The worst kind of unkillable enemy, there's the creature who cannot be mortally wounded, because he has a Healing Factor ramped Up to Eleven. Every injury simply regenerates, from lost limbs, to a blown off torso, to a broken neck. Even when you think you've completely killed him, he comes back, because you missed a bit of flesh that was able to replicate itself into a new body.
This is rarely ever a trait of the hero; it is more often a power a villain has that marks him as being too powerful and resilient for the heroes to overcome, forcing them to find a creative way to beat the villain or gain a Plot Relevant Power Up.
If not properly explained, or given to a character who doesn't really seem worth it, it can become an Ass Pull. No matter how well done, it will have audiences (and characters themselves) screaming "Why Won't You Die?!"
Why every loose cell or drop of blood shed by the character doesn't generate a copy of them is not generally addressed. In a supernatural setting this can be handwaved with some talk about souls or lifeforce, though one still wonders how the various bits go about deciding which one of them gets to do the regeneration. Bonus points if the character/creature in question somehow retains all of his memory and personality (which are stored in the brain - no brain, nothing to restore all that data from!).
Expect this from all sorts of enemies, from demons, aliens, to androids made of Nanomachines.
A form of Nigh-Invulnerability. A character who can regenerate this way and is The Ageless has Complete Immortality. See Shapeshifter Baggage and the necessary Genetic Memory. Also see Good Thing You Can Heal and Immortal Life Is Cheap. Related to Pulling Themselves Together.
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Cell from Dragon Ball Z was a biological android with many of the main characters' abilities coded into his DNA, including Piccolo's regeneration and Freeza's ability to survive in very harsh conditions. Therefore, he doesn't have to worry about dying if... say... the upper half of his body gets blown off. In fact, after initially losing to Son Gohan, he tries to blow himself up and destroy the planet. After Goku sacrifices himself to teleport Cell somewhere far away from Earth, Cell surprises everyone, including himself, by regenerating quite literally from a single cell.
Depending on the translation, Cell's final regeneration wasn't from just any random single cell, but from a group of "core cells" in his brain. (You can see this during his death in the anime.)
Cell has an unusual twist on this; his Saiyan cells grant him a zenkai whenever he recovers from gruelling injuries (such as losing a limb), with the power boost being proportional to the injuries sustained. Applying that to his coming back from a single cell results in him becoming several times more powerful.
The next villain after that, Majin Buu, is a murderous, supernatural djinn, who can reform himself from a single piece of himself, and can reform himself if he is blown apart into a million pieces, or even air vapour. A single cell? All he needs is a single molecule. Planetary explosion? Give him 3 seconds. Blown up to shreds, the shreds then being vaporised just to make sure. Give him 3 god damn seconds. It took a Combined Energy Attack with power from every non-evil, non-dead character in the series up to that point to kill him.
It's interesting to note that his regeneration becomes more powerful (or at least faster) with every transformation. Fat Buu already fit this trope, but it was a fairly slow process. Super Buu was able to regenerate at incredible speed, though there was implied to be mental limits that Gotenks almost reached. Finally, Kid Buu could regenerate seemingly instantly from having his entire body destroyed, with no visible limits aside from complete destruction of every single cell.
Alucard from Hellsing, who can also use his spilled blood as a deadly weapon with no adverse effects. More than once was he reduced to a fine red mist before regenerating fully.
Alucard deserves some elaboration. In the Hellsing universe, when a vampire drains a victim of his life's blood he absorbs the victim's soul into himself. Among other things this gives the vampire a 1-Up. Alucard has feasted on so many battlefields that his extra lives number in the tens of thousands, if not millions, making him effectively unkillable. However, Alucard can turn his "servants" loose as a literal army, which leaves him alone and vulnerable to being killed.
Immortals in Baccano! effectively work this way, although except in the case of their bodies being burned or dissolved, they don't regenerate so much as all of their tissues, any spilled blood included, are simply drawn inexorably back together into the proper form. Even when burned or dissolved with chemicals, however, they can still fully recover - Fermetchecked.
Szilard's done his own research on the subject, and discovered that immortals pull themselves together on a molecular level - set an immortal on fire, and the resulting smoke will return to the burned flesh once regeneration kicks in.
Yakumo from 3×3 Eyes. He was turned into a Zombie Wu, an immortal undead slave, when Pai, the last of the Sanjiyan, sucked his soul into herself to save his life. His Healing Factor is so powerful he was once able to regenerate from being blasted into a bloody mist.
Elder Toguro from YuYu Hakusho does this, but he explicitly says that the only thing that will kill him is by destroying his heart or brain. Simple enough, right? Wrong. He's able to move around all his organs and body parts to wherever he wants in his body and can even ooze through cracks in the ground. Only two characters in the series manage to defeat him: Kuwabara (after transforming his Laser Blade into a Laser Racket in order to squash his entire body) and later Kurama (giving him a Fate Worse than Death by planting on him a monstrous tree that never stops harassing its victim until his or her death, which will never be the case with Elder Toguro).
