Someone Has to Do It

It's a tough job, sometimes a boring job, often a dirty job, but Someone Has to Do It.

Literally. Some kind of supernatural force exists that ensures that there is always somebody doing the job. Once a person takes up the torch, they can't put it down unless there's somebody there to pass it on to. (Situations where it would merely be a very bad idea for the post to become vacant don't count. These are cases where having somebody doing the job is not just a good idea, it's the Law of the Universe.)

Usually, the job comes with exemption from aging and death by natural causes -- although exemption from death by violence may or may not be thrown in. It is often the case that being killed is the only way to leave the job, with the killer being drafted to fill the vacancy.

The job itself is often something of a supernatural nature, such as guiding the souls of the departed into the Afterlife, but not always. Sealed Evil in a Duel is another common form.

Compare to There Can Be Only One. See also Subbing for Santa and Klingon Scientists Get No Respect.

Anime and Manga

 * The second season of Mermaid Melody Pichi Pichi Pitch revolves around a successor being born for, who either died or willingly relinquished her post at the end of the first. The Big Bad interrupts the successor's creation by trying to absorb her soul, and the rest of the season involves Lucia chasing him down to collect all the soul fragments from his angelic Perpetual Molt.
 * The end of the manga, where, leaves a Sequel Hook when you think about it; they need a new.
 * Hell Girl, probably. We don't know if a Hell Girl existed before Ai, but by the third season, she's trying to recruit a replacement.
 * Being the Avatar in Oban Star Racers.
 * In Mirai Nikki,

Comic Books

 * The first story arc in the 2006 Justice League of America series featured an immortal being who wanted to die, but couldn't unless somebody became immortal in his place. (Unlike the other examples, he wasn't doing any important job, it was just that, by its nature, immortality can not be created or destroyed, only transferred.)
 * The Endless of The Sandman seem to work this way. If one of them is killed, a replacement of some sort will come into being.
 * It has been revealed that the Marvel Universe needs three beings to exist, and one of them is Galactus the world-eater. Very bad things have happened when he is destroyed.
 * The Authority: There is always a Doctor. When one dies, the title and powers automatically pass on to someone else.
 * In Fallen Angel, the city of Bete Noire always has to have a Magistrate, as well as several other necessary positions. Doctor Juris seeks to escape his position as magistrate by handing it over to his successor, so he can leave Bete Noire forever.
 * In the Marvel Universe Captain Marvel, the hero goes cuckoo banannas and eventually, with the help of the personification of Entropy, ends the universe. Rick Jones, also somehow existing, persuades Entropy to become the personifcation of Eternity, a long-established entity within the Marvel Universe. The universe, such as it is, reboots.
 * Similarly, Johnny the Homicidal Maniac is only the latest in a long line of "flushers," tasked with guarding a waste-lock for aggression incarnate. Should he die, the universe dies with him, flushing the evil into oblivion. When it reboots, there is a new flusher.

Film

 * Pirates of the Caribbean: At World's End ("The Dutchman must have a captain.")
 * The Santa Clause means that once a Santa is killed, whoever puts on the suit next becomes Santa, right down to the weight gain and beard.
 * "No, sir, YOU are the caretaker. You've ALWAYS been the caretaker."

