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Jedi: you are surrounded, your army is decimated; make peace with the Force now, for this is your final hour. But know that I, General Grievous, am not completely without mercy: I will grant you a warrior's death. Prepare.
In many works of fiction the main protagonist isn’t the only hero doing good out there. These other heroes might be recurring minor characters or part of The Order, and may have little impact on the central narrative, but they are heroes nonetheless, often the greatest warriors in the land, conquering evil without fear.
That is, until the Hero Killer comes along.
Exceptionally powerful, incredibly skilled, the Hero Killer has one purpose, which he pursues ruthlessly: kill the heroes. He will hunt down the greatest knights and he will slaughter them, one after the other. Even the noblest, most powerful heroes will quake in fearat the sound of his name.
What makes him so powerful? Maybe he knows how to exploit heroes' weaknesses. Maybe he was engineered for that very purpose. Maybe he is fuelled by exceptional determination. Regardless, he has earned a reputation for curb-stomping heroes regularly and reliably, often without exception.
Expect a No-Holds-Barred Beatdown and/or Curb-Stomp Battle should his prey be unprepared. Many works will have the heroes train or improve their skills throughout the story, fleeing or avoiding the Hero Killer until such point as the hero is finally ready to face him; even then, they're probably going to need everything they have to stand a chance. If the hero wins the fight, or gives the Hero Killer a run for his money, it may be a Moment Of Awesome.
Closely related to The Dreaded. This character usually invokes The Worf Effect and is often a grim reminder that Anyone Can Die. If the work was fairly light-hearted before this guy appeared, then he overlaps with Knight of Cerebus. He is very often the "Goliath" in a David Vs Goliath scenario. If he's very dogged in his pursuit of heroes he overlaps with Implacable Man. In video games, contrast That One Boss, which is when it is the player and not necessarily the characters that has difficulty beating an enemy.
When adding examples, keep in mind that a character does not necessarily qualify just because they happened to kill one or two heroic characters; they have to have a reputation for killing heroes, or have killed many heroes, or both, to qualify.
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Anime and Manga
D.Gray-Man: so far, Tyki Mikk has put Suman Dark in a Superpowered Evil Side that eventually killed him, Curb-Stomp Battle´d Daisya Barry before having his butterflies eat his heart, killed General Yeegar with the assistance of Road and put Allen in a state that is so close to death, his Innocence had to save him (His arm was already almost destroyed by overloading it trying to save Suman Dark, but Tyki found it neccesary to kill off Suman´s still alive body before his eyes, rip off his arm and disintegrate it, crushing what seems to be his innocence before his eyes and then having a teez eat a hole in his heart). and even when he looked defeated, he instead went into a One-Winged Angel form that took a new character gaining his innocence AND General Cross to show up for them to survive. Hero Killer indeed.
This even extends into Part 3, where he kills Kakyoin and Joseph (at least temporarily) and his Stand ability is so broken that it requires a Deus ex Machina to even stand a chance against him.
In part 3, Vanilla Ice, who makes his introduction by killing off Avdol (for real this time), and proceeding to kill the team's mascot, the loveable stand-wielding dog Iggy of all things.
In Part 5 Diavolo turns up,and justifies his name by killing Bucciarati, Abbachio, Narancia and almost killing Polnareff. That's half the good guy's team!
Part 6 brings us Father Enrico Pucci, who outdoes all hero killers, past and future by eliminating the entire cast of heroes, except for Emporio. He manages to kill Jotaro.
Near the end of Part 7, Steel Ball Run, we have president Funny Valentine. So far he's killed Wekapipo, Diego Brando, Hot Pants AND Gyro Zeppeli, the latter three of whom were thought to be characters with at least a certain degree of plot armor, and one of them being the Deuteragonist.
Seriously, JoJo's Bizarre Adventure could be renamed Hero Killer: The Series and no one would notice the change. The amount of heroic characters that have died makes one wonder if Araki has a We Have Reserves/Kill Them All mentality.
Scar from Fullmetal Alchemist is introduced in this fashion. He kills many high ranking alchemists, and shatters Ed's arm the first time they meet. The Homunculi never lose this title.
Wrath/Bradley. Of all the villains, he has the highest count of likeable characters killed (Dorchet, Loa, Martel, Greed I, Fuu, and Buccaneer, in addition to cutting of Lan Fan's arm, nearly killing Scar, and taking out most of the Briggs garrison and a tank); most people just run after seeing him.
In the 2003 anime version (which went in a very different direction from the manga) Scar retains his Hero Killer status for much of the show, and Kimblee, while a different character, still has this going for him. Envy is another very good example, murdering numerous named characters, dancing pirouettes around the heroes during combat, and ultimately killing Ed during the climax. If you see Envy, book it.
Whenever Ali Al-Saachez of Gundam 00 shows up, you can be sure there will be much asskicking towards the heroes involved. Towards the end of the first season, Ali kills off a total of three named pilots, capping off his killing spree by defeating The Hero of Celestial Being, Lockon Stratos, in mech combat.
By the second season, however, Ali Al-Saachez has been downgraded from Infinite effectiveness to only High, thank to his first defeat at the hands of his enraged mortal enemy, Setsuna F. Seiei. Eventually, his threat against humanity ended for eternity with his Karmic Death at the hands of the second Lockon Stratos.
When you think Gundam, THE Hero Killer by default is Yazan Gable. No doubt about it. Not only is he one of the few pilots who can threaten the life of Kamille Bidan, a fact that terrifies Kamille, but by the end of the series he's managed to kill Jamaican Danigan, Katz Kobayashi, Henken Bekkener, the entire crew of the Radish, Emma Sheen, and Reccoa Londe, than opens ZZ Gundam by cutting down Saegusa.
Yazan Gable expy Rakan Dahkaran of Mobile Suit Gundam ZZ also reaches this status. He's one of the few pilots who can regularly give Judau a run for his money, the protagonists typically flee rather than engage him, and he takes out both Mashymyre Cello and longtime Gundam veteran Hayato Kobayashi towards the end of the series.
Lord Darcia from Wolf's Rain becomes this in the final episodes.
Anytime Battle Royale's Kazuo Kiriyama shows up, everyone you are rooting for is going to die. Well, with the exception of Sugimura, the only character who is badass enough to survive this encounter. Twice.
Pretty much any incarnation of Astro Boy has Pluto, whose entire function is to destroy the most powerful robots on Earth.
The Female Titan from Attack on Titan. While ordinary Titans are quite a problem to deal with, this unique variant proves to be a literal case of More Deadly Than The Male as well as an Implacable Woman. She begins her rampage by slaughtering numerous experienced soldiers, and kills several in unnecessarily cruel ways such as spinning one poor soul to death after she catches his maneuver gear wires. But she's only getting started, and proceeds to slaughter the expert Titan-killers of the Special Operations Squad one by one. In the end, the best the surviving heroes can hope to do is slow her down long enough to escape with their lives.
Neferpitou the first and strongest of the Chimera Ant Royal Guards from Hunter × Hunter. In his debut, Neferpitou killed Kite just to test his strength. Later, he forced Gon to invoke a Deadly Upgrade to defeat him, an act that nearly cost Gon his life.
Meryuem, the Chimera Ant King, is the biggest threat that has appeared in the series. Netero dies in their fight.
Claymore has Priscilla, who in her debut kills the then-#1 Claymore Teresa and from then on proceeds to walk all over anyone and everyone whenever she gets serious.
As well as Rigaldo, who's very first action is to effortlessly kill numerous named characters during the Northern Campaign (including Jean, Flora and Undine) and forcing Clare to nearly awaken a second time in order to defeat him.
Darkseid. If any regular villain in the DCU can inspire dread and single-handedly turns his mere appearance into a major crossover, it's him.
