Someone's put the hit out on you. But no matter, you're armed and dangerous yourself, and when the hitman catches up with you, you get the drop on him, and simply put two in his chest. Problem solved, crisis averted.
And then he gets up, dusts himself off, mutters some kind of pithy one-liner, and comes after you again.
You've just run into the Immortal Assassin.
This character shows up frequently in works of fantasy and science fiction. The professional killer who themselves cannot be killed. Either they are straight-out indestructible, or they possess a Healing Factor that lets them recover from any injury. Why they hire themselves out as hitmen can vary from character to character.
Depending on the universe, the Immortal Assassin may have been given their immortality through supernatural or technological means. Often, Immortal Assassins are much older than they appear, not able to die of natural causes, and as such can have a bad case of "immortal angst" or "immortal smugness". If they hail from a time and place before gunpowder was invented, they may prefer bladed weapons or hand-to-hand combat, eschewing guns as "uncivilized" or "clumsy." As a consequence, they will possess phenomenal martial arts skills, honed from hundreds of years of practice.
Many Immortal Assassins consider themselves absolute professionals, devoted to the job at hand. They see killing as an art form, and make a serious effort to be as close to perfect as they can get. They may adhere to their own version of a "warrior's code," with a strong sense of personal honor. Often, this is because they come from ancient cultures that put a great emphasis on honor and martial prowess, and they see their careers as an extension of that.
Of course, there are exceptions. Some Immortal Assassins are sadists who kill out of pleasure or out of boredom. Being unable to die has left them so emotionally detached from the human race that killing mortals is little more than a game to them, a way to pass the time. Others are simply in it for the money. Some are just Ax-Crazy...
Most of the time, the Immortal Assassin is the villain of the story. They can be a Knight of Cerebus, brought in by the Big Bad when all other plans have failed, a genuine unstoppable badass more dangerous than any our heroes have yet faced. Maybe they're a Diabolus ex Nihilo. Alternatively, they may be a sympathetic character, particularly if whatever circumstances led to their becoming immortal or an assassin (or both) are tragic enough. On occasion, they will even do a HeelFace Turn, join the heroes, and become The Atoner. If part of an ensemble, they may even be used for comic relief; since Immortal Life Is Cheap, the Immortal Assassin can suffer truly horrific injuries and shrug them off.
There may be ways in-universe to defeat the Immortal Assassin. The Assassin may have a carefully-guarded weak spot, or they may be injured so greviously that their healing factor simply may not be able to compensate. Another option is total physical destruction of the Assassin's body, so that there is nothing left for the healing factor to regenerate. Use of Applied Phlebotinum may kill them, or deprive them of their immortality. Alternatively, the Assassin may be neutralized without being killed, imprisoned somehow, or even become Sealed Evil in a Can. Or in some cases, if the Assassin is just in it for the money and has no personal stake in the proceedings, simply calling off the hit will be enough.
There may be overlap with the Implacable Man, particularly if the Immortal Assassin isn't entirely human.
- In Mnemosyne, an assassin keeps trying to kill Rin, and keeps coming back, no matter how many times she herself gets shot, stabbed, and blown up. Fortunately, Rin herself is immortal too.
- Kakuzu of Naruto isn't quite immortal, but needs to be killed five times in one battlenote before he actually dies. He's also one of the oldest living characters; he was exiled from Taki for failing to kill the First Hokage, however many years ago that was.
- His partner Hidan, on the other hand, is pretty much defined by this trope. At the climax of the series he is still alivenote , in pieces, and rotting.
- Deadpool has essentially weaponized cancer to give himself a healing factor to rival Wolverine's, at the expense of his mental health. What flavor of mercenary he is tends to vary from story to story.
- The Saint of Killers from Preacher. Not even God can kill him.
- The Kindly Ones from The Sandman, although they can only be called upon to punish a kinslayer.
- Wolverine, before he changed occupations and became a full-time X-Man.
- The Talons from Night of the Owls probably qualify, since some of them are from at least the 1800s and their healing factor seems to protect them from most methods of killing them. The Court of Owls doesn't even kill them; they instead retire them, bringing them back eventually.
- War Machine faced an assassin who had been granted a wrist mounted counter by the goddess Kali that tracked how many kills he had. If he was killed he would be immolated and resurrected, if he had any kills left. War Machine finished him by leaving him in a collapsing pit.
- La Madrina from Lady Mechanika. She was once an ordinary aztec woman, but when her city and family were butchered by vampires who had emigrated from Europe, she underwent a rite that gave her power from Mictecacihuatl, Lady of the Dead. This turned her into a vengeful revenant with the strength and speed to easily dispatch vampires, and the ability to quickly recover from any injury. In the end, she has to be bathed in Greek Fire and her bones crushed into powder to keep her from slaughtering a Friendly Neighborhood Vampire and his lover, but even then nobody knows if that really stopped her or just bought them time to get away.
