Alice is pregnant. She and Bob are waiting for a healthy child, but they get two for the price of one. Normally this might be considered an extra gift, but in this world/time/place, Alice and Bob's twins are considered a bad omen/bad luck by popular beliefs, religion, or simply superstition. The parents' reaction may vary between reject the child(ren) or love them both but forced to abandon one of them. The aftermath probably will be choose one of the twins and dispose of the other, either by killing the child or leaving the child to his/her own luck. In the latter, sometimes the child could be adopted by other people or just simply became a Wild Child or Street Urchin. But there's always a third option, and Alice and Bob may leave their lands to another one where they can live together as a family... or the twins can escape by themselves.
In cases where the twins had been Separated at Birth and the "disposed" one lives, eventually they will reunite since You Can't Fight Fate, most of the time not knowing the existence of the other or simply considering his/her sibling as dead since always blood is Thicker Than Water and the connection between the twins is stronger. When they finally reunite, the reactions also may vary: this could be a Big Damn Reunion, but the other extreme could be an Abel And Cain scenario. The unlucky one could convert into the Evil Twin or they could be Polar Opposite Twins. Truth in Television that twins may be played by real actors/actresses that are twins- but it could also be Acting for Two, it all depends.
Part of Twin Tropes and Family Twists Index. Compare Don't Split Us Up and Twin Desynch. See also Separated at Birth, in which neither twin has to die but they must be separated. Not to be confused with Cloning Blues or any tropes related. Contrast Sibling Murder where one sibling kills the other.
- In After School Nightmare, it ultimately turns out that Mashiro's gender issues are actually a pair of boy-girl twins where one of them has to die in order for the other to live.
- The story of the birth of Ram and Rem in Re:Zero refers to the belief from Japanese folklore, as their oni's people must symbolize medieval Japan and Japanese demons in particular. And so, in the people of these girls it was customary to kill the born twins, as it was considered a bad omen, and because very often the twins shared horns for two, because of what they had more weaker abilities than their other tribesmen. However, Ram is a very powerful mage, so powerful that she is considered incredibly talented, so the girls manage to survive.. And ironically, all their people are killed after 10 years.
- In the Asgard Saga of Saint Seiya (anime only), there's Syd and Bud, twins Separated at Birth, since in Asgard having twins was a bad omen. Syd was the lucky twin, being raised by his parents and having a good life, and Bud was the unlucky one, being a Street Urchin. When Polaris Hilda reunited their God Warriors, Syd and Bud finally get together, but Syd was the "official one" (God Warrior Mizar Zeta) and Bud was the "shadow one" (God Warrior Alcor Zeta). Always jealous of his twin's luck, Bud became the Evil Twin, but later he changed his ways after Syd sacrificed his life to save Bud's, while also giving him a heartbreaking speech where he explained that he had always known about Bud and was always sad and frustrated over being unable to help him.
- Fai D. Flowright from Tsubasa -RESERVoir CHRoNiCLE-. In the manga it's revealed further in the plot that Fai isn't his real name, but his dead twin's name. In his kingdom, having twins was considered a curse that would harm the country and it's people. After their parents died Fai and Yui's Uncle (the Emperor of the kingdom) told the twins that they could either kill one of them off, or live together and be unhappy, because only if the twins are unhappy could the country prosper. The twins, refusing to harm one another, were thrown into a giant pit reserved for bodies of sinners and criminals. Yui was stuck down in the pit trying to climb the walls with stocking up dead bodies to stand on, while Fai was locked in a tower rising from the middle of the pit; together, but separated. They were never given food or anything else, but their magic kept them alive and young... and miserable. Until at least a decade later when one twin dies and the other gets free, both with a help of an outside interference ... right before their kingdom is completely destroyed anyway, because, turns out, the Emperor wasn't quite right in the head.
- In Shaman King there's Hao Asakura, one of the founder of Asakura clan and one of the most powerful members of the clan; however, his desire to avenge his mother's death and his growing hatred of humans made him evil and was stopped in two lives. His second reincarnation was along his twin brother Yoh; knowing Hao was in this world again, the grandfather of the twins (Yohmei) attempt to kill both children, but since the old man really didn't want to do it, he hesitated for a second... then Hao saw the opening and escaped with the Spirit of Fire to be raised by one of his minions and various years later, joining to a new Shaman King competition after failed in his past two lives.
- In YuYu Hakusho, ice maidens only give birth to daughters unless they've been with a man. Being with a man is completely forbidden. Hina gives birth to twins, one boy and one girl (Hiei and Yukina). Because Hiei is a boy, he's forbidden to live among them and is left for dead. Also causing Separated at Birth with Yukina.
- In Finder, Ascians view twins as such a crime against nature that both are killed at birth.
- Thorgal: In "The Blue Sickness", Thorgal escapes from a desert prison and runs into a tribe of swamp-dwelling pygmies led by the twin brother of the king who imprisoned him in the first place. Their father threw one out to avoid a Succession Crisis, unfortunately he definitely chose the Jerkass to rule. Thorgal eventually leads the brother back, but instead of usurping him decides the throne's big enough for the two of them.
