragon balls are rendered useless.
 * In The Anime of the Game for Persona 4, Personas act more like Stands than in the games. The protagonists do all their fighting through their Personas, and feel any harm that afflicts said Personas.
 * Meldy from Fairy Tail links Gray and Juvia together so she only needs to kill one to kill the other. Despite being burdened by experiencing each other's injuries, the two are too tough for Meldy to defeat, so she resorts to linking herself to them, then attempting suicide.
 * Meldy from Fairy Tail links Gray and Juvia together so she only needs to kill one to kill the other. Despite being burdened by experiencing each other's injuries, the two are too tough for Meldy to defeat, so she resorts to linking herself to them, then attempting suicide.

Comic Books

 * Old Lace, the psychic velociraptor, in Runaways, with her owner Gertrude.
 * The 1970's DC Comics character Kobra had this problem; he and his twin brother suffered the injuries the other one got, so his brother was an enemy he could not kill. Kobra eventually got around this with some Applied Phlebotinum to break the psychic link.
 * In Transformers: Last Stand of the Wreckers, Topspin and Twin Twist have "vicarious perception" (basically a fancy name for this trope).
 * In the original Marvel Comics Transformers series, Megatron and Ratchet ended up fused together after a freak accident. They were separated, but Ratchet wound up seeing (and feeling) what Megatron did.

Films -- Live-Action

 * In the first two Heisei era Gamera movies, a bead salvaged from ancient Mu psychically bonds a girl named Asagi to the title monster, resulting in her sharing proportional injuries when Gamera is wounded. This bond is broken at the end of the second movie, though in the third the enemy monster Irys gets its own psychic partner.
 * In Dragonheart, a dragon can bestow part of its heart on a person, making him immortal—so long as the dragon lives.
 * The So Bad It's Good Jackie Chan film Twin Dragons has him playing twins Separated at Birth. They can influence each other's movements... somehow. The specifics aren't ironed out, but examples include Engineer Fighter Jackie finding his fingers tapping oddly while Musician Jackie is playing the piano in concert, and Musician Jackie being able to fight off Mooks while Engineer Jackie is in a cage because Engineer Jackie starts making the correct punch and kick motions.
 * Big Bad Prince Nuada and his sister Princess Nuala suffer from this in Hellboy II: The Golden Army. This ends in.
 * Played for comedy in Stephen Chow's Royal Tramp. When Stephen finds out they're synchronized, he insists on "testing" them, taking one behind a curtain and leaving the viewer to figure out what he's doing by the reactions of the other one.
 * Cheech and Chong's The Corsican Brothers plays the classic tale for laughs.
 * In the movie Pumpkinhead, those who summon the eponymous vengeance demon feel the pain that the demon causes to its victims, and Pumpkinhead feels the pain caused to its summoner; the only way to kill Pumpkinhead, therefore, is to kill the summoner.
 * Eliott and E.T. E.T. is also able to create a similar link with plants, doing so with a houseplant; near the climax of the movie when he is believed to be dead, the plant starts to wither - but then Eliott sees it bloom again...
 * In The Carpenter, the ghostly carpenter is pained whenever his house is damaged. He's vanquished when the place is set on fire, which causes him to burst into flames.
 * In Star Wars continuity, the Tusken Raiders of Tattoine ride giant, shaggy beasts called banthas, which share a near-supernatural bond with their owners via a combination of training and affinity. A bantha is always the same gender as its rider, and when two Raiders marry, their bantha's also become mates. If a bantha's rider dies, the bantha usually dies shortly after, often becoming feral and violent before dying.