Darkseid's Alternate Company EquivalentThanos. At one point he literally killed half the population of the universe including many superheroes. At another he literally killed the entire population of the universe including every superhero.
Doomsday had this, also in DC Comics, including having won a decisive Curb-Stomp Battle against Darkseid, after having killed Superman. That said, his repeated appearances and increasingly nerfed powers have left him decidedly less impressive, culminating with a recent battle wherein he was torn to shreds by dozens of completely untrained Kryptonians.
Ultron, in the Marvel Universe, a genocidal robot who has wiped out an entire nation on his own. Massively on his own, in fact; on that occasion, he turned himself into an army. And after he had killed everyone in the country, he turned their corpses into cybernetic combat drones to fight the heroes.
Later, he tried to pull the same stunt again, only this time he did it with a galaxy. He also cyber-enslaved a slew of cosmic superheroes - and villains, including the Space Knights of Galador, Gamora (the adopted daughter of Thanos), Drax the Destroyer, Xemnu the Titan, Shatterax, Ronan the Accuser, Nova-Prime and the Super-Adaptoid, several of which qualify as Hero Killers in their own right.
And recently, he returned in "Avengers" in such a way that the heroes - the most powerful hero team in the world, mind - basically were "Oh, no... not again!"
In the same vein as Galactus, the Celestials also scare the crap out of everyone whenever they show up. Fortunately, they don't go around eating planets left and right. Unfortunately, they have a nasty habit of "judging" worlds they have seeded life on (which is most of them), and executing them if they don't like what they see. Yes, they execute entire planets.
In the G.I. Joe Marvel comic book, a SAW Viper misunderstood orders and killed Doc, Thunder, Heavy Metal, and Crankcase. A subsequent vehicle chase led to the deaths of Breaker, Quick Kick, and Crazy Legs. Cobra Commander, who had meant for the Joes to be escorted to the border, was furious at the SAW Viper. The SAW Viper's response? "I just wasted more Joes today than your entire legions have accounted for in nine years!" Cobra Commander throws him a party in response.
The Anti-Monitor, who personally beat Supergirl to death, after having already eaten hundreds of universes, and went on to be responsible for a scad of other hero deaths. He's the standard by which DCU characters judge "evil" and "dangerous". In a mostly successful attempt to avoid Villain Decay, DC only used him three (and a half) times from 1985 to 2010, a full quarter century.
DC has tried to do this with Deathstroke. For the past few years he's been pushed as one of the top villains of the DCU, on a par with Lex Luthor and The Joker. He actually has managed to kill a couple of heroes, including Phantom Lady and the third Atom (and in the trailer for DC Universe Online, he almost kills Batman). Unfortunately, while his powerset (he's basically an evil Captain America with a healing factor thrown in/Deadpool with sanity) should make him utterly terrifying to normals and low-level metas, it's a little more difficult to accept him as a serious threat to the likes of Superman and Green Lantern. His victories against big-name heroes tend to depend on his opponent firmly grasping the Idiot Ball and refusing to let it go (e.g. Flash running straight onto his sword; Green Lantern deciding to swing punches at him instead of flying off and using a ring construct from a distance; or Superman apparently forgetting how his own powers work). Fans have joked that Deathstroke's secret power is a "jobber aura" that makes his opponents behave like morons.
To rub salt in the wound, originally Slade was Genre Savvy enough to AVOID facing superhumans. He just faced the Titans to complete the contract his deceased son had accepted.
Willy Pete, of Empowered, is a particularly nightmarish version. His name is a military term for white phosphorous, and his powers match the name. He's capable of causing an impact as powerful as a nuclear explosion, and generating fire that reaches temperatures as hot as the sun. His favorite pastime is skull-fucking people to death as he eats them, not always in that order, and not always before they're dead. Being a fire elemental, he doesn't need to eat. He just likes to. He goes out of his way to only kill D-List heroes and villains, not because he's weak, but because it makes people underestimate him. He likes being underestimated, as it makes people think he's a pushover. He's the recurring nightmare of Thug Boy, due to killing all of his friends. In Volume 5, he kills eight and a half capes in an instant, and then proceeds to destroy most of the d10, the Superhomeys' space station. He doesn't try for an instant during all of this. He's just that powerful.
In vol. six, they introduced another hero killer, Deathmonger - a superscience necromancer who has enslaved legions of dead heroes. All the supers are too scared to go after him, for fear of adding to his ranks. He's still not as frightening as Willy Pete. Wasn't this supposed to be a "sexy superhero comedy"?
In the Archie Comics' Sonic the Hedgehog, the current Eggman (actually an alternate version of the first one), was this, managing kill his version of Sonic and the Freedom fighters, and when he first made his big return he conquered Mobotropolis and forced the Mobians to retreat back to Knothole, effectively resetting the series status quo to before Robotnik Prime died, though since then he's gone back and forth between how much a threat he posed. A more extreme case is the alternate timeline where Knuckles became a new Enerjak. He became so powerful that he single handily conquered Mobius and defeated every hero and villain that tried to stop him, even Sonic becoming Super Sonic wasn't enough to stop him. Pretty much the only reason why the Freedom Fighters are around in his timeline when Silver shows up there is because he let the Freedom fighters roam around for his amusement.
From the Transformers franchise, we have Bludgeon, a literal Ninja Pirate Zombie Robot. (Seriously, he would have been a pirate in Transformers Animated) In most of his appearances, he's one of the most lethal Decepticon warriors - in the Marvel G1 comics, he eventually ascended to the position of Decepticon leader, and only lost because of The Last Autobot. In the IDW comics, he beat down fan-favorite Soundwave and his casette bots after his own minions, Iguanus and Bomb-Burst, were dispatched. In the Titan UK comics, his Movieverse incarnation is one of only TWO characters to be shown killing his enemies on-panel.
In Spider-Girl Roderick Kingsley, the original Hobgoblin, becomes this after coming out of retirement. He effortlessly beats down Spider-Girl and her allies with minimal help, kills the Venom symbiote, wrecks Black Tarantula's organisation, and gets away.
Durge in the Star Wars: Clone Wars comics and the television show. The most prolific slaugtherer of Jedi prior to Grievous' arrival, Durge killed god knows how many named characters, came frighteningly close to murdering Anakin and Obi-Wan on several occasions, and had to be fired into the freaking sun before he finally went down.
Black Masktried to be this In-Universe, but he wasn't incredibly successful, managing to kill C-lister Orpheus.
The Governor; Rick and his group are terrified of him, and whenever he shows up, there is a good chance people will die. He's responsible for the deaths of Axel, Billy, Lori, and Judith. In addition, he personally kills Tyreese, Hershel, Patricia, and Alice, and cuts off Rick's right hand.]
Also from The Walking Dead, Negan could be counted as one (at least in his earlier appearances—pre-"All Out War" where Rick and co. have learned just how dangerous he can be and have started taking the necessary precautions when dealing with him and his army).
Transformers: Last Stand of the Wreckers: While Phase Sixers are theoretically this (Black Shadow's kills were high, but only showed right before he gets Worfed, and Sixshot, while threatening, never really brings the threat of main character death to the stage), Overlord stands above them. From his very first appearance both Autobots and Decepticons fear him, and when he shows up, people die, by the end of the series, he's killed or ordered the deaths of over 60 bots including Skyquake, Kick-off, Wingblazer, Rotorstorm, and Spin-out. His reappearance in Transformers: More than Meets the Eye continues this trend, even with all of his weapons gone, he still levels the crew and is responsible for six deaths.
In the Pony POV Series, Grogar is this (though he's only appeared in person once). The one time we see him onscreen, Patchruns away from him. Keep in mind that Patch, by this point, has overthrown a number of Evil Overlords who were using the Shards of the Rainbow of Light for evil ends and fought a full grown dragon with nothing but a sword, but runs away from a straight up fight with Grogar. Even though she beats him, he still left her with third degree burns and a limp for the rest of her life. In Dark World he returned, and managed to take on Discord head to head and fair well enough to force an Enemy Mine between Discord and the Changelings. Discord wins, but Grogar is not to be trifled with.