- Pre-Zero Hour Durlans forced twins to fight to the death in the Right of Survival as "only through death can the choice of the gods be surmised". Reep Daggle, who went on to become Legion of Super-Heroes member Chameleon Boy, won the right of survival by killing his twin but regrets it and feels remorse over his sibling's death for the rest of his life.
- Mentioned in Harry Potter and the Methods of Rationality — magical society used to kill one of any pair of identical twins at birth, because their thought processes would be so perfectly in sync that they would come across as one person in two bodies. Exactly why this was considered undesirable is not mentioned in the text, but one could extrapolate any number of reasons.
- Downplayed in Animorphs: When Yeerks twins are born, they are given the same name, but one is designated "primary" and the other "lesser." In Visser Three's case, he apparently felt threatened that his "lesser" twin might catch up to him and purposefully stymied his advancement. (You shouldn't feel too bad for him, though.)
- In The Ear, the Eye and the Arm, the main characters visit Resthaven, an idyllic City in a Bottle that preserves a traditional lifestyle — which happens to include killing the younger member of any set of twins. One villager argues that the visitors can't pick out the parts of their culture that they find objectionable and enjoy the rest, so they promptly escape with the twin that would have died.
- In The Giver, when identical twins are born, the smaller of the two is promptly Released to Elsewhere since having two identical people is at odds with their strictly ordered society. When Jonas sees his father euthanise an infant simply because it didn't match their quota for births, it drives home the scope of the Industrialized Evil that maintains their Crapsaccharine World and he immediately makes plans to escape.
- In King Solomon's Mines, this turns out to be the custom of Kukuanas. Twala, the evil king, was supposed to be killed at birth, but was saved by his mother to usurp the throne once he grew up. Umbopa, the heroes' companion, turns out to be the son of the murdered brother, come to reclaim his kingdom
- In the New Jedi Order, twin births among the Yuuzhan Vong are incredibly rare; their custom is for the twins to fight to the death once they reach adulthood, with the survivor invariably going on to greatness. Supreme Overlord Shimrra is a twin, having fulfilled the custom by killing his brother when they were seven. After learning that his nemesis Jacen Solo is a twin, Tsavong Lah becomes obsessed with capturing Jacen and his sister Jaina and forcing them to perform a Twin Sacrifice to bring divine favor to himself. After Lah dies his successor abandons the plan, figuring that twins from a species among whom such births are common probably aren't worth it, and Lah was letting his ego and personal vendetta overrule his common sense.
- Heavily implied to happen with Donald and Douglas in The Railway Series. The Fat Controller had ordered only one goods engine from Scotland (Word Of God says it was Donald) and their introductory arc had the twins worry about one being sent back to Scotland, where they would be scrapped. Thankfully averted when The Fat Controller hears of the twins' plight and decides both can stay.
- In the Secret Histories series, Eddie learns that this is the basis of a deal with an Eldritch Abomination that powers the Drood family's magic torcs and instant Powered Armor: every Drood is born with a twin, which is promptly absorbed by the Heart to create their torc. He destroys and replaces the Heart via a deal with a more friendly Eldritch Abomination who doesn't demand human sacrifice.
- In Split Heirs by Esther Friesner and Lawrence Watt-Evans, twins and other multiple births are regarded as a bad sign, so when a queen gives birth to triplets she secretly has two of the children taken away and raises the remaining one as her only son. Fate reunites the siblings when they're older and Hilarity Ensues.
- Things Fall Apart: The Igbo people of Umofia believe twins to be a divine curse, and leave both children out in the surrounding forest to die. Some of the characters question the practice, but keep their doubts to themselves. These characters wind up flocking to the Christian missionaries when they come to Umofia, who go into the forest and collect the twins to raise themselves.
- Three Dark Crowns: An Triplet version. Two must die so one ascends to be the Queen. This is a continuing cycle.
- A variant in The Village on the Lake, a Stone Age novel by Sergey Pokrovsky. When twins are born, the younger one is considered to be fathered by a spirit instead of a human father, but is only killed or banished if it is believed the spirit is an evil one. The protagonist is banished due to such a suspicion, but manages to return after presenting "evidence" his father is a benevolent oak spirit.
- Wolves of the Calla: Almost all births in the town of Calla Bryn Sturgis are twins. The titular Wolves ride out of Thunderclap roughly every twenty-three years and kidnap one of each pair of the town's children. Months later, they are returned via train, "roont", deformed into Dumb Muscle Empty Shells that painfully grow until they die young. The purpose of this operation is revealed as The Crimson King harvesting the innate material responsible for Twin Telepathy to feed it to powerful psychics to enhance their abilities he is utilizing to destroy the Dark Tower.
- Subverted in Sabrina the Teenage Witch: When Sabrina discovers she has a twin sister, Katrina, a series of trials are held to determine which of the twins is evil; Sabrina fails the trials and tradition dictates that Katrina must push her into a volcano. Katrina does so without hesitation, but it's actually a Secret Test of Character; Katrina's willingness to kill Sabrina exposes her as the Evil Twin and she gets sentenced to imprisonment in the Other Realm, while Sabrina survives the fall and is declared the good twin.