Literature

 * It's one of the major plot points of the Dragonriders of Pern books: Newly-hatched dragons bond with the candidate they feel the strongest empathic connection to ("Impression"). From then on, the pair can communicate telepathically and influence each other's emotions. If a dragon's rider dies, that dragon will immediately suicide by jumping between permanently. If a dragon dies, the shock will often kill his rider outright (or in one particular case, leave them brain dead).
 * Humans and their daemons in His Dark Materials.
 * Many examples from Mercedes Lackey's Heralds of Valdemar books, the foremost being the telepathic and empathic bond between Heralds and their Companions. Similar bonds exist between twins (those with mind-magic, anyway) and lifebonded couples.
 * The twin version appears in the novel The Bumblebee Flies Anyway.
 * Another twin version in Eldest: When Roran
 * One sub-story in American Gods features a pair of African twins, sold into slavery aged twelve and separated when they're sold on. They never see each other again in the flesh... but when the boy is bitten by a poisonous spider and loses an arm as a result, his sister's arm withers for no reason her owner's doctors can explain. The brother also appears to the sister one last time after his death.
 * Also used between the twins in Kate Forsyth's Witches of Eileannan series; Isabeau and Iseult are psychically linked, meaning that when Isabeau is being tortured, Iseult feels severe pain and passes out. It later comes in useful when Iseult goes into labor.
 * Roz Brickman from Patrick Tilley's Amtrak Wars series shares the injuries inflicted on her brother Steve.
 * In one of Tamora Pierce's Protector of the Small books, Neal forces a cloak on a man that makes him feel the pain of the people he beats after he finds bruises and broken bones all over the man's indentured servant.
 * She loves the twin version too. In Circle of Magic, Jory says that every twin in the (admittedly magic) world knows where their twin is and if they're hurt. In the Tortall Universe, when Aly is kidnapped her parents are constantly writing to her twin Alan to ask him if she's still alive, as he would know the moment of her death.
 * Damon Knight's science fiction story Rule Golden about an alien who comes to Earth and uses some Applied Phlebotinum to change all mammals on the planet so that whatever they do to something they also feel. Slap someone and you both feel the pain. Kill someone and you have a heart attack and die. Sit on a hoard of food while other starve around you and feel the pain of their hunger. The title of the story is the reverse of The Golden Rule, attributed to Jesus of Nazareth in the Biblical book of Matthew: "Do unto others as you would have them do unto you."
 * Stanislaw Lem used a similar plot point for his short story "Altruizine." Hilarity Ensues.
 * The protagonist of Charles Stross' Jennifer Morgue is "destiny entangled" with his partner... this grants them useful tricks like telepathy and sensorium sharing (definitely Fetish Fuel when one half of the partnership is a succubus...) but with the downside that they
 * A major point in Faith of the Fallen is the so called maternity spell, which causes that effect... useful for taking hostages.
 * Percy and Grover's empathy link.
 * In Bronze Dragon Codex, Tatelyn and Simle get this due to a Starjewel.
 * Septimus Heap: A sought-for art for Dragon Riders

Live-Action TV

 * In Fringe a boy becomes synchronized with a fungus. If it dies, he dies, and he senses what it senses (light, heat).
 * Farscape
 * An episode features a pair of bracelets that cause one of the wearers to feel what the other is feeling. Even injuries. This is used by the crew as insurance when dealing with Scorpius.
 * Later, a greedy doctor purposely synchronizes pairs of Moya's crew so that he can charge them a huge fee for the cure. Which of course only he knows how to make.
 * In Red Dwarf, the Justice Field reflects any criminal acts back upon the actor. If you try to light the sheets on fire, you end up lighting yourself on fire (and the sheets stay unburnt). "It's the ultimate deterrent," as Rimmer says. This situation leads to a marvelous fight in which hitting your opponent hurts you instead. The bad guy, a Simulant (think the Terminator with really bad BO) is too dumb to realize what's going on. So he tries shooting Lister and throwing a knife at him. Twice. Lister even draws a target on his groin and presents it for kicking. Then he hands the Simulant a few giant vases, to be smashed over Lister's head. The Simulant then tries to strangle Lister, thus strangling himself. With the bad guy defeated, The Cat turns up, armed with a spade...
 * In the dramatization of Longitude, one of the cranks seen by the Longitude Committee suggests using a "powder of sympathy" to link dogs in this way. The idea is that a dog on land would be injured at noon each day, causing its counterpart aboard a distant ship to bark in pain. In this way the sailors could keep track of Greenwich Mean Time. Not surprisingly the Committee didn't think much of the idea—Untruth in Television, you might say. (Of course, they didn't think much of John Harrison's practical methods either.)
 * In one episode of Star Trek: The Next Generation, Capt. Picard and Dr. Crusher are (slowly) synchronised by alien technology implanted into their brains as a prisoner control device (they experience debilitating pain and nausea if separated too far). Slowly because the implants gradually get more effective as they "learn" their hosts' brains. Mostly though, the whole episode is an excuse to get them to talk to each other about their feelings.
 * Used in one episode of Legend of the Seeker.
 * In The Vampire Diaries
 * Multiple artifacts in Warehouse 13 have this kind of effect.
 * The Globbor monster on Mighty Morphin Power Rangers had an energy link to Ninjor, making the Rangers fearful to attack him.