Nightmare Eclipse/Paradox is this in Dark World as well. Literally, as she's killed the mane cast sans Twilight (who she needs alive) several million times either directly or indirectly. The only reason the mane cast stand a chance against her is Rancor's intervention being a Spanner in the Works, Rarity having all six Elements of Chaos, Rainbow Dash, the Princesses, and Minty Pie arriving to help, and Discord giving Twilight memories of the previous loop, making Paradox unsure of what to do since she doesn't know what memories she has.
Clover's Gaiden story has its de facto Big BadThe Master (not that one, who also exists in this series), who ends up killing Clover's party member Slipknot/Scout, and nearly kills Clover herself if not for the accidentally intervention of the Dragon Prince Consort Bahumut.
In Mega Man: Defender of the Human Race, Bass inspires fear in Mega Man, and manages to give both him and ProtoMan a good thrashing before barely losing or leaving due to outside circumstances.
Wily later builds Enker, Punk, and Ballade, the Mega Man Killers. They live up to their name; in-fighting is the only reason they don't succeed in their first encounter with Mega Man.
Later those two forces fight. Bass wins, and Mega Man is even more afraid.
In The Immortal Game, one of the first things King Titan does is to kill Princess Celestia. Actually he just beats her within an inch of her life and gives her to Terra to torture, but he could full well have killed her. He is such a terrifying force that even his Puppet Avatars are enough to dominate Princess Luna.
Syndrome and his Omnidroid from The Incredibles are sort of a Hero-Killing team; Syndrome builds the robot, lures supers to his island, and repairs and upgrades the robot whenever it is defeated, while the Omnidroid does all of the actual killing as it was programmed. Syndrome does this to eliminate competing supers so that he can exploit built-in weaknesses to publicly defeat the robot and look like a hero in the eyes of the people, but the Omnidroid figures out how to defeat Syndrome and tries to kill him too.
Bryagh, Ommadon's second in command from The Flight of Dragons is dreaded by even Sir Orrin and manages to literally kill off almost the entire Five-Man Band, sparing only Peter because he wasn't there. Granted, they get better, but that does little to change it.
The Dark Knight has a straight version of this trope in the form of The Joker. He outright murders former GCPD officer Brian Douglass (one of the vigilantes of Citizen For Batman), kills several known Gotham City cops and other law enforcement including Commissioner Loeb and Rachel Dawes, and finally causes Dent to slid off the slope of sanity, leading to several more deaths, including Dent's own death.
In Bodyguards and Assassins the final assassin is a highly skilled martial artist before whom all the bodyguards, all capable fighters in their own right, can only desperately throw themselves in his way in an attempt to buy time.
In X-Men Origins: Wolverine, Victor Creed murders about 80% of the cast. His resume includes: Wolverine's Only Friend John Wraith, Wade Wilson (though Stryker somewhat brings him back brainwashed), Silverfox, twice. Bradley and finally Blob (at least that's what's implied by his dialogue with Wraith). Meaning the only main cast members he never killed were Wolverine, Gambit, Agent Zero and Stryker.
In X-Men: Days of Future Past, Future Sentinels are downright unstoppable; even a combined team of half-a-dozen really powerful mutants can't defeat them. They kill most of the characters in the Bad Future, some of them more than once, and the only way to escape from them is to run before they even find you.
In Highlander, the Kurgan hunts other immortals to steal their power. All of the immortals are potential hero killers, but the Kurgan stands out among them.
The Forsaken from The Wheel of Time start out this way, before the Villain Decay sinks in. Moiraine estimated any one of them could sense magic being used within a couple of miles, and tear the user apart without even blinking. As the series wears on, most of them get picked off with increasing ease. However, at the Last Battle, Demandred very much plays this one straight, as does Graendal to a lesser extent. Demandred even calls himself "the dragonslayer", which is the series terminology basically translates as "Hero Killer".
The Mirror from Day Watch, who killed Tiger Cub by literally stripping her flesh from her bones after incapacitating several other high-level Night Watch magicians.
Nicodemus from The Dresden Files. He has killed at least a hundred Knights of the Cross and only two have ever gotten away with their lives, and of those, one was permanently crippled.
The one who escaped without being permanently crippled so angered Nicodemus that even after Nicodemus had him tortured to death Nicodemus was willing to give the good guys a chance to stop his plan to turn the Archive (a repository of all humanities knowledge, and thus possessor of the launch codes to every nuke on earth among other things) into a Denarian just to get the guy's sword, Fidellachius.
If you are face to face with Lord Voldemort and your name isn't Albus Dumbledore, you either run like hell or kiss your butt goodbye.
Bellatrix Lestrange later became a Dragon form of this with several significant deaths (Sirius, Dobby and Tonks) under her belt before she was taken out.
A lesser one can be found with Antonin Dolohov, one of Voldemort's tougher underlings. In the first war, he was the Death Eater who slew Gideon and Fabian Prewett, who were both powerful and accomplished wizards. In the second, he seriously injured Mad-Eye Moody and almost kills Hermione in Order of the Phoenix and is, according to Word of God, the person who slew Lupin in Deathly Hallows.
Whatever enemy that Eragon fights in the climax of each book tends to turn into this. In two instances, he's needed help in Brisingr, the Shade he fights in the climax not only overpowers him at first, but also Saphira and Arya, the other case, enemy in question used to his friend and let him live. In universe, the Ra'Zak and Shades as a whole are built as this, with the Ra'Zak acting as Galbatorix's personal assassins, and Shades, Brom states that prior to the series, there were only two instances of somebody killing one and surviving.
And the Ra'zac, who are The Empire's Rider-killers. It turns out that their primary prey is humans, and they have a number of abilities to that effect. Pretty much the perfect weapon for sending after young Riders, or anyone else who Galbatorix doesn't like.
Murtagh starts entering this role in the second book, especially at the climax, where he slaughters the dwarf king and his magician bodyguards with a single spell before soundly trashing Eragon and Saphira, but letting them go in a moment of mercy.
The Sword of Truth has some examples. The Dreamwalker, a weapon from the Great War, has power over someone directly proportional to their magical talent, with a few exceptions. He can brutally mind-rape (in every possible sense of the expression) sorceresses and (off-screen wizards), but he can barely touch any of the muggles (who all have some tiny amount of magical power anyway, only a few people in the world are truly devoid of any magic at all).
Ship Breaker: Nailer's father, Richard Lopez, turns out to be one of these. A sociopathic ex-gladiator with a serious drug addiction, Richard doesn't seem that dangerous as first. However, after stepping into the role of main antagonist he proves himself exceptionally deadly. The rest of the adults at the beach are terrified of him, Tool, a Half-Human Hybrid bred for war describes Richard as the "perfect killer" and would prefer not to face him, and in their final confrontation, Richard walks all over Nailer, cutting him to ribbons and gloating as he does.
More than a dozen show up across The Black Company. The first book has the Ten Who Were Taken, plus the Lady and the Dominator, with the Shadowmasters and the Voroshk turning up later. Of particular note:
The Limper kills literally thousands of extras, as well as Bomanz and Silent.
Shapeshifter killed Tom-Tom at the start of the series.
The Dominator kills so many Company Brothers that the book ends with the Company's roster at seven members.
Lisa Bowalk finally kills One-Eye for his involvement in her mentor, Shapeshifter's, death, after more than twenty years of feuding.
The Goddess Kina Herself kills Goblin personally and millions of others by proxy.