- Subverted and averted in "The Vampire Diaries": The Gemini Coven is revealed to have an merge whenever an set of twins turns 22 which is problematic for Liv & Luke in the sixth season. However, Luke found a loophole where he didn't have to go through The Merge with Liv. Instead he performed the merge with his older brother Kai, as they were both biologically 22 years old (Kai qualifies as Human Popsicle due to be stuck in a prison world for 2 decades) and shared a bloodline as brothers despite the fact that they weren't actually each other's twin. Kai emerged as the winner to everyone's chagrin. Kai would later kill his own twin sister Jo while simultaneously having an hand in Liv's death which made Luke's sacrifice basically All for Nothing.
- Grim Hollow: The Laneshi are a race of merfolk who have a cultural affinity for Necromancy and a strong case of Blue-and-Orange Morality compared to the land-dwelling races of Etharis. Their tradition dictates that when twins are born, the second-born twin is sacrificed in a necromantic rite that transforms them into a spirit-guide and bonds them with the first-born.
- Orc culture in Arcanum contains a variant of this, according to the manual. Twin births are a more common event for orcs than for humans, but triplets are seen as an unnecessary burden, as the mother, with her two breasts and arms, is seen as only being capable of raising two children at once. The solution to the problem is for the father to pick the weakest looking newborn and eat it, in celebration of his own fertility.
- In Fatal Frame II: Crimson Butterfly, Minakami Village had the ten year annual Crimson Sacrifice Ritual, where one twin had to strangle the other to death in order to keep the Hellgate beneath the village sealed. After a failure that caused the entire village's destruction, the restless ghosts of the inhabitants try and force Mio to do the same to her own twin, Mayu. Canonically she does end up doing it, which plays a role in the third game, in which her guilt makes her become a victim of the Tattoo curse. Other endings do have both twins survive (one at the cost of Mio's eyesight), one in the remake has them choose to be Together in Death after being too late to stop the Repentance, and another in the remake has Mayu kill Mio instead.
- Played with Bridget from Guilty Gear games, appearing since X2. In the series, the androgynous character was born male with a twin brother in a village where the birth of same-gender twins is considered bad luck; therefore, her family named and raised her as a girl to protect her. When Bridget grows up, she decides to prove herself as a bounty hunter to show people that she was not cursed, and succeeds in lifting the superstition - not that it helped her from developing some rather severe gender identity issues by Guilty Gear -STRIVE-. Ultimately, she concludes that, if proving herself as a man didn't do anything to make her feel better, then some part of her wants to be a girl - and so she comes out as transgender.
- When the Sonozaki twins were born in Higurashi: When They Cry this was meant to occur. Their grandmother ultimately didn't kill them, but the youngest was sent away. To make matters complicated, Shion and Mion often did Twin Switches as children. One day they did it at the wrong time and were permanently switched, due to the heir being given an "oni" tattoo on her back. Thus, the girl known as "Mion" was really born "Shion", the younger sister who wasn't killed at birth. No one besides them know of the switch.
- In Unsounded's Gefendur faith twins are bought from their parents at birth and raised with other twins, apart from society. When they come of age, the oldest is enrolled in the clergy, while the youngest is poisoned and eaten by the clergy, state officials and anyone with enough money to pay at one of two annual events.
- In various aboriginal tribes around the world, especially in Africa, having twins was not just a bad omen, they were (and still today are) considered the same human disease as malformed babies, so the children had to be expelled from the tribe or killed, depending on the culture. Sometimes the shamans or wizards of the tribe would be the ones who rejected the babies, but sometimes, the parents themselves wouldn't want them. Sadly, this happens a lot even today in Madagascar, where twin babies are usually abandoned and killed because of superstitions, and families who keep their twins may be exiled from their village.
- In 1800s Japan, it was rooted in the belief that a demon had replicated the real baby and was in the process of trying to take its place-although perhaps it was only a more socially acceptable way for impoverished families to justify getting rid of the two extra mouths to feed. There are quite a few tales of the spirits of twins left in the wilderness to die coming back as Yōkai or vengeful ghosts (yurei/onryo).
- During the Middle Ages (in Europe), there was a common superstition that multiple births meant the mother had been unfaithful to her husband. That might be a milder form of this trope. However, the superstition may have been discredited by the late 12th century, since in the French lai (poem) Le Fresne, when a knight's wife makes this accusation to another knight's wife who had twins, it is considered slanderous, and the wife who made the accusation bore twin daughters.
- Giant pandas give birth to twins in about half of pregnancies, but the mother only cares for the stronger one and lets the other one die, since she is unable to produce milk for both as she does not store fat.
- A Body Horror example is the "Vanishing Twin Syndrome" or "Fetal Resorption", in which one of the twins is condemned to death...in the mother's womb, being assimilated by the survivor twin. The most fitted (and horrible) example in fiction is Stephen King's The Dark Half.