Puppet Shows

 * The Dark Crystal: The urRu and Skeksis are connected, so that when a member of one race dies so does one of the other (so their numbers are always the same), and when a Skeksis is injured one of the urRu suddenly sustains an identical injury. The nature of this connection is a subtle clue to the film's ending.

Tabletop Games

 * Warhammer 40,000
 * Titan Princeps suffer physical pain when their titans suffer battle damage. Not merely the Princeps, but the key members of his crew also suffer such pain, in varying degrees, particularly the Moderati.
 * In a similar manner to the above, Tau Battlesuit pilots suffer from this, including 'battlesuit psychosis', in which they are so linked to the suit, so used to it that they cannot function without the suit's sensors as filters for their own senses, and if the suit is damaged, (say by the loss of an arm), the pilot will find themselves unable to use that arm until the suit is repaired. The Tau also have a 'bonding ceremony' which can provide a kind of synchronicity between its members, to the extent that the death of a member of the bonded team can drive the survivors insane (by Tau standards), although the more survivors there are, the less trauma, apparently.
 * Dungeons & Dragons
 * Some powers can induce synchronization between two living beings, like the psionic science "Fate Link" or the "Death Link" spell. Both creatures will lose Hit Points if one is wounded, but death isn't necessarily automatic for both if one is reduced to zero. Can be useful as a bargaining chip (to dissuade an enemy from getting violent), or to wound a creature otherwise immune to many forms of attack. Can also work in reverse, rejuvenating both creatures by using healing magic on just one.
 * Depending on the interpretation, this opens an exploit when by constructing a triangle of "Fate Links", any damage to one participant results in infinite damage to all of them. There is at least one build that deliberately inflicts this on themselves. Combined with the "Masochism" feat and "Delay Death" spell, the character gets a single round with an infinite bonus to all rolls.
 * A minor synchronization also exists between a magic-user and his familiar, although the exact effects vary from one Edition to the other. Same applies to some other Bond Creatures, such as homunculus. Though it doesn't concern mere wounds, a wizard will suffer badly if his familiar or homunculus is killed, and a homunculus never survives his creator's demise.
 * Likewise, the Shaman takes damage when her spirit companion is destroyed, and the companion vanishes if she falls unconscious.
 * Some races in D&D have their Life Force linked to an item, plant or place, mostly nature spirits or descendants of some such, with direct inspiration from folklore or mythology. Nereids carry a shawl that contains their soul, and someone seizing it and threatening to destroy it can control the creature. Dryads and Hamadryads are linked to a single tree in the forest. Bamboo and River Spirit Folks, from the Oriental Adventures, are synchronized with a specific bamboo grove or river, respectively. Damaging a linked tree or place will weaken or wound those spirits, and destroying it will kill them.
 * Some elven priests have spell "Hamatree" that permanently invests one's lifeforce in an old oak - much like a dryad or hamadryad. The caster then can transfer damage to the bond tree's Hit Points (the tree got a lot of them and regenerates slowly, but reliably), teleport to the tree from any other healthy tree at will and speak with it at will. Drawbacks are that fire damage is transfered 2:1 and damage to the tree from any other source is automatically copied to the caster. The tree dies with the caster, and the caster will save-or-die upon the tree's death.
 * There are several spells that reversibly remove the subject's heart (which still beats) to a prepared container and (usually) replace it with a stone duplicate, which gives the subject some advantages and reserves a fresh piece of body for resurrection, but also means the risk of being killed instantly and without any warning, at any range, via destruction of the heart.
 * 4th Edition actually has an entire race built on this—the Dvati, a species consisting completely of identical twins, with "souls that burn so brightly one body is not enough to contain them". They don't share damage exactly, but are played as one character (with their HP divided in half between two bodies) and have some limitations on what both can do in one round. If one Dvati twin dies, the other takes unhealable ability damage from the shock and loss of the other half of their soul until either they die too or the other twin is raised/resurrected.
 * In the Ravenloft setting, Dr. Mordenheim and Adam are linked in such a way that they have to be killed at the same time, else they regenerate from any injury.
 * The Twin Souls merit in Mage: The Ascension lets you combine spheres and boost Quintessence when in contact, and keep track of each other relatively easily when not. On the down side, if one dies, the other must make a tough Willpower roll to survive the psychic shock.

