Curious phenomenon where there's only one true descendant of the some legendary hero/villain or chosen one. Usually it's only that one person who can save the day or bring about The End of the World as We Know It. Because Destiny Says So, of course. And often it has to be the male line, or whichever keeps the last name. The fact that there's multiple branching into different last names doesn't count genetically for some bizarre reason.
This is curious in itself. Most family trees branch quite a bit due to multiple children having multiple children (and families of important lineage in particular tend to make a point of having a "spare" heir around to avert any potential Succession Crisis). In fact, if you look far enough into the past, you reach a point where every human being alive then is either the ancestor of everybody or nobody who is currently alive. Here apparently only one child was born per generation. In older legends, it was explicitly stated that only the firstborn "counts", but the socioeconomic systems that fostered that kind of thinking withered away, leaving only this trope.
Similarly, the sole true descendant is generally treated as essentially equivalent to their ancestor, having the same powers, personality, ethnicity, and so on. Never mind that, if the legendary hero lived twenty generations ago, he had roughly a million contemporaries who are just as closely related to the modern-day version as he.
Note that, in the present generation, if there is another sibling, he or she tends to be recruited by the Big Bad as the Evil Counterpart.
Occasionally it will be explained away that the Big Bad found and eliminated the rest, and the Supporting Leader happened to get to the last one in time.
Possibly related to the Identical Grandson. Only one in a generation can take the "legacy."
See also Only Child Syndrome. The last offspring of a Single Line of Descent is by definition the Last of His Kind.
Raideen: Akira Hibiki is a descendant of the ancient people of Mu and he must help Raideen save the Earth. With Akira, the line of descent doesn't extend very far. His mother is the queen of Mu.
Naruto: Averted with the Sage of the Six Paths, who spawned the Uchiha, Senju, and Uzumaki clans. In the case of the Uzumaki and Uchiha clans, Naruto and Sasuke being the only descendants is justified because they are also the only survivors of the clans being annihilated, though it turns out with the Uzumaki that Karin is another survivor. In addition, the Hyuga family has multiple lines, which is a major plot point.
The Nakatsukasas in Soul Eater. The eldest child is expected to inherit the Morph Weapon Dark is Not Evil/Chaotic traits, and Masamune took issue with the fact he did not (though he got the 'dark' bit right). Odd, in that the Demon Weapons are the result of experimentation and that Tsubaki's family is quite clearly not the only example. In fact, the others have branched out into more modern and outlandish interpretations of the word 'weapon'.
Enforced Trope in the Nasuverse, where magi generally only pass on magic to a single heir, even if they have more than one children, in order to (according to Tohsaka Tokiomi) discourage magic-related Sibling Rivalry (the Aozaki siblings are an exception to this, but then again, they also seem to have decided that they can't coexist in the same universe). Defied Trope in the case of Tohsaka Tokiomi, who upon siring two talented daughters, made Rin his heir and gave Sakura to be the heir to the heir-less Matou house, which had no unfortunate consequences for her at all.
The Raregroove family in Rave Master doesn't boast any cases of multiple children. Understandably, since they are a bloodline of Cosmic Playthings who apparently tend to lose anyone they cherish in very brutal ways. Because of this, Lucia ends up being the sole descendant of Acelia, the last human not to be created by Star Memory.
JoJo's Bizarre Adventure starts off this way, but beginning around the Stardust Crusaders arc there are multiple Jojos running around at once. And then the concept of who's descended from whom starts to get tricky...
The DCU: Both used and averted with the immortal Vandal Savage, who has multiple descendants apparently including Arsenal whom he harvests for organs, and a person he specifically identifies from amongst them as his daughter Scandal, leader of the Secret Six. According to Scandal, her legitimacy is down to the fact Vandal married her mother.
Star Wars: Legacy seems to hint that this happened. It's been a hundred fifty years, and the only Skywalkers alive are Cade and his immediate relatives. It's hinted that the Organa Solo line got absorbed into the Antilles Fel line, whose descendant is the Emperor. An upcoming comic series will cover the fate of the Solo-Djo line.