The Guyver can do this. Aptom from Guyver can also do this, although he requires Zoanoids to alter into more of his body mass (rather like The Thing, under Film). As a bonus, this also lets him use those Zoanoid's powers. The "why doesn't every part of him regenerate into him" issue is addressed - this will happen if they have access to sufficient biomass, but all copies of Aptom have a Hive Mind and can merge or split at will. Eventually, a villain finds a flaw in his power: after Aptom is nearly killed, his non-sentient remains are exposed to a Zoanoid which contains a fake "Aptom brain" and absorb it. His cells do not regenerate his brain due to thinking it's already present, thus allowing the villain to control Aptom's body through the fake. Sho questions why "Evil Aptom" keeps using an unwieldy Wave Motion Gun while fighting him, and realizes that he has been using it to destroy any pieces of Aptom which were separated from the main body.
Deoxys from Pokémon is shown to have this ability as long as its brain, the gem-like substance that protrudes from its chest (Which is pretty much indestructible anyway), is still active. While it can regenerate lost limbs in just a couple of seconds, it has been shown that regenerating its entire body can take up to several years.
Tomie, the title character of a J-Horror manga series by Junji Ito and its subsequent film adaptations, can do this. She can also speed the process up by infecting the bodies of other women and turning them into copies of herself. A scene in an early storyline implies that she is able to do this by virtue of being extremely radiotrophic, somehow converting background radiation into mass as her growth was accelerated by a doctor's attempt to kill her with a radiation machine used to destroy tumors. Throughout the story its been proven that fire will kill Tomie for real and unable to regenerate (burned flesh are dead tissue)
Genji Ikusabe from Busou Renkin's spear has the ability to regenerate his entire body very rapidly, even if the body itself is completely destroyed. Admittedly, it has the weakness that he has to be holding his weapon at the time.
Rin, a rare protagonist example from Mnemosyne (due to encountering a spore of the Yggdrasill tree) has virtual immortality, including the ability to recover from any injury, even getting sucked through a jet engine. Of course, being able to regenerate and stay alive under all sorts of conditions can be... bothersome.
Naraku from InuYasha becomes like this (he starts off able to regenerate, but with limits). At one point he's reduced to his head, one hand, and miscellaneous bits of flesh and has regenerated fully within a chapter.
The Homunculi from Fullmetal Alchemist seem to be able to do this when their Philosopher's Stone is intact. Case in point: after one of his subordinates is grievously injured during a fight with the homunculus Lust, Roy Mustang attempts to hit two birds with one stone by forcibly removing Lust's Philosopher's Stone to kill her and using it to heal his friend. When the Stone is removed, Lust's body disintegrates, and Roy thinks he has won. Cue utter horror when Lust's body begins to reform around the Stone in his hand.
There is a limit to how much they can regenerate, as their stone's power is limited, so you can kill them permanently if you do enough damage. Which is a lot.
In the case of the Homunculi, the issue of why lost body parts and/or blood don't regenerate into copies is handled by the fact that a homumculus's Philosopher's Stone is the source of all its power. Thus, any part not connected to the Stone ceases to exist.
Nurarihyon from Gantz. Lets face it, without the gravity gun against this guy, you are screwed.
Toriko has Elg of the Bishokukai, who fused with a legendary horse called a Heraku and as such can regenerate from a single cell, and do not cut him up into pieces because each piece will regenerate into an entire copy of him. He dies anyway after Tengu Buranchi electrocutes all of his copies long enough that the Heraku's regenerative powers can't keep up with the damage.
The Darkness had this once after Jackie set a warehouse and a few mooks on fire. The Darkness survived by hiding in a tooth cavity; not knowing what else to do, the Darkness rebuilt Jackie.
Lobo of DC Comics, an alien mercenary who deviates back and forth from a Psycho for Hire to a Heroic Comedic Sociopath, can revive himself from a single drop of blood. He once cloned himself an army this way, but, y'know, being all "sociopathic," they all killed each other until one remained (in non-bloody ways, like suffocating each other).
One of the potentially toughest JLA foes was always the Shaggy Man, a nearly-invulnerable, ten-foot-tall bionic android that could regrow any limbs it lost almost instantly. Luckily, it was utterly mindless and therefore easy to trick. Eventually, a crazed general dying of brain cancer transferred his mind into it. He hasn't shown up very often since, probably because they're running out of ways to get rid of an unkillable, unsubduable villain with the mind of a brilliant tactician.
Part of this may be because General Eiling found out too late that The Mind Is a Plaything of the Body. The Shaggy Man's tiny brain did not have a good effect on his mental facilities, and the Shaggy Man mostly just slept a lot. And yes, this does show a bizarre combination of the mind as the brain and as something beyond physical.