The First Law trilogy contains the aptly named Fenris the Feared. After radiating menace before the Open Council in The Blade Itself, he proves his status as a member of this trope toward the end of Before They Are Hanged, killing one of the most heroic characters in the series, Rudd Threetrees.
Kerrigor in Sabriel is the most powerful Greater Dead in the history of the Old Kingdom and has spent the last several centuries waging a war of attrition against the Abhorsens, the lineage of necromancers tasked with laying the undead to rest. He spent most of that time winning because he'd turned his mortal body into a Soul Jar so that he could never be fully banished while it existed, and also wiped out the Royal family except for one member and threw the Kingdom into chaos while he was at it. In the end, he's defeated only after a grueling battle that nearly kills Sabriel and Touchstone and even then, he's only turned into a Sealed Evil in a Can, not destroyed.
An interesting in-universe example occurs in Anthony Ryan's novel Blood Song. The protagonist, Vaelin al Sorna, kills another country's champion during an invasion and is subsequently dubbed "Hope Killer" (the guy he killed was the avatar of hope for the entire nation) by the enemy.
The forces employed by Apex are deadly and really show why Apex is the best. Condor effortlessly defeats Wolf twice, only leaving him alive because his contract says so and he doesn't what to kill off a good opponent with potential. The Justice, an extremely dangerous mech with six barrels filled with shotgun shells, and its pilot, Victory, claim the lives of Andre, Caruso and Red. And finally, the nameless dragon at the end kills Bob, Filch, Murray, Ry and Blaze with Baron, another dragon, finishes of Roy.
The evil Priest Caleb from the end of the show. He delivers a No-Holds-Barred Beatdown to Buffy, Spike, Faith, and Kennedy, brutally breaks Rona's arm, kills two Potentials, and finally gouges out Xander's eye.
Spike himself has personally killed two Slayers, a fact he takes much pride in even after his Heel-Face Turn.
Angelus often invokes this, or at least threatens.
The Beast of the Apocalypse.
Glory. The Scoobies spend the entire season running from her in terror. And her actions indirectly lead to Buffy's death in the Season 5 finale.
Almost an entire season running in terror. Glory's Mind Rape of Tara is what initiated Willow's firstRoaring Rampage of Revenge. But the Scoobies still have to plan out literally every single step they take during the final battle with the Hell-bitch (the overall strategy of which amounts to "desperately try to keep her busy without getting killed for a few minutes").
Vampire Xander kills Wishverse Angel.
The Masterkills Buffy herself. Twice, once in the real world, and once in the Wishverse. And supplementary materials reveal that in the past he's killed one Slayer and sired another.
Jubal Early the Bounty Hunter in Firefly. He shows up without warning, methodically making his way through the ship, taking down everyone in his path with little difficulty, and makes it perfectly clear that he's not afraid to kill innocent people to get what he wants. He probably would've succeeded, or at least thinned out the cast, if he hadn't seriously misunderstood River; she was the only member of the crew he couldn't anticipate. On account of being psychic and crazy.
Sylar from Heroes, who besides being one of the most powerful characters in the show's mythology has also racked up by far the highest number of main character and supporting character kills.
Darkseid on Smallville was held up as one, but failed to impress in person. Zod might also count, and Lex has been headed in this direction since Season 7 or 8.
Though the only one to actually kill a hero was Slade. Of course, since it was Hawkman, who is kinda low in the power department (nothing like Clark's Combo Platter Powers or Green Arrow's wide variety of weaponry) none of the heavy-hitters in this series exactly come to dread him.
Icicle Jr., Adaptational Badass extreme, manages to be much more kill-tastic against (admittedly out of practice) JSA members.
Inverted on Robin Hood. As the Sheriff's Dragon no one was even remotely afraid of Guy of Gisborne, least of all Marian who played him like a violin throughout the show. Robin and the rest of the outlaws ran rings around him - so no one saw it coming when at the end of season two he brutally stabbed Maid Marian to death.
John from The Fades. Before the other characters learn his name, he's known as the Angelic Killer because he's racked up such a high body count. In fact, he's responsible for the deaths of multiple major characters before the end of the first episode.
The demon, Meg Masters from Series/Supernatural is an example of this trope. In Season One alone she was able to come dangerously close to killing Sam, Dean, and John, kill two hunters, and captured John. Next season she killed another hunter and almost succeeded in killing Dean and Jo. She was later responsible for the deaths of Ellen and Jo. It is also worth noting that she was the first demon to successfully defeat an angel.
Lucifer could also count. During his single season on the show he was able to slaughter a group of pagan gods with ease and effortlessly killed Castiel, Bobby, and Gabriel (even though all three were brought back or turned out not to be dead later) and gave Dean arguably his worst beating on the show.
Older Than Feudalism: Typhon in Greek myth may well be the Ur Example. The child of Gaia and Tartarus, Typhon was, Depending on the Writer, a god with dozens of snake coils for tentacles, or a towering giant with 100 dragon heads. Either way, he stormed Olympus by himself, defeated Zeus (tearing out his sinews in the process) and drove all the other gods into hiding in Egypt. It was only in the rematch that Zeus was able to defeat him, and even then Typhon wasn't killed—just buried under a volcano, where he awaits the chance to break out again.
In Norse Mythology, any of the major players at Ragnarok, including Surtr, Nidhöggr, Fenrir, and Jörmungandr could qualify for this trope, given the sheer number of bodies that they leave behind. Fenrir and Jormungandr are likely the two best examples, as they are feared long before the apocalypse goes down, have established rivalries with the gods, and take the two strongest gods (Odin and Thor) with them when they go.
In Marvel's The Mighty Thor comics, Jormungandr retains this title. Anytime he shows up Thor is in for a very, very bad day.
Surtr is more or less described as one in the mythos proper: "At the end of the world he will go and wage war and defeat all the gods and burn the whole world with fire." He also personally kills Frey in the process.
Ivan Koloff: In 1971, the "Russian Bear" did what few people thought possible: defeat then-unbeatable WWWF World Champion Bruno Sammartino cleanly, in the center of the ring, on his home turf at New York City's Madison Square Garden. Koloff completely dominated the match and, after knocking the Living Legend down with a clothesline, dropped the top-rope knee on his prone body. Surprisingly, an exhausted Sammartino never attempted to lift his shoulders. The crowd sat in stunned silence as Koloff celebrated in the ring with his manager, the evil Captain Lou Albano … and rumor had it that an announcement that Koloff was the new champion was never made for fear of angry fans starting a riot.
The Iron Sheik: Easily defeated then-WWF hero Bob Backlund to win the WWF World Championship. Backlund, who had held the title for nearly six years, had been injured by the nefarious Sheik a few weeks earlier during a Persian club-lifting challenge, and when it came time for the title match, Backlund – who already had at least one nationally televised pinfall victory over the evil Iranian – was no match for the Sheik. Not to worry: a new hero was waiting in the wings … .
Hulk Hogan: Of course, the Hulkster had several wrestlers attempt to break him as well, most notably King Kong Bundy, André the Giant and Earthquake. Even though both Bundy and Earthquake injured Hogan, and André scored a controversial, Hogan always averted the trope … until one June day in 1993, when the Japanese sumo wrestler Yokozuna flattened Hogan, in dominating style, in his final match during his most famous WWF run; Hogan sold being burned in the face as he was led from the ring following his loss … and for nine years, that was the last image WWF viewers had of him – a broken, defeated hero with the villain and his stooge, Mr Fuji, gloating in the ring over their victory.
This was essentially the point of Randy Orton's "Legend Killer" gimmick; he severely injured several of WWE's most famous and legendary personalities, in some cases even forcing their retirement. He's recently been doing something similar with his head punt of DOOM, which has put several people on the injured/reserve list for months with concussions. It's wrestling, so you can't quite stretch Kayfabe to cover killing the faces, but Orton comes about as close as you can.