In the newspaper comic The Phantom, natives and pirates are led to believe that the masked crime fighter is the same man living for hundreds of years. He is, in fact, the son of the last guy. I don't believe they indicated there was ever more than one child, no girls, and they always were fit to take over.
Deconstructed in Preacher; in order to keep the bloodline of Jesus Christ pure, his children and their offspring were only allowed to mate with each other. The end result, after generations upon generations of this... is a gibbering, deformed freak who barely has a clue what's going on around him.
Herr Star: Son of God or son of man, you can't fuck your sister for a thousand years and expect anything good to come out of it.
A character in Dogma is the only member of Jesus's brother's bloodline (through a sibling of the "ancestor" instead of the "ancestor" himself).
In Underworld. The Lycans are searching for a human descendant of the progenitor of both werewolves and vampires, but they need one with a specific gene and a wall filled with crossed over photographs tell us that there has been a lot of failures before the events of the movie. But that's because all they have to go on to find that one descendant is his last name, Corwin, which isn't exactly rare or unique. Depending on how long they've been looking for this guy, and killing the ones who don't meet their specifications, the Lycans may be the reason there's only a Single Line of Descent.
It's not clear if they look anywhere else besides Hungary. The novelization mentions that Michael's grandfather emigrated from Hungary to the US and Anglicized his name from Corvinus to Corwin. Michael only decided to go back to the Old Country after his wife died. It may have been sheer luck that the Lycans have found the right person.
Explained in The Covenant that only the firstborn males in each generation receive powers. There may well be many descendants of the original (male) witches of Salem, but only five males with the Power per generation (there are five families).
By which point, however, Aragorn basically was the last direct descendent of Elendil, thus re-invoking the trope.
In Harry PotterVoldemort is the last descendant of Salazar Slytherin, one of the four founders of Hogwarts. It is Justified by explaining that Slytherin's descendants became so obsessed with keeping the line "pure" that they repeatedly married within the family.
In the movie of The Da Vinci Code, there is only one surviving member of Jesus's bloodline. (In the book, though, the corresponding character has a sibling, and it's stated that they aren't the only line of descent, just the most reinforced and "qualified". That apparently wasn't dramatic enough for the movie, though.)
Deltora Quest establishes that by tradition, the ruler of Deltora has only one child, meaning during the time of Deltora Shadowlands, Adin (the first king) has only one heir. If that heir dies childless, the power of the Belt will be lost forever. It's eventually subverted. Lampshaded/Invoked Guess why the tradition got started.
The Belgariad book series featured the bloodline of the Rivan king, which only had one child per generation for centuries. This is justified due to prophecy keeping the family small so as to be easier to hide from numerous enemies. Seeing as almost the entire family was killed when they were secure on a small island surrounded by guards, this may be a good idea. Interesting in that the subject matter of one of the prequel books covers how the family was hidden/protected.
Except in Polgara the Sorceress, wherein Cherek wives of the descendants of the Rivan kings always had lots of babies. Either they all died, or Garion has bunches of n-th cousins running around... (He does meet a first cousin at one point, so that's probably the case.)
The House of Usher is a classic example, as Poe describes them as having produced no side-branches and declined to a point where only two siblings remained.
M.P. Shiel's "House of Sounds" used the same idea, with a family that had once numbered in the millions dwindling down to three. A partial subversion, in that the declining bloodline is traced through both sexes, with the Last of His Kind descendent being born to parents deriving from two distant branches of the family.
In the Hollow Kingdom Trilogy, both the Elf and Goblin Kings only ever have one son apiece. None of those messy succession debates here, no sirree!
Lampshaded in Prelude to Foundation, where a noblewoman claims she should be the Empress because her family descends from an ancient ruling house. A nearby historian remarks that with said dynasty ruling 5,000 years ago, half of the galaxy is their descendants.