Characters who can change their shape or are made of something inanimate, such as Sandman, Hydro Man, and Clayface, are often implied to be able to do this.
Superman villain Brainiac is constantly shown to be able to survive from the smallest remaining bit of his circuitry or computer program. This is taken to an extreme in the DCAU, where the tiniest bit of Brainiac always survives, is stored somewhere, and manages to take over more technology and reform himself completely. He is then defeated, and the processes repeats itself.
Master Mold from the X-Men acted much like Brainiac, storing backups of itself in out- of the way places.
And its future "descendant", Nimrod, can rebuild itself if even a single molecule survives.
Isstvan, from the comic adaptation of Myth Adventures by Robert Asprin and Phil Foglio, gets blown up by a spell, and then rather squelchily reassembles. Nothing remotely like this happens in the original novel.
Wolverine once regenerated from a single drop of his blood landing on an incredibly powerful MacGuffin that was making the villain of the month all powerful. As a result, his healing abilities were supercharged to the point that he could regenerate from a single cell. He was also still touching the item when he popped his claws, (to destroy it) making it reasonable to assume he either deliberately or subconsciously used its power to re-adamantiumize his skeleton.
During the Civil War Saga, he was immolated down to his skeleton and regenerated. Later comics apparently had him trade in some of his healing factor to return from the dead.
Deadpool also has regenerative properties. Unfortunately for him, it causes his cancer cells to regenerate rapidly too, causing him to be horribly disfigured and, well, not exactly sane.
Slippery B'eeef, the villain of The Authority: Kev had this ability.
Dr. Manhattan in Watchmen puts all these guys to shame. As Jon Osterman, he got disintegrated by radiation, to the point that they had nothing to bury. Somehow his consciousness survived and, in a few months, mastered command of matter to the point that he could remake himself from essentially nothing. Later, he is completely disintegrated again, and is back (and huge) in 5 minutes.
Doctor Manhattan: I'm disappointed in you, Adrian. I'm very disappointed. Reassembling myself was the first trick I learned. It didn't kill Osterman. Did you really think it would kill me?.
The comic book character The Sentry has been able to unwillingly and unconsciously regenerate his entire being from nothing.
Daredevil of Earth X can do this—including ending up as many copies of himself after being torn apart by a mob. He even makes it into a circus act in which people try every method imaginable to kill him. Too bad for him he's a Death Seeker.
Swamp Thing once lost an arm, which grew back (for the first time in his book). Later, he met a mindless clone of himself which had grown from the lost arm. During the later Alan Moore run of Swamp Thing, he could dissolve his body at will and re-appear from any form of plant life whatsoever, anywhere in the world. (He later got exiled into space with help from Lex Luthor, though, but he managed to find a way to return.)
Modular Man from Tom Strong isn't quite at this level- he is, technically, mortal, and can be killed- but functionally he's identical: if even one of his modules exists, he can use it to make more. And he posted the plans for free download all over the internet...
Doomsday, The One Who Killed Superman, regenerated from being reduced to a skeleton. Lex Luthor helped by pouring some of Superman's blood on the bones to give him some raw material to work with.
It was later revealed in Doomsday's back story that he was created with the ability to die and then evolve an immunity to what ever killed him upon resurrection.
Let's not forget Mister Sinister. As an aside, quite apart from this healing factor he has back-up plans in the event that someone does manage to kill him, namely that he secretly experimented on several characters in their childhoods (including Juggernaut, Professor X and Sebastian Shaw) to allow his mind to be transferred to them telepathically while their DNA is overwritten with his. So even if you stop him returning from a single cell, he'll still come back. Not helped by the fact that all three of those characters are quite hard to kill anyway, and one is nigh-invincible.
In the Silver Age, The Flash (Barry Allen) was a rare heroic example, as he was able to reconstitute himself from extreme fates such as being vaporized, apparently due to his complete control over his molecules. The one thing he couldn't save himself from, however, was being converted into energy in an epic Heroic Sacrifice.
Hulk's Maestro incarnation was capable of regenerating from dust.
In the Godzilla (2014) tie-in comic Godzilla: Awakening, Shinomura's body is composed of thousands of individual cells that will grow into a new Shinomura if left alone.
Dark King Joe Dark and Chaos Alex both utilize this trope in Clash of the Elements, but it is stated in-story that the best way to counter their regeneration is to burn their cells faster than they can restore them, which Alex Whiter does with his Blazing Overlord Fist
The Dark World Mane Six in the Pony POV Series due to their Elements of Chaos (which Discord replaced their hearts with) giving them Complete Immortality so long as the Elements aren't destroyed or forcibly removed from their body while they're whole. Unusually, the ones that have turned good keep the power. Unfortunately, most of their enemies have the same ability if not better and the one that didn't knew how to deal with it and had something nearly as good, so it's not as big of an advantage as it'd otherwise be.