Literature

 * In one of the Grimms' Fairy Tales, "The Devil's Three Golden Hairs", one of the incidental characters is a man who is doomed to keep ferrying travellers across a particular stretch of river. In the end, having learned from the hero of the story that he can escape his task by tricking one of his passengers into becoming ferryman in his place, he resolves to try the trick on the next passenger who comes along -- and, as luck would have it, the next passenger to come along is the villain of the piece. All live appropriately ever after.
 * In Piers Anthony's Incarnations of Immortality series, various Anthropomorphic Personifications are jobs of this nature, each with its own rules. Some examples:
 * The Grim Reaper remains in office until he becomes complacent or suicidal and is killed by one of the people he has come to collect. That person then takes his place.
 * Time holds his office for exactly as long as he lived before taking office. (It is left to Fate to ensure that there is a new candidate standing by when his time runs out.)
 * Fate, appropriately, gets to choose for herself the time of her service and the identity of her successor. She also, unlike her colleagues, gets to return to life when she retires, instead of passing on (leading to one character, over the course of the series, being recruited as Fate, serving time, retiring, living out her life, and then getting recruited as Fate again).
 * Gaea is the same way. She can only be removed if another Gaea steps forth and if she's willing to give up the office.
 * War only ever gets replaced in the unlikely event that, at any given moment, there isn't any war anywhere in the world. The moment does not have to last very long, and indeed usually doesn't.
 * Good (i.e. God) gets replaced if a majority of mortals involved in the dominant religion on Earth change their idea of the greatest being of good. YHWH was the Incarnation of Good before God, who apparently replaced him when Christianity became more popular than Judaism.  The actual rules for Godhood aren't very clear; Hinduism's gods are implied to also be active.
 * Evil (i.e. Satan) also remains in office until he's killed or kills himself. It normally defaults to the most evil person in the world, but it can be claimed before that happens, which is how a good man ends up with the job.
 * In The Giver, someone must act as the Receiver of Memory and hold all of the community's memories of the past in his or her own mind. If they die before passing them on, the memories escape and infiltrate everyone's mind... and as this is an emotion-free Utopia, their minds aren't able to cope (imagine a wide-scale human Logic Bomb). For this reason, Receivers are forbidden to undergo voluntary "Release".
 * In the Discworld novels, there is a deity for everything, but since there's only so much belief to go around, most of them don't manifest. If a major source of belief were to be removed, other small gods would start to reappear, as seen in Hogfather: after the titular Discworld equivalent of Santa Claus is "killed" (for want of a better word), the leftover belief allows beings such as the Oh God of Hangovers, assorted creatures that eat small items that always go missing (such as socks and pencils), and the Cheerful Fairy to appear. And that's only the start of the trouble, which is why Death, of all people, steps in to take the Hogfather's place on Hogswatchnight.
 * Death himself has a similar job. Should he slack off, someone else is pulled into the Grim Reaper's role. And if he's fired, people who are supposed to die become undead instead (hilarity ensues).
 * If Death even deviates a little from the Duty, Susan can and will get pulled in to do her grandfather's job. She is not happy about it.
 * Another Discworld example: it has been observed that kingship is instantly passed to the new monarch upon the death of the reigning king/queen. In a world powered by tropes, this can have major plot importance (and would be great for use as a faster-than-light communication signal, if the bar hadn't closed while the wizards were discussing it). For instance, Pteppic, heir to the throne of Djelibeybi, notices himself manifesting some pharaoh powers before he even gets news of his father's death.
 * Closely related is a disturbing variation from Lois McMaster Bujold's The Hallowed Hunt. The ancient spell
 * The Dresden Files features an extremely powerful being called the Archive, which contains the sum total of all written knowledge on Earth. The job is passed down from mother to daughter - and, tragically, the current incarnation is an eleven-year-old girl whose mother killed herself shortly after her birth to avoid carrying the burden.
 * The Archive herself said that the knowledge transfers at the moment of death leaving the parent brain dead, but she dosn't really mind as her mother lives on inside her head. Currently fans don't know who's right.
 * Harry will occasionally quip that this trope is why he's Chicago's resident combat wizard/PI. A number of villains call him on it. Usually when they're explaining how he couldn't have ever won in the first place.
 * Minor example from Jonathan Strange and Mr. Norrell - towards the end, Lascelles kills a mysterious guardian in Fairyland, but is then forced to take his place until he is killed, its implied his predecessor threw the fight, and that this happens to everyone who passes by.
 * However, almost immediately afterwards someone comes by who's powerful enough not to care about such rules, and may well have just blasted him to bits and carried on.
 * In Jack Chalker's And The Devil Will Drag You Under, a magic gem is guarded by the ghost of the last person who tried to steal it. The ghost is substantial enough to hold and use a sword, but not substantial enough to be hurt by one. He stands guard until the next thief arrives--then he kills the thief, freeing himself and recruiting his replacement.
 * Chalker also uses the same concept in The Dancing Gods series: a person who has control of the Lamp of Lakash can have the genie of the lamp grant one wish. If the person makes a second wish while still in possession of the lamp, however innocently, they become the genie and the genie (themselves a former wisher who made the same mistake) is freed and returned to their previous form.