Video Games

 * In the Mega Man Battle Network series, Lan and his Net Navi MegaMan.EXE often synchronize with each other.
 * A particularly extreme example occurs at the end of Battle Network 3, when Lan
 * From Battle Network 4 onwards, it becomes a gameplay effect: Damaging enemies just before they attack causes this, which gives you double damage on your next attack.
 * It's even more extreme in the manga, where the consciousness of the Operator and Navi actually merge. In addition, any damage done to the navi in the virtual world is transferred to the operator, be it a small cut, a dismembered limb, or full-out deletion/death. The concept in general is taken a bit further than usual, with Full Synchro being explained as eliminating the delay between a command being issued and it's execution, which is absolutely necessary to the most powerful of the autonomously operating Net Navis, who don't have such a delay in the first place.
 * In Star Wars: Knights of the Old Republic 2, the Jedi Exile is synchronized with one of the characters in their party, causing any status buffs either one receives to affect the other. There is talk of severe consequences to the Exile should the character die, but the game wisely doesn't inflict damage on both characters in combat if one is hit. It is, in fact, a plot point that you do not take damage from the other if it is in the heat of battle.
 * The bond there was based off a similar (but weaker) bond between Bastila and the player in the first game. That one was not strong enough to serve as a physical conduit, but only served as a plot point to justify a series of Heel Face Revolving Door rotations.
 * In Final Fantasy IV, Cecil and Kain murder Rydia's mother by killing her conjured dragon. In the DS remake, Pochika/Whyt is a substitute for the party Summoner, and is thus bound by the same rules. If it dies, she does as well.
 * Which makes for a Fridge Logic moment when you have Rydia Summon Magic a monster that attacks by blowing itself up.
 * Bloodline Champions has the Psychopomp's Soul Link ability, damaging and causing an effect on enemies hit to also cause their allies hit by the effect to take an additional 20% of the damage they personally take while the effect is on. The Guardian had an ability called Thrust, which would cause an ally you hit to have part of the damage they take while affected be done to the Guardian, with the damage the Guardian taking from that being also reduced.
 * In The World Ends With You the characters have to synchronize themselves with another player, if they want to win the game. This, however, comes with all of the above explained consequences. This works the same way during battles, as enemies and Players alike share life force on the top and bottom screens. If Neku defeats an enemy on the bottom screen, it is also erased on his partner's side.
 * In addition, eating food raises the Sync Ratio of your partner (somehow?). The lower the Sync Ratio, the less effective they are in battle, with a 0% sync resulting in insultingly bad performance. Yeah, the more they like the food, the better; they gives audio cues as to how each food item appeals to them. But this is a temporary effect, fading within a few battles; for increased sync that lasts longer, Neku and partner should really wear the same brands.
 * The final boss in Planescape: Torment is . Both it and the protagonist are immortal and one way to end the game is to
 * Mechwarrior pilots wear neurohelmets which link their consciousness to their Mechs and lets the Mech use the pilot's sense of balance to keep upright. It also functions as a security system, to prevent other pilots from stealing a Mech. The link is explicitly and wisely not two-way in most occasions, but "neurofeedback" can result from ammunition explosions. It is described as extremely painful, and frequently renders the pilot unconscious from sensory overload. If it happens too much in a short time, it can even kill the pilot, similar to how a strong shock across the heart can stop it.