The Alicorns and Draconequi both are capable of surviving and healing from unholy amounts of damage. It would seem nothing short of being vaporized can actually kill one. And even then, their natural forms are spirits that can live on past death, though killing them does seem to prevent them from just returning to the mortal plane. The only exception was Discord, who was shattered after being turned to stone. A fail-safe Havoc put in place allowed his spirit to remain in the mortal plane and possess a pony, whom he used his powers on to turn into a clone of his original body. Even then, it takes a massive amount of damage or special weapons to actually pull off under most circumstances.
One of the more dangerous qualities of The Thing (1982) (the shape-shifting Body Horror one, not the space carrot), provided it has biomass to feed on. The kicker? It only needs a single cell to consume you from the inside out.
Species: At one point a single cell is injected with the alien genetic material. It immediately starts dividing and growing, but is killed before it finishes turning into one of the Sil-type monsters. At the end of the movie it's implied that any part of one of the monsters can regenerate into a whole.
This is how the title monster in the B-movie Reptilicus is created. A dinosaur tail, found frozen, regenerates into a full dinosaur once it thawed. It then goes on the rampage in Denmark. In the end, it gets put to sleep where they can then Kill It with Fire. Almost. A foot survived destruction, implying that it will regenerate and return again.
In The Fifth Element, the Action Girl protagonist Leeloo is regenerated from scratch in a rather cool sequence near the beginning of the film.
The doctor who restores her practically quotes this trope when explaining the process, saying "A couple of cells are still alive. It's more than I need."
In the "Sorcerer's Apprentice" segment of Fantasia, Mickey Mouse can't stop the enchanted broom from flooding the room with water, so he chops it to bits. Unfortunately, each of those bits turns into a broom - complete with buckets already filled with water! - ready to continue the job.
At the end of The Spirit, it is implied that the Octopus, who has just been blown up all over an alleyway, will be able to regrow himself from a single finger.
The immortals of the Kage Baker Company series of books, well...it takes a LOT of destruction to get any of them to the point where they can't be brought back.
The traditional way to kill vampires in western literature involves staking, beheading, then burying at a crossroad at midnight with garlic in their mouths. They can still come back. Usually, the only way to truly kill them is to burn the body and scatter the ashes to prevent them from reforming, usually into moving water.
Discworld jokes about the various and complicated methods of killing vampires by having vampires from different areas have different weaknesses. Furthermore, all vampires on the Disc can be brought back to life with a drop of blood - even if they were burnt to ashes. In fact, one vampire is a photographer for Ankh-Morporkh's newspaper and gets turned to ash every time he uses the flash. He carries around blood in a breakable vial so he instantly gets revived when this happens. Consequentially, one group of particularly bad vampires is threatened with having their ashes thrown over the Rim into outer space.
Margaret Weis and Tracy Hickman had monsters that could do this in The Death Gate Cycle. You had to stab a "chaodyn" in the heart, because if not they would heal and the blood that had dripped out of the wound would spawn a second chaodyn. (Killing them without spilling blood worked too.)
Blessed with Suck example from Orson Scott Card: Planet of Treason. One of the families exiled to an isolated world has genetically-engineered themselves to a state in which they can regenerate organs, so that they can sell them to eventually buy their way home. Occasionally a mutant like the protagonist appears whose regenerative abilities approach this level — but he also grows extraneous body parts that eventually cripple, then kill him. At one point, his guts are ripped out, producing a feral copy.
The book Jason X The Experiment has Jason Voorhees coming back after being reduced to little more than a hunk of flesh due to being incinerated during atmospheric reentry. It takes him a couple years (even with help from nanomachines) though.
The Animorphs frequently make use of the morphing power's healing properties. Their fights often go like this: Attack; get torn to shreds while decimating the enemy's defenses; de- and remorph; carry out the mission. Rachel in particular seems to lose and regrow limbs on a regular basis.
Their regeneration ability becomes a major plot point in one book where Rachel morphs into a starfish and gets cut in half. She returns to her human form, but so does her other half, creating split personality Rachels, a decent, kind-hearted girl, and a total ruthless bitch.
Greg Bear's Blood Music takes a different spin on this. The noocytes are able to encode the squishy chemical versions of people into molecular memories based on DNA/RNA. The noocytes can rebuild and upload the original person - or an amalgam of anyone who wants to participate - back into a squishy flesh body should they so wish. After encoding and absorbing almost all biomass on North America, there are uncountable replicas of everyone to the point that there is no possible way to erase every copy.