In Legends of Dune, Camie Boro claims to be the only descendant of the last emperor of the Old Imperium. Given that the Old Imperium fell over 1000 years before, this is not very likely.
Played with in the Mithgar series with the "Lastborn Firstborns". A pair of mortal lovers become involved in a quest, but after a point, they're going to have to wait centuries to carry out the next step. They get married and pass their lore and quest items down the generations, hers to her firstborn daughter and then her firstborn daughter and so on, his to his firstborn son and so on. When the destined time finally rolls around, we end up with two characters who each look very similar to the ancestor whose lines of descent they've been following, but are no longer closely related. So they meet up, fall in love, and continue the quest, just like their ancestors. Then one of them dies, breaking the line of firstborns.
In Simon Hawke's The Wizard of novels, Billy Slade is the one and only descendent of Merlin Ambrosius.
Averted in Mikhail Akhmanov's Arrivals from the Dark and Trevelyan's Mission with the bloodline of Paul Richard Corcoran. Corcoran himself has two daughters, and each of his descendants has at least two children. However, the farther "down" the line we go, the less his Faata genes manifest themselves. The most common form is unnatural longevity coupled with sterility until 40. A few individuals do get his Psychic Powers as well. Sergey Valdez, Corcoran's great-grandson is notable for having these powers and passing them to both of his children. The last book of the Trevelyan's Mission series reveals that the protagonist Ivar Trevelyan is a 1000-year removed descendant of Corcoran with latent abilities.
In Tales of the Branion Realm, the DeMarian royal family rarely has more than a dozen members while other noble families may have hundreds, but has nonetheless managed to survive for 900 years.
Played Straight and Averted in A Song of Ice and Fire: The Starks, Arryns, Tullys and Baratheons are all confined to half a dozen or so members at the most, and the inheritances of their various regions are threatened because of it, while the former ruling Targaryen family is down to one (confirmed) member. Meanwhile, the Lannisters, Martells and Tyrells are all bursting with wide and diverse family trees, with mentions of separate family branches being established.
Carrot Ironfounderson is apparently the last living descendant of the Discworld's ancient Kings of Ankh. This has been supported by evidence uncovered by Edward d'Eath in Men at Arms, and by Dragon King of Arms' inability to find a better alternate successor than Nobby Nobbs in Feet of Clay. In Carrot's case, the lack of other heirs is at least implied to have been engineered, as he was the Sole Survivor of an attack on a caravan in which the long-lost royal sword had been carefully hidden.
Live Action TV
A rare female example in Charmed. The Charmed Ones are three sisters who are the last decendents of 17th century witch Melinda Warren. Apparently, up until the birth of the protagonists, there never was more than one daughter per generation.
Kind of subverted in season 4 when we find out about Paige. But of course nobody had thought of her yet in season 1 when the trope was established.
Unless you accept that there is no limit to the number of siblings as long as there is at least three sisters. Otherwise providence would have to wait until the mother reached menopause to grant the powers to the sisters, because more siblings might still be born. Besides we find out in That 70's Episode in season 2 that the sisters had their powers from birth - presumably because they were always going to be witches, it's just the Charmed gimmick that wasn't (and couldn't be) established until Phoebe's birth.
In the Star Trek: The Next Generation episode "Sub Rosa", we learn that a "ghost" named Ronin has been bedding Dr. Crusher's female ancestors for centuries, moving from mother to daughter, and is now moving onto Crusher following the death of her grandmother Felisa Howard. The logical flaws start with the fact that all her female ancestors were apparently named "Howard", indicating that Crusher is the first woman in her family to lose her maiden name, despite her living in the twenty-fourth century and the ancestor this started with living in the seventeenth century.note It is established that the Howard family is from Scotland, where it was commonplace for women to keep their maiden names until the twentieth century. However, the premise is still logically flawed as it requires that the Howard women passed their maiden name on to their daughters. Not to mention what would happen if a Howard woman failed to produce a daughter. (Would Ronin have to move onto Wesley after Dr. Crusher died?)