 * In Metroid Fusion, it's stated that when Samus puts on her semi-organic Power Suit, it bonds to her nervous system, which is why no one can remove her suit but her; this leads to problems when she needs emergency medical attention, like when the X Parasite infect her through the suit. It's also why when she is infused with Metroid DNA to save her life, her suit grows a layer of Metroid flesh; it couldn't be removed entirely and she had to be treated with it on. Fan speculation states her relationship with it may be more symbiotic: it's the reason why she feels pain when her suit is hit, why she regains health if she picks up energy, and why she can go hours or days on end non-stop without rest or eating (she siphons energy from her suit).
 * Pokémon has the ability "Synchronize", which works in that if the Pokémon with that ability gets poisoned, paralyzed, or burned, then their opponent gets poisoned/paralyzed/burned, too.
 * There is also the move Destiny Bond. The Pokemon that makes the move places a bond of fainting on the opponent that K.O.'s it on the next turn which makes the K.O.er faint.
 * In Tsukihime, the maids Hisui and Kohaku are known as Synchronizers. It is known they can give other people energy and maintain their sanity  Also Shiki and SHIKI in Hisui's route connect during dreams and feel the pain/take the damage of the other. Unfortunately, SHIKI has something of a Healing Factor...
 * in Fate/stay night.
 * In Shin Megami Tensei I, the Hero is linked to . After the End, he remains linked to her reincarnation, and whenever there is a full moon he takes damage from.
 * Rynn and Arokh make a "Bond" in Drakan: Order of the Flame that effectively synchs their hit points with each other (in-game it is described as "merging their existences into one"). It doesn't seem like a fair trade at first, since Rynn is a mere mortal and Arokh is a nigh-unstoppable dragon but apparently, only killing Arokh kills Rynns as well, not vice versa. This is also supported by the fact that the human who originally made a Bond with Arokh (a certain Heron) has been dead for centuries when Rynn meets Arokh.
 * It used to be in Super Mario World that Yoshi eating a powerup would affect Mario. This is not longer the case in New Super Mario Bros. Wii, but in that game, using a held powerup affects all players, unlike powerups found in levels.
 * In Warcraft 3 tauren spirit guides could use spirit link to bind several people together. However it was used defensively, because instead of the damage being duplicated it was split between them, causing an enemy's attacks to be dispersed though a larger group of people and thus easier to keep people alive through.
 * In World of Warcraft it is used both offensively and defensively:
 * A Holy Paladin's spell "Beacon of Light" causes any heals they cast to also heal the one carrying the beacon. Protection paladins have an ability that takes part of damage dealt to raid members and transfers it to him.
 * San'layn encountered in the Icecrown Citadel have the ability to link two members of the raid, with any damage dealt to one also being dealt to another.
 * The Wracked Slaves in Zangarmarsh frequently attack in pairs, with a visible "tether" between the two of them. Any damage inflicted on one is divided evenly between both, resulting in the pair lasting twice as long (but also resulting in both dying at the same time).
 * The Interactive Fiction game Savoir-Faire has a magic system based around linking objects so that what happens to one happens to the other as well. People can also be linked, and it's said that members of feuding aristocratic families are often linked to one another as a sort of hostage thing, so that one family can't harm members of the other without also hurting their own relatives.
 * BioShock (series) 2 reveals that the first series of Big Daddy would fall into a coma and die if their Little Sister were killed. Or if they just wandered off too far.
 * In Kid Icarus Uprising, Dark Pit is connected to Pit, since he's really the physical manifestation of Pit's "dark side" (not that Dark Pit is actually dark - more neutral than anything). When Pit for three years, Dark Pit is unconscious the whole time - so he realizes that he's connected to Pit, much as he dislikes it.
 * In Kid Icarus Uprising, Dark Pit is connected to Pit, since he's really the physical manifestation of Pit's "dark side" (not that Dark Pit is actually dark - more neutral than anything). When Pit for three years, Dark Pit is unconscious the whole time - so he realizes that he's connected to Pit, much as he dislikes it.