One of the Star Trek: The Next Generation novels mentioned a race that fought a brief war with the Federation over this trope. Because they were effectively immortal, one of their more revered ways of learning things was by dying, because you usually at least learn "That was a bad idea." Because they thought every race had the same trait, they didn't realize that they weren't doing the Federation any favors by killing them.
In On Stranger Tides by Tim Powers, Blackbeard could regenerate from even the smallest part of himself, even a drop of blood, so long as it fell into the ocean.
Live Action TV
Kolchak: The Night Stalker (1974) episode "Primal Scream". An ancient cell sample discovered in the Arctic regenerates into a murderous ape man.
Grim reapers in Dead Like Me are effectively impossible to kill, being already dead. In the movie the cast try to kill a renegade reaper by shooting, drowning, and finally dismemberment followed by cremation and launching the ashes into space.
Jack King in Reaper could form a new self from just a hand. Possibly less, we never saw him try.
Ultraman Mebius has the villain Imperizer who have to, at first, have his entire body vaporized to kill it, otherwise, it would regenerate. Later it is toned down to destroying a certain part of it to prevent regeneration, but it is still Nigh Invulnerable beforehand. Worst part, they are the Big Bad's Mass Produced Minions.
The Replicators in Stargate SG-1 come close. It only takes one active Replicator to make an entire swarm. Fortunately, they are limited by local resources, and there need to be at least enough active blocks to form a working entity. They can't actually regenerate from a single cell.
The Doctor Who villain Eldrad first appeared as a fossilized hand, then having absorbed some nuclear radiation, turned into a walking hand. Eventually it regenerated into an entire person.
In a very similar vein, there was a time when the Doctor himself had a previously-severed hand put away somewhere. Then, when Ten tried to stop his own regeneration, he shunted the regeneration energy into said severed hand. The hand then grew a not-so-clone of Ten (he was more Human than Time Lord). It should be noted that the Doctor himself averts the trope. While he can regenerate from fatal damage, there are known limits to this ability and there are a number of ways to outright kill a Time Lord.
Captain Jack in Torchwood is not invincible, but always comes back to life after being killed, no matter how severe the damage. Even when in Children of Earth he is torn to shreds by a bomb planted inside his chest, and the only thing remaining of him is an arm, a shoulder, and part of his head, he regenerates fully within a few hours. He is conscious for most of it and it is very painful.
GoGo Sentai Boukenger: The helmet of Creator King Ryuuwon is actually a Clingy MacGuffin that granted Ryuuwon immortality as a humanoid reptile, as well as the power to survive falling off a high cliff, getting blown up while piloting his dinosaur-mechs two, maybe three times, and suicide. However, this power fails Ryuuwon when Bouken Red destroys it during the two's final duel, causing Ryuuwon to revert back to his original human form.
Space: 1999 episode End of Eternity features Balor, who is imprisoned forever on an asteroid by his immortal race. At the start of the episode he quickly recovers from horrific explosion and rockfall injuries. At the end, he's blown out onto the Lunar surface (or into space - the episode glosses over this). The question of what happens to a From A Single Cell individual left in a vacuum is never addressed.
The Supersoldiers in The X-Files. Even after being crushed and mashed to pieces, they can regenerate from a single vertebra.
A similar version appears in one of the more thorough methods of disposing of vampires. When you burn the body after staking it, chopping off its head and stuffing its mouth full of holy wafers, you have to drive back any rats or beetles that scurry out of the fire. It's not entirely clear what would happen if it did get away though.
In Hindu Mythology, the demon Raktabij could birth a new form from every drop of blood. Kali solved this by having the demons strangled. Part of the origin myth of the Thuggee cult.
An alternative version is she cut off his head, then drank all the blood that flowed out.
The Hydra in Greek Mythology. Cut off one head and two grow back unless cauterized with fire.
Any Dungeons & Dragons creature with regeneration works this way unless you take its Hit Points to -10 with whatever form of damage it can't regenerate (that's when a creature is officially considered Killed— -1 to -9 HP is only considered Dying). A joke D&D item exists called "Troll in a Can" that uses this trope to turn a chunk of trollflesh into self-regenerating field rations. Wizards of the Coast ruled that eating a troll will eventually cause a new troll to grow inside you and EXPLODE OUT OF YOUR BODY. (Unless it's well done on a bonfire, since trolls do not regenerate fire damage and can be safely killed by it.)
The tarrasque has this ability. The only way to defeat it is to beat it into unconsciousness and then use the most powerful spell in the game to make sure it doesn't wake up. (Good thing it sleeps 99% of the time.)
And as of Fourth Edition, the Monster Manual implies that the only real way to kill a tarrasque for good is to send it into space. Reducing its HP to 0 just sends it back to slumber beneath the earth.
Second Edition required the tarrasque to be reduced to -50 hit points, requiring the use of +1 or better magical weapons, then a Wish spell or equivalent to be used wherein you wished for the tarrasque to never return. Depending on your DM at the time, sending it into space may be a lot easier.