In The Secret Circle, The coven are all descendants of six witch families (twelve in the books). How six families managed to intermarry for 300 years without inbreeding is a hefty bit of Fridge Logic. It's later explained that there are other witches and circles within Chance Harbor, allowing for new blood to enter the coven, and while the Circle is made of six witch families, the previous Circle contained twelve witches (a member from each of the families and their spouse).
Religion and Mythology
Subverted trope in The Bible, in that Jesus needs both father and mother to be both descendants of the entire Davidic dynasty. Through his mother he gets biological descent, and through his adoptive father who has direct male descent he becomes legal heir. Not to mention that Jesus has several brothers (and there's a possibility that his cousins on his mothers side may also have the same descent).
Strongly averted in Legend of the Five Rings where the direct descendents of the two dozen or so gods and heroes that founded Rokugan number in the tens of thousands. And that's not counting the two million or so samurai that carry those gods' and heroes' names through fealty. It does appear in the first Imperial line, but only because when a new Emperor takes the throne, all of the other candidates must renounce their family name and be adopted into one of the secondary Imperial families. Interestly, this does lead to the logical conclusion of the Imperial family being wiped out in the first story arc. An heir turns up Moses in the Bullrushes-style in the third arc... but he's the new Big Bad.
Warhammer's Tyrion and Teclis have always been the only direct descendents of the first High Elf Phoenix King - Aenarion the Defender - from his first marriage to Astarielle over seven thousand years ago, and hence the direct inheritors of his bloodline and destiny. However, this trope was averted in the 7th edition High Elf army book (and the Tyrion and Teclis trilogy written afterwards) when it became clear that they're only the last surviving descendents, and the rest of Aenarion's heirs were systematically murdered by a Greater Daemon out for revenge against the bloodline of the elf who slew it. Given the extreme longevity and slow reproduction of elves, it helps that there have only been a handful of successive generations born during that time.
Averted in the Assassin's Creed series, where Ezio and Altair are from different parts of Desmond's family tree, and Ezio is known to have at least one descendant Clay Kaczmarek, aka Subject 16 who is from an illegitimate child. And then there's Connor Kenway (AKA Ratonhnhaké:ton), who is half-Mohawk, another ancestor unrelated to the other two. Word of God also explains that they were intentionally averting this trope.
In the Castlevania series, only the Belmont family has the power to vanquish Dracula when he rises every 100 years (though there have been many exceptions). Each time he rises, there are only one or two Belmonts around to do the job.
The Morrises, a family that stepped up to the plate to battle Dracula when the Belmonts went missing, are said to be related to the Belmonts. The relation isn't pure, however, and they aren't able to effectively wield the Vampire Killer whip without significant cost to themselves.
Exception: The plot of the cell phone game Castlevania: Order of Shadows involves three Belmont siblings, two of them women. But only the guy gets to take on Dracula, and the game isn't considered canon anyway.
It is stated that the villagers in Castlevania: Order of Ecclesia are of the Belmont lineage, but don't have their last name.
In The Elder Scrolls IV: Oblivion, the realm is protected from the land of the Big Bad by Dragonfires, product of an ancient pact. At the time of the game, the last Septim to be Emperor is assassinated, laying the realm open to invasion by the Daedric forces. The main plotline of the game involves finding the hidden son of the last Septim and getting him the Phlebotinum he needs to start the Dragonfires again. Or, at the very least, to stop the invasion... Though it's an aversion as well: the murdered Emperor had other sons, three of them, in fact, all of whom were taken out in similarly nefarious assassinations shortly before the game begins (making the Cyrodiilian Secret Service the worst bodyguards in history). Martin survived because, as the bastard son of the Emperor, his existence was kept so secret that even he didn't know who he was.