Mark Henry in his recent heel turn. He took out The Big Show and Kane by breaking their legs with a steel chair and a squash, leaving them out of action for weeks and a few months respectively. He even went to the point of even giving Randy Orton who was World Heavyweight Champion a hard time. The result at Night Of Champions? He becomes World Heavyweight Champion!
In the dying days of WCW, Scott Steiner was this, credited with putting Sting, Diamond Dallas Page, Kevin Nash, Booker T and Sid Vicious out of action by "injuring" them.note At NWA Clash of the Champions IX, November 15, 1989, Steiner botched a fallaway slam on Sid during the Steiner Brothers-Skyscrapers (Sid and Dan Spivey) match, resulting in a collapsed lung. Sid was out for months and was replaced by a young guy coming in from Memphis who was given the name "Mean" Mark Callous.
Brock Lesnar: 2014 is shaping up to be the former NCAA champion's greatest year in wrestling, and it's easy to see why. The Beast ended the Undertaker'sWrestleMania winning streak at 21, in arguably a hard-fought victory. But the greatest moment was yet to come, as he completely dismantled and dominated John Cena to win the WWE World Championship at SummerSlam 2014; Lesnar rarely, if ever sold any of Cena's offensive moves while crushing the now former champion with 16 consecutive suplexes. After both matches, Lesnar's manager, Paul Heyman, used Moment Of Awesome promos to put over Lesnar as a conquerer who cannot be beaten and that the fans' heroes are not only ineffective but inept in taking down the Conquerer.
Jasper Stone in Deadlands (the dude on the cover of the main book). He ends up specifically assigned to this role by the Big Bads.
It's only level 20, but the Heroslayer Hydra in 4e is meant to fill this role. It's designed to make "tank and spank" strategies useless or near-so by dishing out a lot of damage from multiple attacks, and getting a large damage bonus against anyone who "marks" it and tries to force it to attack them. The ability is appropriately enough called "heroslayer."
Mephistopheles from 4th and older editions of D&D. Other archdevils do as well, but Mephistopheles is the most visible.
If an entire dungeon can count, the classic Tomb of Horrors module and Forgotten Realms' Undermountain definitely belong on this page.
The Tarrasque was created as basically the single most powerful monster in the world that didn't live in another world. It was essentially Godzilla for D&D. It had resistance or immunity to a great deal of attacks and had a number of special rules that made characters have to jump through a variety of hoops to kill it and make it stay dead. In practice, however, players found a number of weaknesses, such as its lack of ranged attacks and vulnerability to energy draining, causing many hardcore fans to brag about the time they killed the Tarrasque.
Orcus was supposed to be this in 4th Edition, but this mostly led to people custom-building parties to beat him and then bragging about it.
Dragons also tend to be more powerful than their Challenge Rating would suggest, partly so that encounters with them would be more memorable.
Elminster, the iconic wizard of the Forgotten Realms setting, was not intended as an encounter for players, but because his official stats are available, many players try to pit themselves against him. Dragon magazine got tired of players writing in to tell them that they had slain Elminster, so it included a mass response saying that Elminster was way too smart to be killed and that he was playing a trick on the player characters.
This is the point of the 'character-killer' build for heroes in Warhammer and Warhammer 40,000. The Eldar Farseer's Mind War psychic power is a particularly good example.
Skulltaker from the Daemon books is quite literally made for this. The fantasy version has Killing Blow on 5+ and the 40k version wounds anything and inflicts Instant Death on 4+. Of course, a significant amount of characters are immune to Killing Blow or Instant Death, which reduces his utility somewhat.
The fantasy version also had Karnak, an who had the ability to mark an enemy unit and re-roll and failed hits or wounds against it.
The Blood Angels's Sanguinor has the ability to choose one enemy Independent Character and re-roll To Hit and To Wound dice in close combat against said character for the game's duration.
Also from the Blood Angels is Mephiston, who has an ability that forces an enemy character in contact with him to take a leadership test with a minus 4 modifier, and he gets to re-roll any failed To Hot and To Wound rolls for the round of combat if they fail, which coupled with the fact that he has Force Sword, means whatever he's fighting is likely to die in that unless it's not immune to instant death. Of course, Mephiston is strong that enemy characters will generally die against him regardless.
The 5th Edition Tyranid codex brings us the Swarmlord, a unique Hive Tyrant that the Hive Mind reincarnates when a hive fleet can't beat its prey through normal means. Not only is the thing a cunning strategist, it wields four energy-charged swords, and was capable of beating Marneus Calgar, Chapter Master of the Ultramarines, in hand to hand combat (given, Hive Tyrants are large, so that may not be as hard as it seems).
In game, the Swarmlord follows this in that is swords force an enemy to re-roll successful invulnerable saves and inflict instant death, though the Swarmlord is so expansive it's probably only worth using in an Apocalypse game.
6th edition codex Chaos Space Marines brings us the Murder Sword, nominate a target, when the bearer is in base to base contact with the target, the sword becomes double strength, ignores armor, and gains instant death. Wait for enemy player to challenge, swap the challengee with a model in base to base with the challenger, laugh manically while your daemon prince has a fun time
In-universe, Fulgrim. So far, canonically, including the Horus Heresy novels, he's killed one Primarch, came within inches of killing a second (admittedly one on his side), and is ultimately going to mortally wound a third. Any Traitor Primarch could qualify if the loyalist heroes they've butchered get counted, but Fulgrim goes the extra mile by being the one with the most Primarch blood on his hands.
Exalted features one of these in the corebook in the form of Octavian, a Second Circle Demon. For comparison, the Solar Exalted are designed to be bastions of good, the righteous god-kings of Creation, with all the corporeal and celestial power that comes with the office. Octavian carries three of their severed heads on his belt- and as a Second Circle, there's all the Third Circle Demons above Octavian even before you reach the Yozis...
Hilariously, due to the glitchy nature of early Second Edition, he's only a mild threat, and thanks to sorcery, it's quite possible he never actually gets as far as a fight-a comic has an impatient Arianna blast him with Adamant Circle Banishment, sending him screaming back to Malfeas. One assumes he doesn't consider being banished an actual loss, since his opponent cheated. A better example would be the Wyld Hunt, the organization of Dragon-Bloods devoted to, among other things, killing Celestial Exalts before they become a threat to the Realm.
This is also the primary role of Day Caste Abyssals and Scourge Caste Infernals. Unfortunately for their respective groups of Big Bads, the signature Day Caste has as his primary goal getting into Harmonious Jade's pants, and the signature Scourge is so arrogant he even annoys Adorjan.
The Black Knight from Fire Emblem: Path of Radiance. You could choose to fight him in the earlier levels where he shows up, but it suggests that you flee, and for a very good reason. Every single character you throw at him (yes, even Ike) will be unable to dent him and will soon find themselves dead in one blow. Only towards the end of the game does it let you know that you are ready to face him, but even then it's one hell of a challenge.
Orbital Frame Anubis is introduced at the end of Zone of the Enders: despite having just taken out The Dragon, it's made abundantly clear that Jehuty facing off against this thing would be suicide. You don't get a chance until the sequel, and the bulk of that game is spent finding a way to even the odds—the first few times you run into each other, your only real option is to avoid dying until you can escape. By the time you're ready to face it, it and its pilot have gone into One-Winged Angel mode. Of course, Naked Jehuty is also a One-Winged Angel of sorts, so it balances out.
Metroid Fusion adds a Survival Horror bent to the game by setting up fixed encounters with the SA-X, a malevolent Samus clone created from her parasite-possessed power suit. Most of these encounters can be avoided with stealth and caution, but if you attract the SA-X's attention - and unavoidably in one case - then your only option is to run for your life. The situation is exacerbated by the fact that Samus' Fusion Suit uses cold-vulnerable Metroid DNA, and the SA-X happens to have Samus' Ice Beam equipped. Eventually, Samus is able to go toe-to-toe with the SA-X, defeating both it and its Clipped Wing Angel form.