Mortasheen zombies work this way, given their absurdly powerful healing factor and the fact that their consciousness is spread out all over their body.
A similar abilty is possessed by the Hosts of Werewolf: The Forsaken. These are bizarre hybrid spirit creatures (most commonly the Azlu and the Beshilu) who are able to either take on vaguely human forms or hide inside human skins. If a Host is destroyed, its body will shatter into dozens or hundreds of its animal form (which will look like mundane animals), one of which contains its soul. They all need to be destroyed to be safe or else they will eventually consume enough to regrow into their monstrous forms (and usually be pissed at whatever killed them).
In World of Darkness: Immortals, we have the Purified, who have this as one of their main abilities. You see, they've turned their souls into spirits, and like the Hosts above, just killing one's body simply sends his mind to the Shadow, where they can rebuild it even if it was completely destroyed. They're a perfectly viable character option (far more so than the blood bathers and the body thieves), and this ability is mitigated by the fact that the Spirit World is rather dangerous. That still means a halfway competent Purified is going to be alive for a very long time (this is a book called Immortals after all), but it's something of a weakness.
The Ananasi and Kumo (werespiders) in Werewolf: The Apocalypse could break into swarms of normal-sized spiders (called the Crawlerling form). If a single spider from the swarm survived, it could eventually hunt enough to increase its mass to the point that it could resume human form. (Unfortunately, the Ananasi in such a situation would probably lose almost his entire memory...)
In GURPS, this is the 100-point level of Unkillable. For just 50 points more, you don't even need that one cell.
While the Necrons of Warhammer 40,000 don't have cells, being automatons, they fit the spirit of this trope, able to regenerate or be repaired from any damage whatsoever. Well, at least originally; newer versions seem to have Nerfed that ability significantly.
Tyranids are this trope on an interstellar scale, when considering that their Hive Fleets and Hive Mind are really just one big body. If even one vessel manages to survive and reaches an ill-defended planet or three, they can use the biomass to rebuild the entire fleet.
The latest incarnation of the Dark Eldar Codex indicates that Haemonculi can regrow people from small amounts of flesh, leading to many Dark Eldar leaving a bit of skin or a finger in a Haemonculus' care when heading out on raids to act as insurance should they perish.
Also, the Perpetuals. They don't even need an entire, intact cell; they can regenerate from complete and total atomization. The only Perpetual to have canonically died is Ollanius Pius, on account of Horus' Cessation of Existence attack, but even that much has the fandom going back and forth on the He's Just Hiding option.
On a larger scale, we have the Phyrexians. As long as a single drop of glistening oil remains, they can be reborn continuously, forever.
Chimeras - Lunar Exalted who have gone bat-shit insane - can learn this. The Knack is called "Perfect Regeneration", and also solves the question about "which part regenerates"; the Chimera's spirit will always occupy the largest remaining part. This lends itself to some interesting ideas about how you could permanently kill it with relatively mundane methods, but since we're talking about an Essence 6Axecrazy shape-shifter-slash-titan-killing-weapon-gone-rogue, it might just be easier to pop a Total Annihilation on it...
And one of the Lunar Exalted's most characteristic traits is their ability to tank obscene amounts of damage, so you had better be ready to pop another half-dozen different WMD-level spells, just to be sure - they also excel at adapting, so just spamming one is unlikely to work too good. And it might still take cover in an otherwise inaccessible pocket dimension, or space-warp into the Wyld, or... You get the idea.
Xenosaga's Albedo Piazolla may be the single most indestructible example on this page. It takes a Cosmic Entity to kill him permanently, and even then, he comes back. The third game ends with him asleep in his twin brother's subconscious mind, with the implication that this might not be a permanent arrangement...
One of the hardest bosses in Xenogears is Miang Hawwa's Omnigear Opiomorph, which had been transfigured from Organic Technology into essentially Grey Goo by the game's Big Bad Krelian. After the party beats it, it promptly snaps back up to fighting position like nothing even happened to it. Cue Miang gloating about how the entire Gear can restore itself from a single atom if need be.
Touhou's Fujiwara no Mokou and Kaguya Houraisan. The Hourai Elixir they drank granted them immortality and the ability to come back to life even if their bodies were completely destroyed, and beyond that. Yuyuko has the ability to kill anything by simply inviting it to its death, and once attempted to use it on Mokou. She couldn't even find her death. Turns out the Hourai Elixir simply banishes their death at every point in their timeline, ensuring they can never die no matter what. Even if they could get someone to kill them before drank the Elixir, it would fail - their deaths would have been retroactively banished. It's just that good at averting death.
Marisa, being Marisa, lampshades it by asking what every one of us has always wondered: if we split either in half, which half will regenerate?