Done in Chrono Trigger, where only the Royal Family of Guardina is said to be descended from Ayla and Kino, ancestors some 65 million years back. Then again, over the course of 65 million years, pretty much everyone can trace their lineage to them at some point.
In The Lost Crown, William Ager was a villainous example. Unusual in that he died of tuberculosis with no heir to pass on the family secrets to, hence let down both Destiny and centuries of bloodthirsty tradition.
Fire Emblem's Jugdral timeline has holy blood passed down from the Crusaders. Word of God explains that if the child of someone with "major" blood (and is able to use the Crusader's Ancestral Weapon) gets "minor" (just some stats bonus) or "major" blood is completely random (birth order and sex play no part) and that the blood can be determined by a birthmark, this doesn't explain why only nobility seems to have even minor blood.
It's mentioned in the backstory that all of the crusaders that were given both power and the Holy Weapons of the gods started most of Jugdral's current nations. The few non-royals to have holy blood have gone astray in some fashion: Briggid, who is the princess Adean's twin sister, was kidnapped by pirates as a child (major Ulir blood), the dancer Sylvia was orphaned at a young age (minor Blaggi blood, and a relative of the priest/ruler of Edda/possessor of major Blaggi blood, Claud, according to Word of God), and at least one of the royal families lacks holy blood completely (Prince Jamke of Verdane). There is also the cursed Loptyr bloodline - the descendants of Maira, a traitor to the Loptyr Empire and possessor of Loptyr's blood, all live their lives in the hidden Spirit Forest in order to avoid both being ostracized and the possibility of the Loptyr Empire's revival. The fact that Deidre and Arvis both have minor Loptyr blood because their mother left the forest drives the crux of Geneaology of the Holy War's plot.
Seemingly averted in Fire Emblem Awakening, as the descendants of Ylisse's first Exalt (a.k.a. Marth) all bear the Brand of the Exalt, and those with the Brand can both wield the Falchion and perform the Awakening Ritual. Chrom's younger sister Lissa does not have the Brand, having her fear that she was a bastard child, but the fact that her Kid from the Future Owain has it raises questions about why hers never surfaced. Also, in Lucina's support conversations with whomever her sibling is, it is covertly revealed that they can wield Falchion as well, but they never can do it in-game.
Averted all over the place in Blaze Union. Medoute, the descendant of Gill the legendary dragonslayer, mentions repeatedly that she's from the secondary line. Characters also discuss that due to the Imperial bloodline being so diluted and spread throughout the people, it's not unusual for children qualifying as Brongaa's descendants to be born in all walks of life; the interesting thing about Gulcasa is that he's the first pureblood to be born in centuries.
Also averted in Tales of Symphonia, where the family of the Chosen is huge, has many branches, and is scattered all over the world so that if something happens to one branch, the bloodline will still exist.
In Dragon Quest VIII, the Big Bad is tracking down the descendants of the Seven Sages who originally sealed him away, as their deaths are the only way to release his original body. The game establishes that only one descendant in each most recent generation is the heir (as Jessica's brother Alistair held the bloodline whereas Jessica just received magical strength), with an odd exception in the case of Marta; while she had a son, he decided to practice medicine in a nearby town, figuring his skills would be put to better use there, and forfeited his status as an heir in the process (which managed to throw the Big Bad off for a bit).
Inverted in Fable III, only the youngest of the Hero of Bowerstone's two children gets any Hero powers.
In Mark of Kri, the marks are passed down through the first born descendants of six separate families, one mark per family, with at least two of the families having willingly died out to prevent their marks from being passed down any further. In an interesting turn of events, however, it is The Hero's younger sister who bears the mark, even though he is the first born, though the second game hints that they may not actually be related.
One of the conversations in Poker Night 2 reveals that Brock Samson of The Venture Bros. is one of only two descendants of Ash Williams's one-night stand with Sheila back in the 13th century. Of course, it could be just GLaDOS messing with them.