Dark Samus, especially Metroid Prime 3. The game introduces three other bounty hunters. By the end of the game, Dark Samus has killed and absorbed all three of them.
Barbatos Goetia from Tales of Destiny 2. The first and foremost thing he's known for is killing the prequel's hero Stahn and he's off to claim the heads of other heroes, namely Philia and Garr. His legend as a Hero Killer has even stretched to Tales of Vesperia, where he reappears as a bonus opponent in the battle arena and his name is Killer of Heroes. Indeed, it turns out that his entire purpose in life is to kill the heroes of the first game under the orders of the Big Bad to remake the timeline. Nothing less can be expected of one voiced by Norio Wakamoto.
Every time Vladimir Makarov appears in the flesh, without exception, a Player Character either dies or gets critically wounded, usually along with other important characters as well. He exceutes Allen at the end of "No Russian", he guns down Harkov when hijacking the President's plane, he sets up the ambush that kills both Kamarov and Soap, he shot Yuri in the gut during the flashback and he finished Yuri off at the end of the third game. Basically, anytime Makarov appears, it is bad news.
Though he doesn't appear when he does it, he detonates the nuke that kills Jackson and Vasquez in "Shock And Awe", and his chemical bomb kills the Davis Family
Nikita Dragovich in Call of Duty: Black Ops. One of his introduction in Reznov's flashback showing him killing Dimitri Petrenko, the Player Character of the previous game. Also his dragon, Kravchenko, tries to be one, but almost always foiled, even when he returned briefly in Black Ops 2, when he again, failed to kill Mason and Woods, even then he indirectly killed Bowman.
Raul Menendez in Call of Duty: Black Ops 2 is an even straighter example. Just count how many named characters he killed, more than half are playable.
Childrich of Suikoden V can also serve as one, particularly during the Final Duel against him. The player can elect to take along several other heroes with personal grudges against him, who then all try to call Leave Him to Me at once; you can select which one fights him... and they can die if he beats them, forcing somebody else to step in and take up the fight.
Nyx from Persona 3. Ryoji tells the heroes, at least half a dozen times, that "Nyx cannot be defeated." He's right. There is no way to defeat Nyx. The main character puts his entire being into an ultimate attack, effectively commiting suicide, just to make her leave.
That's because Nyx isn't so much an entity as the entire concept of death. As My Life Is A Goddamn Mess puts it "You can't defeat Nyx, much like you can't defeat the color blue."
Five words: "Be careful, I sense Death!" The Reaper will chase you down and annihilate you as early as the first floor if you spend too much time dawdling on a single floor in Tartarus. Even when you get to the end of the game, right up to the final boss, he'll only downgrade from an unbeatable foe to an extremely difficult one without either a serious amount of level grinding or the Armageddon fusion spell. And EVERYONE knows how monstrous the Reaper is, as evidenced by your allies special 'I just got in a fight on my own' lines, and Fuuka's standard pre-battle lines instead urging you to run like hell.
Lavos from Chrono Triggerkills the main character, Crono. It's entirely possible to proceed to beat the game without him, unless you carry out a sidequest to bring him back using the titular Chrono Trigger.
Alma in F.E.A.R.. It kind of helps that bullets do jack against ghosts.
Alma is a special case in that she's an accidental Hero Killer. In F.E.A.R. she just wants to HUG Point Man because he's her son but her presence is so lethal that even such a harmless action can kill. It's kind of heartbreaking - all she wants is a hug, but letting her do that will kill you, so you have no choice but to shove her off. And in Project Origin she's got a gigantic crush on Beckett, and wants to mate with him. That, incidentally, is also why at several points in the game she actively lashes out against anyone who's even looking at you funny. She ends up getting what she wants and nailing Beckett in the ending. He somehow survives the intimate contact, which probably means he's the most badass character in the series.
The Collectors in Mass Effect 2 can be seen as this, though it depends a lot on player choice and skill. From the first scene, you know they're bad news when they kill your character from the first game. In later story encounters they tend to be very difficult fights, and the team has to pull out a lot of stops just to survive; in the final mission against them, it's possible for everyone to die, and without excellent planning, at least one or two characters will die. In one case a Harbinger-controlled Collector deals the deathblow.
Mass Effect 3 gives us Kai Leng, who had previously established his credibility in the Mass Effect novels. Unfortunately his stealth-assassin skills don't translate well to direct confrontation, making him look less effective in the game proper.
The novelization takes it further and even discusses the trope. Not knowing it's actual name, Jill refers to it as the "S.T.A.R.S. killer".
Final Fantasy VII: Sephiroth. You spend most of the first disk following Sephiroth and observing dozens of bloody murders in his wake, including several key NP Cs, two impaled party member, and the Midgar Zolom hanging from a tree.
While he gets his actual significant kill in the game, it's really in the movie Advent Children that he shows the power backing up his claim for the title. As soon as he appears, he forces Cloud to stretch himself to twice as superhuman feats as he has so far (and that's saying something), while he himself doesn't even get out of breath. Especially in the extended version, where he beats Cloud within an inch of his life before the inevitable Heroic Second Wind kicks in. Cloud has already defeated opponents like the ridiculously powerful Remnants of Sephiroth and a gigantic dragon god, but his reaction upon first seeing Sephiroth can be described as "Eep!"
Final Fantasy VIII has the interesting case of X-ATM092. It seems invulnerable because it keeps repairing itself, and it is unbeatable the first time you fight it. The main characters can kill it off, but that takes time, and that timer on the screen is counting down to a game over.
Sin from Final Fantasy X. Cuts a swathe of destruction and leaves nothing but ruins in its wake wherever it treads. Still, that does not deter the many organizations that rise up to Spira's defense and try to defeat him. Just ask the Crusaders how well that went....
Witches in Left 4 Dead. Normal zombies, even in a horde, are fairly easy to survive; smokers and hunters are usually just an annoyance; tanks are deadly and very scary, but an effective team can take them down without getting hurt; but the only appropriate response to a witch is Don't Ask, Just Run.
As time goes by and people learned proper tactics for handling witches, serious Villain Decay set in. High level players outside of tournament matches have made a sport of killing witches in the most outrageous ways possible, such jumping over her, spinning around, and shooting her point blank with a shotgun for an instant kill. In competitive play, if something's going to kill an entire team of survivors, it will be a tank.
Tri-Edge/Azure Kite. The dude is just... plain... unstoppable. Pretty much every appearance he makes involves someone getting Data Drained and comatose. He's only defeated twice in the series, which barely slows him down at all.
Starcraft: Sarah Kerrigan as Queen of Blades. She has used her Swarm to kill six people with major roles, all of whom were badasses of their own rights, was the mastermind behind the deaths of two more and drove a last one into suicide after defeating him in despite being seriously outnumbered. The sequel also sets up the Hybrid to be this, although they hadn't yet has the chance to present this, but whenever the protagonists encounter them, they are a source of fear and can cause Non Standard Game Over by killing your hero.
Arthas Menethil, The Lich King, from Warcraft and World of Warcraft. He killed many named characters and many of them are badass in their own right.
Although it comes back to haunt him in a big (not to mention literal) way. His runeblade Frostmourne steals and entraps the souls of everyone it kills (oh, and his, too). At the climax of his encounter in Wrath of the Lich King Frostmourne is shattered by an attack and all the souls in question are freed — including his own.
Mephiles the Dark from the infamous Sonic the Hedgehog (2006). In the Last Story, he actually kills Sonic to get Princess Elise to cry, setting the chain of events that nearly destroyed time itself.