Fairies have the same level of regeneration ability in the series. However, they are idiot mooks subject to The Fog of Ages.
Jenova from Final Fantasy VII can apparently regenerate indefinitely so long as some cells still exist.
Which is what provided the retroactive Sequel Hook: Cloud (and others) are infected with Jenova cells, and thus Jenova and Sephiroth are not truly dead, but, in fact, possibly more powerful than ever.
Roa from Tsukihime can be like this, but only when the conditions are absolutely right (midnight during a full moon, as Arcueid found to her chagrin, when he regenerated from his ankles up).
Arcueid herself is able to construct an entirely new body after being sliced into seventeen pieces by Shiki near the beginning of the game.
It is explained at some point that in case of most powerful beings (like Shinso and Dead Apostle Ancestors) the wounds do not regenerate by normal means (i.e. cell division cranked Up to Eleven) but rather by reversing the flow of time around the missign body part (or even missing body, for the most powerful of them returning from the state of complete non-existance is just as simple) effectively making the wound to never existed. Of course most of those creatures also have abnormal Healing Factor just in case someone attacks them with means note like Conceptual Weapons that prevent recovery by time reversal.
The Bacterians from Gradius work like this. Every time you defeat them, they regenerate and come back stronger.
At the end of Prototype, Alex Mercer saved New York from being nuked by flying the nuke out over the ocean. Unfortunately, he didn't escape the blast, and a few mangled fragments of him wind up in Manhattan. A crow eats the pieces; which then eat the crow. Alex rebuilds himself from the inside out.
Every single enemy in Metroid Fusion works this way. Since they're all X-Parasites mimicking various creatures, they revert to the cellular form when defeated. The only way to kill that form is to absorb it (which only Samus, or a metroid, can do). If the cell form isn't absorbed, it recreates its body or infects the killer. Some can even spawn more cell forms, creating even more monsters. Needless to say, had anyone other than Samus been there, the universe would have been screwed.
Ridley also seems to have this, as his entire body routinely explodes, though Super Metroid was the canonical death of the original Ridley. A clone was made in Other M, which the X Parasites infected in Fusion.
The Metroid PrimeAKA Dark Samus also has this quality, suviving everything up to and including being vaporized and then caught in the destruction of an alternate dimension immediatelly afterward. The 100% completion ending of the first two games of the Prime trilogy basically just show the player that, yep, it's still alive and kicking.
The Flood of Halo are this trope on a much larger scale. From a single spore the Flood can rise again, and even if its Gravemind is destroyed, its communal memory remains.
Once Hard Mode hits in Terraria, the Corruption and the Hallow spread constantly. As long as one block of Ebonstone or Pearlstone exists within 4 blocks of a block of dirt or stone, they will continue to spread.
In the Powder Toy, most forms of 'exploding' life will regenerate no matter what you do.
Xykon from The Order of the Stick is able to regenerate from any injury as long as his Soul Jar still exists. After his first (and so far only) defeat, he was notably ably to regenerate his whole body from essentially nothing.
This is a standard ability of all Liches in DnD.
Schlock from Schlock Mercenary kind of does it here. When 80% of his body mass is incinerated in an explosion, the remaining mass is able to re-assemble into a sapient individual- but Schlock loses most of his mind in the process, (it's spread throughout his body) and regresses to a child-like state. It's implied that while his memories are gone for good, in time he'll eventually just grow up into an adult again.
Or he would if he hadn't Time Traveled back into the past and merged with his alternate-universe counterpart. Thereby also answering the question of what happens if he gets split in half.
In Problem Sleuth, Demonhead Mobster Kingpin's 2nd form had 3 health bars, all regenerating extremely quickly. However, if you knock out a health bar, it's gone for good. His 3rd form had enough bars to, from the center of the universe, physically break into hell and he could regenerate entire bars just about whenever he wanted to.
The SCP Foundation's SCP-682 is seemingly unkillable, though it's not for lack of trying on the Foundation's part. Various attempts at destroying it have reduced it to as little as 1% of its original body mass before it adapts and regenerates.
Darkwing, as a parody of the T-1000, in Ducktalez 6
As a result of meddling by a dark god, the trolls of Tales From My D&D Campaign can regrow from nothing more than their jawbone. Since the jawbone itself cannot be destroyed by any known force whatsoever, this renders trolls utterly unkillable.
Amazo in Justice League and Justice League Unlimited is made of Nanomachines, and thus he can reform from any injury that doesn't completely destroy him. However, this was subverted at first because he only gained the ability to do this after he copied the Martian Manhunter. Before that, a small explosive in his brain really would have killed him.
This trope was double-subverted in a later episode. In an Enemy Mine assault against Amazo, Lex Luthor and the League use the robot's blueprints to build a weapon capable of disabling its nanomachines. However, it's soon revealed that Amazo knows about this limitation, and had evolved itself beyond the nanomachines' tech threshold, rendering the weapon useless.