- The focus of the Ranma ½ story Relentless, though the killer is a demon (or even Eldritch Abomination) known as the Reikoku, sent to perform an assassination, rather than a normal assassin who happens to be immortal. Its weaknesses: it takes a day to come back to life, and if a single person can kill it three times, it will be banished back to its home dimension. Unfortunately, each time it comes back to life, it will have developed an impenetrable defense against any attacks used against it the previous time, plus it will have developed brand new attacks, including tailor-made Flaw Exploitation attacks. It might seem like a good idea to destroy its corpse, but that just brings it back to life instantly. Oh, and if any of your friends attack it in order to help you, they get added to its hit list. And three total kills amongst you and all your friends aren't enough to finish it, it has to be three kills by the same person. Have fun!
- Inverted in What About Witch Queen? - while the assassin is absolutely mortal, his quarry proves much less so, to the surprise of them both.
- Many characters in the Highlander universe have made their living being Immortal Assassins, but most notably the Kurgan from the original film. He hires himself out to Clan Fraser in their battle against Clan Macleod, in exchange for the privilege of killing Connor Macleod himself.
- Terminator: Most terminators are specifically designed to be unkillable killers; the only way to make the T-800 in Terminator 2: Judgment Day seem vulnerable was to create an even deadlier terminator: the T-1000.
- The character "Jaws" from James Bond's The Spy Who Loved Me and Moonraker appears to be immortal. He gets a pyramid dropped on him, bites a shark (not the other way around), drives off a cliff, slams into a cable car terminal, drives a boat over a water-fall and falls out of a plane. The mooks with him drop like flies, but he just dusts himself off and goes back to work. The last we see of him, he's parachuting(?) back to Earth with his girlfriend.
- Karl Ruprecht Kroenen, the Big Bad's dragon in Hellboy, is a quasi-undead Nazi ninja kept alive by clockwork cybernetics and sheer villainous willpower. His blood has long since turned to dust, making him Immune to Bullets, and he keeps showing up again and again no matter what the heroes throw at him. By the end of the movie he's been taken out of the picture, but it's highly likely that, although trapped in an And I Must Scream situation the last time he's seen, he's still alive and available for a return in a sequel.
- The Gholam of Robert Jordan's The Wheel of Time is basically an Anti-Magic T-1000 designed as a literal Mage Killer—so terrifying that only a handful were ever made. Only Anti-Magic artifacts like Mat's medallion (and probably starvation) can possibly do any damage, and that's not really a practical solution. It ultimately does die some time after being forced into the Void Between the Worlds that is the Skimming dimension, however, but not without a lot of ingenuity.
- Ghost, of R.A. Salvatore's The Cleric Quintet series, was a relative weakling who used a magic artifact to switch bodies, kill the target who is now in his weak body, and then heal it again using a magic ring of regeneration.
- James Spector, AKA "Demise," from the Wild Cards series. Originally killed by the Wild Card virus, he was resurrected with an experimental cure. He gained both a powerful healing factor and the ability to "project" his agonizing death onto anyone he made eye contact with, essentially killing them with a look. He was decapitated when a political assassination went wrong, and his body was destroyed by cremation when it became clear that he was trying to GROW A NEW HEAD.
- The Night Angel from The Night Angel Trilogy by Brent Weeks. He/she/it is literally immortal assassin although the immortality comes at a terrible price: For every time he dies, a person of importance to him dies in his stead. Cue a whole lot of immortal angst and be done with it.
- The Mercenary from The Bartimaeus Trilogy, while not strictly immortal, is incredibly resilient. Through his huge Healing Factor and resilience to magic, he survived a long fall from a cliff, survived being smashed under a huge rock and survived magic that could destroy demons, let alone humans. Unlike most examples, he is not the main villain. Although even he can die to a lot of very powerful, concentrated magic.
- Messers. Croup & Vandemar from Neil Gaiman's Neverwhere are a pair of Humanoid Abominations who stalk after Door, one of the main protagonists of the book, under orders of an unknown employer. They react to injuries with amusement or annoyance, they teleport seemingly at whim, and may even possess limited time travel abilities, and they eat human flesh. Their true nature is never explained.
Mr. Croup: If we are injured, do we not bleed?
Mr. Vandemar: ...No.
- In The Lord of the Rings, when the Nazgul aren't Sauron's generals, they are his assassins. All nine were tasked with killing Frodo and retrieving the Ring from him, and it took intervention from the biggest heroes and healers of Middle-Earth to thwart the attempt.
- Lexx has an undead assassin, Kai, as one of its main characters. He is completely indestructible, and his long-term goal throughout the series is finding a way to die for good.
- Kai eventually faces an even more dangerous and indestructible assassin, the Divine Executioner Vlad.