LeChuck from the Monkey Island series. Though he plunders, tortures and even kills hundreds upon hundreds of people (and even forms his own skeletal army from these corpses), his plans to rule the entire Caribbean keep getting thwarted upon many defeats (and deaths) at the hands of the main protagonist Guybrush Threepwood. It is not until Tales of Monkey Island that, after spreading his own Pox on the entire Gulf of Melange (while becoming human himself), LeChuck proceeds to kill bounty huntress Morgan LeFlay by stabbing her with her own blade in Chapter 4, and then saves Guybrush's life from the gallows... only to kill him with the Cutlass of Kaflu later on in the chapter when all the Pox is absorbed from the entire Gulf of Melange into the legendary Esponja Grande, which LeChuck then uses to reabsorb the Voodoo powers back into himself and then, while using the power of his hypnotized monkeys, to gain his powers from the Crossroads and become the Pirate God capable of unleashing destruction upon the entire Carribean.
Its even more apparent if you check the gravestones at the start of Chapter 5. As well as some Grave Humour, you'll find the names of several characters from previous chapters. It's heavily implied he killed them all after getting his powers back.
General RAAM from Gears of War. He kills Minh Young Kim in his very first cutscene, and then, in the RAAM's Shadow DLC, he offs Alicia Valera, and almost kills Jace Stratton, not to mention the countless Gears and Onyx Guards he murders.
The Dahaka in Prince of Persia: Warrior Within is a being whose only point in existence is to be this. When a being somehow avoids their own fate, in and of itself a rather admirable fate, a Dahaka is created to destroy them. The Prince can only defeat it with the Infinity+1 Sword; otherwise he cannot even harm the beast.
The Dahaka is so well known for this, a lesser-known band called Merkabah actually has a song about it, appropriately named: "Dahaka". A transcript of the Metal Growled opening (lyrics are difficult to retrieve on the internet, and no easier to understand): "Sometimes it was whispered; sometimes it was read: the secret incantation that filled with the enemy with dread. If you knew it, if you used it, you could reach back to [unsure]. But stop and [think?; if one can see it]: Sole Master Of Your Destiny You Would Be. Dahaka is released. Dahaka: the Guardian of the Timeline. [A sentence; possibly "Eternal pursuer for fiddling with time."] NO ONE ESCAPES THE DAHAKA."
Etrian Odyssey features an entire army of hero killers in the form of FO Es. Although they're not particularly important to the plot, they do get a textual introduction in each game warning the player not to foolishly try to fight them, which in this series is not an empty threat. Players that don't get the hint often mistake the first FOE they come across for a particularly nasty Early Bird Boss. (a mistake made easier by EO's general Early Game Hell) Avoiding them requires three things: the realization that you are supposed to avoid them, knowledge of their movement pattern and prayer that regular enemies won't slow you down as you run the hell away.
Grand Theft Auto IV:The Lost And Damned has an unusual example with Niko Bellic, who kills Johnny's friends Jason Michaels and Jim Fitzgerald, whose assassinations are missions in the original game! What makes this example strange, the first spoiler aside, is how Niko never antagonizes Johnny and acts as an ally in two missions (where, in their final mission together, Johnny remarks that he wouldn't want to piss off Niko, after witnessing him slaughter dozens of mobsters on his own) and Johnny and the Lost never find out what he did.
And then in Grand Theft Auto V, Johnny himself shows up when the game introduces Trevor. He lasts about a minute before Trevor hits him in the face with a bottle and, while he's on the ground, stomps his face in.
Nippon Ichi's games (including the Disgaea series and Makai Kingdom) has Overlord Baal, an evil conglomerate of souls who possess others' bodies to spread terror throughout the Netherworlds. He is almost always the most powerful enemy in each game, only reachable through a lot of leveling and workoing your way through secret options, and is not only very high level, but has stats that are jacked up far above what a normal character would have at that level, plus awesome equipment. In Makai Kingdom, he's even safely tucked into the absurdly overpowered Battleship Yoshitsuna, meaning you'll have to take that out before you can get a shot at Baal himself.
Borderlands 2 has Handsome Jack. One of the sidequests you get in the beginning of the game reveals that in-between games Jack killed Helena Pierce, a major NPC in the first game. Later on, he ends up killing Bloodwing and more importantly, Roland. There's also Wilhelm, who is stated to have been capable of defeating the Vault Hunters of the first game.
In Prototype, interestingly enough, the player character is the hero killer, for a given value of 'hero', that is. The game compensates through timed missions and zerg rushes
The Shivans in Freespace. They show up right out of nowhere in the middle of a war and get to effortlessly slaughtering both sides with their nigh-invulnerable ships, forcing their victims to band together just to survive. Though the resulting Alliance does score decisive victories against them, it's eventually revealed afterwards that those victories are nothing but small dents in the Shivans' sheer military might.
Vile in his first appearance in Mega Man X (though he suffers from Villain Decay in his subsequent appearances.) He starts out the game utterly trashing X in a Hopeless Boss Fight, requiring a Big Damn Heroes moment from Zero to drive him away. Near the end of the game, X has likely picked up tons of upgrades and gotten the weapons of all the other Mavericks...and he still can't beat Vile. In fact, this time Vile beats Zero beforehand, requiring Zero to pull a Heroic Sacrifice after X's second beating just to destroy his Ride Armor and give X a fighting chance.
The Stalker also qualifies, at least in the early game; a Tenno wearing a modified suit of Excalibur armor with unique weapons and determined hatred for their target. Rather than being a specific boss fight, The Stalker can just show up at any point in almost any mission if the player is a certain rank and has a 'mark' from killing a boss.
Fallout 2 has Frank Horrigan, a titanic Power Armored super mutant, who's the Enclave's best soldier and the Final Boss of the game. The first time he's seen is when he slaughters a random townsperson and his family. Whenever he shows up, somebody's going to die, including the talking Deathclaws in Vault 13 (who he punches in half with his bare hands) and Matt, the Brotherhood of Steel member manning the San Francisco outpost. When you finally go toe-to-toe with him at the end of the game (and you have to, because unlike the previous Final Boss there is no talking your way out of it,) you find he has 999 HP (for comparison, the Final Boss of the previous game had half that,) 10 of every stat (even though in-universe he's not particularly smart,) and a plasma gun and melee weapon that are the most powerful energy and melee weapons in the game, respectively. Thankfully, the player can turn the Enclave base's turrets on him and gain the help of a nearby Enclave squad to help them out.
Shay Cormac, the protagonist of Assassin's Creed: Rogue is a former member of the mostly heroic Assassin's Order, who defects to their archenemy, the Templars, making him a rare protagonist version of this trope. In the game, he ends up killing Adéwale, who was the protagonist of the Freedom Cry DLC and is responsible for reducing the Colonial Assassins down to simply Achilles, the mentor of Assassins Creed III. As the epilogue reveals, he was the man responsible for killing Arno Dorian's father in Assassin's Creed: Unity.
Jack Noir, the Disc One Final Boss of Homestuck, fulfills this role, along with being the Knight of Cerebus. So far, his tally of protagonists killed includes Bro, John (twice), a second alternate universe Dave, the Dreamselves of most of the Trolls, a thousand Aradia clones, Mom and Dad, Rose, and Alpha universe Dave. He also killed his own king and queen, slaughtered two entire armies, destroyed three inhabited moons and twelve planets (the latter offscreen) as well as all of the Exiles save PM and WV (who only survives thanks to being healed by a dead alternate timeline Feferi), along with that entire universe.
Also Gamzee, after sobering up, is a bit of a downplayed version of this trope. His killing spree sees two victims before he is calmed down, but he is still certifiably insane and his friends are constantly on edge since they never know when he'll snap again.
Kore from Goblins was feared by the main cast of goblin adventurers even before they had met him. In their first encounter with the dwarven paladin, they were barely able to wound him. Notably, that fight ended with the death of Chief.