Live Action TV

 * The Doctor Who serial, The Keeper of Traken, involved a planet where someone had to sit in a control chair to keep the planet running smoothly.
 * In Buffy the Vampire Slayer, "into every generation a Slayer is born." A girl, chosen from  is selected to fight demonic forces in order to prevent Hell from taking over Earth. Once the current Slayer dies, the next takes her place.
 * Subverted twice in the series: once when Buffy dies Because Destiny Says So, only to come Back From the Dead with a little CPR (which means now there are two Slayers) and once at the end of the series, when.
 * In The Lost Room, the Prime Object aka The Occupant is a person, just as indestructible outside of the titular room as the other Objects. The only way for him to stop being the Occupant is to enter the room and have someone kill him, at which point they become the new Occupant.
 * In the Tales from the Crypt episode "Let the Punishment Fit the Crime", the absurdly strict town of Stueksville has apparently only one public defender. It seems the way the job is filled is that if an Amoral Attorney passing through happens to get arrested, the current public defender tries to get their sentence reduced to "public service", in which case the attorney takes over the job and the previous public defender is finally allowed the sweet release of death.
 * The Twilight Zone
 * TOS episode "A Game of Pool". After his death, Fats Brown has to show up and play a game of pool - to the death - against anyone who wants to prove he's the best pool player ever. When someone finally beats him, Fats is released, and the guy who beat him takes over.
 * The New Twilight Zone (1980's) episode "The Curious Case of Edgar Witherspoon". A man must maintain a patchwork contraption to keep the world stable. He's eventually replaced by another man who becomes fascinated by the machine.
 * In the classic TZ episode "Paladin of the Lost Hour", an old man carried a watch that was stopped at 11 o'clock with him at all times. He needed to find someone who would accept the watch before he died; otherwise the watch would start working again and.
 * Lost: In Across the Sea, Mother implies that someone has to act as protector of the Island. Later, when Jacob is killed before a new protector is chosen, Jacob sticks around as a ghost until takes the job.
 * Subverted in Star Trek: Voyager on a couple of occasions when the crew runs into waste disposal ships. While the aliens claim this is their situation, Janeway openly critiques them, suggesting that they simply aren't trying hard enough to come up with better options. She's proven right in one instance, where Voyager actually gives the alien technology that would render their current waste disposal methods unnecessary. But the alien decides he won't use this technology since it would ruin his profit margins in the waste disposal business (We'll ignore for the moment the lucrative business opportunity he could exploit by patenting this technology).

Tabletop Games

 * The permanent death of a darklord in the Ravenloft setting often results in the next most suitable villain within the domain being impressed into that role, like it or not. As the alternatives would be for the domain to either be divvied up by the neighboring darklords, or to simply disappear and take all its inhabitants with it, such a hand-off to a new Big Bad may well be the preferable option.

Video Games

 * In one of the endings of Princess Maker 2, you discover that the post of Satan passes on to the most fitting mortal should the Lord of Darkness himself ever die. Unfortunately, you only find it out when your daughter kills him and assumes the title.
 * Overlordhood in the Nippon Ichi universe seems to work that way -- the only ex-overlords present
 * The Nameless One in Planescape: Torment can find the Silent King of the Dead Nations completely dead. He can be replaced, but a new King would have to be someone still alive, and that person will be bound to the Throne. If the Nameless One chooses to do it, it's a Nonstandard Game Over.
 * The Fall From Heaven backstory has this typically happening with gods and particular elements of creation (represented in game by the different magic types.)
 * A rare sci-fi example comes in the form of the Shadow Broker of Mass Effect. The Broker deals in trading secrets and information to high and low profile clients. He's been in power for so long, traded so many secrets, and has so many pan-galactic governments playing his game, all of civilization on a galactic scale would unravel like a tapestry without the Broker weaving secrets through societies.
 * The Guardian of the Balance in The Longest Journey and Dreamfall is a mortal who serves a lengthy term (measured in centuries) keeping the twin worlds of Stark and Arcadia from colliding. Afterwards he or she returns to mortal life.

Webcomics

 * In the spirit of The Santa Clause, Bun Bun of Sluggy Freelance inherits the title of Easter Bunny by inadvertently killing the previous one. Then he accidentally gets stuck with the sentient Shadow of Groundhog's Day. Eventually, he just decides to run with it and starts whacking Anthropomorphic Personifications of holidays left and right, seeking to form a one-rabbit majority in the Council of Holidays.
 * DMFA has the Phoenix Oracles, of which there are 42 - when "killed" they instantly reincarnate in a different location and travel back to one of their temples. There are also the Fae, of whom there are exactly 2,438,165. New Fae can only be born when an existing one chooses to die, apparently due to a finite supply of souls.
 * At times, Sal feels this way about being Future High Priestess of Phred. On the plus side, she gets incredibly good luck. On the down side, she gets incredibly bad luck.

Web Original

 * In the Global Guardians PBEM Universe, there is always an Archmage, he mystical defender of Earth. When the previous Archmage dies, is incapacitated, or gives up his power as Archmage, the next Archmage is chosen immediately and gains all the powers and responsibilities of the office.

Western Animation

 * In an episode of Chip 'n Dale Rescue Rangers, an oil lamp containing a genie appears. If anyone wishes to free the genie, he takes the genie's place inside.
 * In an episode of the Jumanji animated series, Alan and the kids finally manage to (apparently) kill Egomaniac Hunter Van Pelt by luring him into a Bottomless Pit. However, Peter ends up transforming into a miniature version of Van Pelt since, according to the latter, "There must always be a Van Pelt. It's the rules of the game." After turning Peter back, the original Van Pelt manages to climb out of the pit somehow, resuming his old job.