- Warehouse 13 features Marcus Diamond, The Dragon to Season 3's Big Bad. Technically he can't be killed because he was already mortally wounded a long time ago, and he's using an artifact that rewound time for his body to just before his death. He's literally living on borrowed time.
- The Highlander series featured several. Christoph Kuyler claimed to be the most prolific assassin in history, as well as the less theatrical Anthony Galen and Kamir, the last of the thuggee assassins.
- The Caretaker from Alphas is virtually unkillable due to his high bone density, which he's able to regenerate so long as he has access to calcium. The downside of this is that this makes him very heavy, which Kat uses against him to drown him.
- River Song from Doctor Who was abducted as a newborn and raised as a Tyke-Bomb to assassinate the Doctor. Having been conceived in the TARDIS, she possessed the ability to regenerate if killed, and had a lifespan measured in centuries.
- Dirk Gently's Holistic Detective Agency: Bart Curlish, holistic assassin, essentially has official Plot Armor. Bullets swerve to avoid her, guns pointed at her at point blank jam, a knife hits her pommel first and bounces back... But only if she's doing what the universe wants her to do, which usually is killing bad people. When she mistakenly tries to kill Dirk she gets stabbed, and when she refuses to kill Suzie Boreton she gets blasted with her new magic wand.
- The protagonist of Warren Zevon's "Roland the Headless Thompson Gunner" is a Norwegian mercenary who is betrayed and killed by a companion, but instead of dying he becomes some sort of avatar for Cold War-era proxy warfare ("Time stands still for Roland 'til he evens up the score"), going wherever there is unrest and discord.
- Deadlands give us Jasper Stone, an evil Harrowed who has progressed into a full-fledged Abomination, who works for the Reckoners themselves and shoots down dangerous heroes at their bequest. He was statless for a long time, for the explicit purpose of not being killed by the party.
- Liegenstrasse in Warhammer 40,000 is a result of splicing human and alien genome and the prime case of Gone Horribly Right - with Casting a Shadow, Healing Factor and full Anatomy Arsenal, she proved nigh-impossible to take down and it was only managed by entire assassin temple. Maybe...
- From the point of view of the enemies, ANY player character is one of these. Even if they manage to kill you, you just resurrect at the nearest save point and keep on coming. A few examples below are justified since their means of resurrection are actually addressed within the narrative of the game:
- Raziel from the Legacy of Kain series. Killed and resurrected twice already before the events of Soul Reaver, by the time the player meets him he is a soul-devouring revenant animated by an Eldritch Abomination for the sole purpose of bringing down Kain and his empire. His physical body is essentially a half-rotted corpse, without skin, internal organs, or even a lower jaw. And if this body takes too much damage, he simply shifts to the spectral plane until he builds up enough life energy to return. And since time stops on the material plain when this happens, from anyone else's perspective he just pops out of existence and then immediately back in again at full strength. The Elder God specifically says he is "beyond death." He can't be killed because there is simply nothing left of him that's killable.
- Jack and Subject Delta from the BioShock series: functionally immortal because the game's equivalent of save points - the Vita-Chambers - are coded to their specific DNA. No one else in Rapture can use them, except Jack's father, of course.
- The player character from The Elder Scrolls V: Skyrim can seem like this, having two methods (a perk and a mask) to automatically heal from near death.
- Having in-universe and plot relevant justifications for the playable character's ability to come back from death is pretty much a staple in FromSoftware's games:
- The Slayer of Demons from Demon's Souls becomes bound to the Nexus after the first time they die, granting them Complete Immortality. This basically makes them the only hope for the world.
- The Chosen Undead, the Bearer of the Curse and the Ashen One from the Dark Souls series are all undead, and merely rise again every time they fall. The same deal goes for all enemies as well, who are all undead and respawn once you've rested at a bonfire, but have gone on for so long that they've gone Hollow, essentially losing their sense of identity and turning into mindless husks that instinctually attack you. And as "Assassin" is a class you can pick, you can be a literal version of this trope.
- The Hunter from Bloodborne becomes bound to the Hunter's Dream upon their first death. Being a Dream Walker in a world where dreams and reality have some serious overlap, the Hunter just wakes up again every time they fall, very much as if their death was a mere bad dream.
- The Wolf from Sekiro has been bestowed by his Lord, Kuro, the Divine Heir, with the Dragon's Heritage, a blessing that gives Wolf the power to come back from the dead. In fact, you can come back more than once; should an enemy kill Wolf, he may choose to expend a token and rise on the spot, or give up and resurrect at the last savepoint, though the latter form of resurrection spreads a Mystical Plague known as Dragonrot throught the land. Being a ninja, Wolf is also a literal example of the trope.
- The eponymous bad guy from The Horribly Slow Murderer with the Extremely Inefficient Weapon.