Web Comics

 * Two of the main characters of Darken, Gort and Mink, share such a bond. Mink is rather annoyed by it, due to Gort's tendency to charge into trouble with his sword raised, often commenting "He's going to get us BOTH killed!" Gort was the one who agreed to the creation of this link, in order to save Mink's life, and nobody's entirely sure why (since he's basically a supervillain), though allegations of a romantic relationship between the two HAVE been made.
 * Killroy & Tina are linked thus. They also share super powers as long as they're within five kilometers of each other.
 * Girl Genius: Gil and Tarvek wind up synchronized by a "Si Vales Valeo" procedure intended to save Tarvek's life.
 * Happens both ways in The Dragon Doctors. When Kili's soul is plunged into darkness and imminent death by an evil spirit, her empathically-bonded lover Greg nearly dies along with her, but is able to use his connection to save her and himself.

Web Original

 * Whateley Universe example: Tennyo and Generator are now psychically linked so when one get really badly hurt, the other feels it. Not really helpful so far.
 * Trinton Chronicles Aurora is linked to her duplicates the same way by feeling their pain and their death, kind of screws her up mentally.

Western Animation
"Stewie: I'm Tomax and this is Xamot. We're twins who can feel each other's pain. Brian: (after Stewie steps on his foot) Ow! Stewie: No Brian, I'm supposed to say 'ow!' You ruin everything! Brian: (after punching Stewie) Ow."
 * Demona and Macbeth from Gargoyles were synchronized by a magic spell that caused them to feel each other's pain whenever they are in close proximity, implied to be intended to keep Demona loyal as Macbeth's protector. Since they came to hate each other's guts, there has been a series of violent confrontations where they're essentially beating themselves up by fighting the other. This is the source of their immortality compact, handed out by the mystical tricksters that granted the wish; they cannot die unless one kills the other, at which point both will. It's seemingly Macbeth's life goal to do just that.
 * Tomax and Xamot from G.I. Joe are an example of synchronized twins. It seems to end at pain (and possibly pleasure), though—Xamot has a facial scar that Tomax lacks.
 * Referenced on Family Guy when Stewie and Brian pretend to be them.


 * In the fourth season of Ben 10, Ben gains an alien form called Ditto that can split into multiple copies of itself, with the catch being that they all think autonomously, albeit with the same personality, but are synchronized so that pain inflicted on one Ditto is felt by the others.
 * Ben 10 Alien Force gives us Echo Echo, essentially Ditto with a sonic scream. He doesn't have Synchronization, with the duplicates acting more like kage bunshin no jutsu, as seen in "Singlehanded".
 * Captain Planet and the Planeteers: Captain Planet is synchronized with Gaia (whenever she's hurt, his powers fail). Gaia is linked to the Planeteers (if they turn on her or abandon her, she loses her powers). And of course the whole team is connected through Ma-Ti, and Gaia is synchronized with the whole Earth... oh... as they say on Avatar: The Last Airbender, everything is connected. Example: When Gaia temporarily lost her powers, Captain Planet was depowered and couldn't dissipate, meaning that the Planeteers rings didn't work.
 * The subject of the Men in Black episode "The Psychic Link Syndrome".
 * Wakfu: Amalia (like any Sadida) is somewhat synchronized with her animated puppet, which she uses to fight (though this doesn't transmit wounds; in fact she can use it as a shield). This turns out quite problematic in episode 7, where Amalia gets poisoned and thus delirious, and the puppet becomes a nasty, evil little pest.
 * In one episode of Futurama, Bender feels what Leela feels due to an emotion chip installed.
 * In Kung Fu Panda Legends of Awesomeness, "Bad Po" has this happen between Po and his Evil Twin, used for genuinely hilarious effect.
 * In Transformers Prime, Starscream makes an army of clones, only to find out that he feels whatever pain they experience. He discovers this when Bulkhead beats one to death.

Real Life

 * Quantum entanglement: Albert Einstein famously derided the concept as "spooky action at a distance". But it turned out that this phenomenon really exists.
 * Some animals form symbiotic mutualisms with other animals. A well known example is the clownfish and the sea anemone. The clownfish is able to hide from predators in the anemone's tendrils, while in return, it eats any dangerous parasites that might endanger the anemone.