Subverted in Girl Genius. Part of the plan of Zola (the fake Heterodyne) is to ally with and marry a direct descendent of Andronicus Valois, the Storm King. When Gilgamesh skeptically points out that, if the legends are true, half of Europe is descended from the Storm King, she angrily specifies that it's a descendent who the Fifty Noble Families of Europe will recognize.
Later, when she explains more of the plan to him, she specifies why this one's confirmed, and it has something to do with the Mongfish family being specially gifted in the biological sciences.
Subverted in Digger. The ghost of Helix, a wombat who's been dead for a thousand years, give or take, immediately pegs Digger as a descendant. He had nine sons a thousand years ago, so it's more likely than not.
Parodied in The Adventures of Dr. McNinja, where a demon can only be slain by one of "pure Belstein blood". There is only one left, who's deformed and in a wheelchair due to generations of inbreeding.
Enforced in China by the government's one-child policy, to keep the already humongous population from completely exploding. It's been estimated that without this policy, there would be 400 million more Chinese people.
Not a spoof at the time, as the true scope of Genghis's genetic legacy didn't come to light until after Adams passed away. He was just making a funny contrast between Prosser and the Great Khan. That makes this Hilarious in Hindsight.
Also, the book specifies that he was a "direct male-line descendant" of Genghis Khan, apparently meaning the more improbable lineage in which the long chain from Prosser to Genghis Khan consists only of men.
Some 30% of Europeans have Charlemagne, or Charles the Great, as their ancestor. Charles was not a first-class fornicator - he had ten children - but all his offspring survived and had in turn children.
This is averted with pretty much anyone who had descendants, if you go surprisingly not far back in history. A paper in Nature demonstrated that if a person has four or five grandchildren, either their line tends to die out within a few generations, or the number of descendants begins to increase exponentially. Someone living two or three thousand years ago will either have no descendants at all alive today, or be an ancestor to a lot of people.
Averted with Sophia of Hanover who has over 5000 legitimate descendants. Out of these, all who are not Catholics or married to Catholics (still over 1000) are in line to the British throne.
Because the members of European royal families tended to marry members of other European royal families, just about every royal in Europe is descended from her.
Because her descendants were mostly royalty, and that royal men tend to have at least one mistress, there are even more illegitimate descendants. William IV of the UK alone had ten illegitimate children by his mistress, the actress Dorothea Jordan (he would have married her, but there would have been an uproar; remember Edward VIII? Now put that a hundred years earlier). Among those illegitimate descendants is David Cameron.
When it comes to European royalty, the grand champion of averting this trope might well be Edward I of England (the legendary "Edward Longshanks" of Braveheart fame), who lived between 1239 and 1307 and fathered sixteen children, nearly all of whom survived into adulthood and had children themselves. According to geneologists who have studied his family line, the man now has close to four million living descendants around the world (most of them are in the United States, Canada, and Brazil of all places...)
Sometimes intentionally attempted by dynasties to limit the number of people vying for the throne, land or fortune. Different cultures handled this in different ways:
In Europe, this often came in the form of sending surplus sons off to monasteries and various church jobs, ensuring no legitimate children. Alternately, younger sons would be sent to the army, where they would fight (and in many cases hopefully die on the battlefield); this, incidentally, is where we get the term "cadet" for a trainee officer ("cadet branch"=junior line of a noble house). If the line were about to expire, a monk or priest in such a position might be permitted to leave the Church and marry.
In Ancient Mesopotamia, aristocratic sons could end up as castrated but high-ranking officials at court. The Chinese sometimes did much the same thing.
Royal relatives in Ethiopia used to live in Gilded Cage prisons on top of high buttes in the wilderness, to ensure their genetic lines remained strictly under the reigning monarch's control.
The Hapsburgs of Austria and Spain tried keeping it in the family. The last member of the Spanish line ended up being saddled with a truly depressing array of physical and mental disabilities and was probably sterile to boot, so it's safe to assume they didn't exactly find the winning formula, either.