The Itchy and Scratchy parody of The Sorcerer's Apprentice has thousands of Itchys regenerating after Scratchy chops him up into fine dust. Unfortunately for Scratchy, they regenerated after he inhaled some of that dust.
Bushroot in Darkwing Duck, as a plant/duck/thing, can regenerate even if there are only tiny vines left of him. Through the whole series, he's been mulched at least twice.
This once happened to General Skarr on The Grim Adventures of Billy & Mandy when he became a plant like being after coming in contact with Grim's scythe. At the end he is blown through the air by an exploding lawn mower and is chopped to pieces by a helicopter and we see him slowly regenerate later.
In the SpongeBob SquarePants episode "Franken Doodle", said doodle recreates its body by redrawing itself with a magic pencil.
Danny Phantom has two dangerous enemies with this kind of power: Undergrowth and Nocturne. Apart from instant recovery from Danny's attacks, this trait is made apparent when Undergrowth managed to (offscreen) completely regrow his body after it was blown to nothing but a weed and Nocturne was able to pull himself back together after Danny froze him solid and shattered him.
The Big Bad of Generator Rex Van Kleiss is able to rebuild his entire body using the nanite-infused soil of his homeland. He did this near the end of the first episode after Rex smashed him to pieces.
In Ben 10, one of his original 10 forms, Ghostfreak, turns out to have this ability. Unfortunately, this means the mind of the original alien whose DNA sample is in the Omnitrix is present within that sample and capable of restoring himself. And he's not a good guy.
Sea squirts can literally regenerate from a single blood cell.
Some species of flatworms and starfish have impressive regenerative abilities, and although they can't grow back from a single cell, a single individual can survive being cut in half and both halves can independently develop into complete, separate individuals.
A few examples from the plant kingdom — Cultivars are specific clones of certain plants (such as a Fuji Apple, or a Red Dragon Venus Flytrap). You can't grow these by just making seed and hoping the new plant is identical to the parent — you have to cut off a piece and force the plant to spit out new roots.
Another plant example is Micropropagation — that is, taking a single cell of a host plant, putting it into some special goo (essentially extremely sugary jelly), and letting the cell clone itself a few thousand times, all through a process called Tissue Culture. It's difficult to do, being extremely expensive and time consuming... At least it was; nowadays you can do it in your kitchen. Isn't science grand?
Not to mention the innumerable weeds/invasive plants that just excel at that asexual propagation stuff. Any plant cell from an asexually-reproducing plant has the potential to become a new plant. Many a gardener have probably screamed Why Won't You Die? whilst uprooting creeping charlie on their hands and knees.
Caulerpa is an extreme example. It's normally a seaweed (technically a single-celled algae, making it a literal example of the trope) native to the Caribbean and a few other tropical seas. However a combination of a mutated variety capable of surviving in subtropical and temperate waters, a natural toxicity and the ability to grow back into a full plant from the smallest fragments has turned it into one of the most dangerously invasive aquatic specieson the planet. Because of its incredible regenerative ability, the only effective way found so far to combat a Caulerpa outbreak is to sterilize the entire outbreak area of all life and let the native life recolonize the resulting aquatic desert.
Sacculini carcini is a hellish parasite that combines properties of the Alien and the Puppet Masters or the Goa'uld. Its larval form attaches to a crab and injects just a few cells, which grow into a sluglike thing that lives in the crab's abdomen and grows a set of tendrils throughout the crab's body, including into its eyes. It controls the crab's behavior also, making it eat constantly to feed the parasite and making the crab care for the parasite's eggs. In a female it seems to be activating maternal behaviors already there, but male crabs take on the same behaviors when infected.
All totipotent cells, most commonly found at the start of an organism's development before mitosis gets too far under way. Prior to a certain point, splitting the developing zygote apart results in identical twins.
Sponges contain totipotent cells throughout their bodies when mature, potentially giving them the full version of this trope.
Nematodes. Not only can they regenerate from a single cell, they can eat themselves till being reduced to one cell if they don't find food, and then regenerate from that single cell left.
Cancer. If there's even just a few cancerous cells left in an individual, they'll continue to rampantly grow and divide and continue the disease. This is why metastasized cancers are so dangerous, it's extremely difficult to ensure that all the abnormal cells have been destroyed or removed, and that there aren't any more floating around in circulation somewhere.
Most notably the Hela cells, the first immortal human cell line. They were taken from Henrietta Lacks's cervical tumour in 1951 and they're still going. They will also invade and take over any other cells you happen to have growing about the lab - they've done millions of USD worth of damage that way, but it's sort of balanced out by also being responsible for every significant medical discovery in the past sixty years.
All single-celled organisms can technically do this, but it takes away the point when it was a single cell to begin with.