Xykon from The Order of the Stick has killed many heroes before the start of the series, including Lirian, Dorukan, and Fyron, that last one prompting Roy's father to pursue a quest of vengeance that Roy takes up upon his death. In the series itself, Xykon kills Roy too, and then the entire Sapphire Guard. Later, he curb-stomps soul-spliced Vaarsuvius in spite of their insane power.
After the Order are seemingly killed, Goblins hand out T-shirts saying, "I killed a PC and all I got was this lousy T-shirt."
Lord Dragos from The Beast Legion falls invokes fear in everyone he comes across.
564 from Chiasmataand the Chiasm ARG counts. While mostly offing civilians, 564 has exactly one survivor from one of his attacks. He has a bodycount in the thousands, and is bar none the most powerful superhuman in the setting. He's more dangerous than a man who has turned the southwestern United States into a glass bowl.
Nilhil sat in the same place and still managed to kill waves and waves of would-be attackers.
Deathlist of the Whateley Universe. He's got the name because of the huge list of superheroes he has already killed, including Champion. The only person we know of who has survived this killer cyborg is Lady Astarte in the Halloween story, and she had help from a Reality Warper, a precognitive, and a really huge blaster. She still couldn't polish him off.
The Meta from the Red vs. BlueRecollection trilogy, especially in the first part of the trilogy (Reconstruction) and the final episodes.
Lilith from Shadowhunter Peril. Her wrath is inescapable, and every single character knows that when she appears, it is time to run. Even Valentine knows that he shouldn't mess with her. She is one of the few characters in the entire story that can fight on equal terms with an angel (and possibly win). She is completely ruthless and has absolutely no qualms about killing any of her own family: she shoved her hand through her son Umbra's chest, and squeezed his heart till it exploded, then dropped him unceremoniously onto the ground and laughed, all just so she could psychologically disturb the watching Resistance.
Oblivion has no surpassed Lilith as the Hero Killer of Shadowhunter Peril. While Lilith is powerful in her own right, it's revealed that she CAN be defeated by Umbra and Nicholas if they work as a team (Umbra distracting her while Nicholas powers her up and then hits her with a dragon made completely of fire and wind). Oblivion is no such thing. He beat Umbra within an inch of his life and tossed Nicholas aside. Even Puriel, who is the most powerful character on the Light side, is pushed to his limit whenever he meets Oblivion. It's really shown in the final battle, where Oblivion effortlessly kills Ra (a giant phoenix god-king who commands an impressive army of 9-foot-tall phoenixes) in the first few moments of the final battle. Then he finally kills Nuriel, an angel of God who is, for all things considered, Shadowhunter Peril's God of Blizzards. This manages to drive Puriel into a Heroic BSOD. When Puriel snaps out of it, his ensuing battle with Oblivion rips up half the city and it only ends when Puriel manages to rip out Oblivion's heart and then finally kill him. And even then, there's evidence that he might not be totally dead.
Abbadon is also shaping up to be one of these, what with being the Demon of Destruction and all.
Levi Cole in The List is a professional superhero assassin. Though many heroes underestimate him, his reputation seems to have grown since the beginning of Season 2, when he killed the second in command of the Superior Six
Worm has the Endbringers—Behemoth, Leviathan, and the Simurgh—which routinely kill at least one out of every four parahumans that fights against them. Behemoth in particular is known in-setting as the "Herokiller" for its exceptionally high body count. While Leviathan can do the most geographic damage and the Simurgh is arguably the Big Bad of the Endbringers, Behemoth is exceptional at removing humanity's defenders and simultaneously irradiating the surrounding area.
The Slaughterhouse Nine collectiely also qualify, as they kill or inflict a Fate Worse Than Death on several main characters. In particular, Bonesaw tortures and nearly kills Grue, one of the Undersiders, soon after the Nine arrive in town.
She also managed to literally kill Aang (i.e. the Hero), even if he was brought Back from the Dead almost immediately.
To a lesser extent, Combustion Man.
Amon in The Legend of Korra, though his most important quarry isn't always needed to be killed. Immediately, at least.
"I told you I would destroy you."
The Red Lotus is Book 3 of Korra. They can wipe the floor with the White Lotus, and any bender who stands against them is as good as dead or captured. And being Bomb-Throwing Anarchists the world as a whole doesn't stand that much more of a chance against them. They also manage to basically kill the Avatar, but Suyin saves Korra at the last possible second from the metallic poison, much like Katara did Aang.
"Do you have any idea the power that these criminals possess? Individually, they can take down any bender. Put them all together, they can take down the entire world."
The Worf Effect included having his vitals turned into chunky salsa by Samuel L. Jackson. He shouldn't even have been alive, much less being able to fight off Obi-Wan for a good ten or so minutes.
Apparently, the retcon of his character in Star Wars: The Clone Wars was only conceived because George Lucas' interpretation of Grievous was that of a mustache-twirling Smug Snake, not the Jedi-slaughtering cyborg from the series.
The Brain and his Brotherhood of Evil in Teen Titans, who track down young heroes all across the world and turn them into Human Popsicles. Much of the fifth season is spent following the core team as they try to recruit these heroes first.
Nox, the Big Bad of Wakfu. His strongest creation, the clockwork monster Razortime, manages to become this trope in one episode. The moment it appears, the heroes realize it's really bad news. All of their efforts are pretty much useless against it and even Rubilax!Grovy is no match for it. Raxortime literally becomes a Hero Killer near the end of the episode by killing Grovy.
Oberon, in Gargoyles, hands down the most powerful being in-universe the Gargoyles face. Even the combined strength of the clan, Xanatos, Fox, Reynard, and Puck couldn't stop him.
The enchanced Archmage also had shades of this, taking a cackling psychopth and adding power nearly equal to Oberon's on top of it. He was stopped, but only through his love affair with the Villain Ball and the fact that Goliath figured out his Achilles' Heel.
The energy monster in episode 18 of Sym-Bionic Titan qualifies as this. The heroes were completely powerless against it because it was completely impervious to their attacks and could drain the energy out of machines and permanently drain the life out of them so they couldn't be recharged (considering the heroes use robotic battle armor and one is a robot, you can see why this counts). Becomes literal at the end of the episode by killing Octus. It takes the full fire power of the G3's ship and the explosion of the space station it was in to finally destroy it.
The Skullmaster from Mighty Max. In the series backstory successfully conquered the Earth, and Max's predecessor couldn't beat him and was only able to seal him in the center of the Earth. Most of the appearances he makes have the heroes doing some running from him, and in the finale he kills both the supporting heroes and the most Max could do was create a Gainax Ending
Vilgax from Ben 10. Word of God is that his atrocities include destroying 4 planets and creating a black hole, and the dialogue in the series indicates that everyone is scarred to death of him. When he first starts to act at the end of the 1st season, Ben's Badass Grandpa tells him to run when he sees him, which turns he should have as Vilgax promptly stomps him. When he makes a return in Ben 10: Alien Force, though for reason acting under rules of Galactic Conduct, these rules mean he gets to fight some champions from a planet and if he wins, the plant is his. Till returning to Earth, he never lost.
In the 2k3 TMNT, the Shredder maintains always manages to maintain himself as the turtles most feared enemy, notably in part because they're never able to beat him without any outside help. Special mention to the fact that he's actually beaten them on two instances despite them having Splinter with them in the late case because in his new exo-suit he flat out curb stomped them. The demon Shredder that appeared in the lost season also fits, with the Turtles trying to avoid fighting him till they can get more allies to help, and even when they do, and they have Karai draining his strength, it takes Deus ex Machina appearance by Hamato Yoshi's ghost to save them.
Taken to the extreme in Turtles Forever when he's freed by the 1987 Shredder and Krang. '03 Shredder discovers the existence of every incarnation of the TMNT in existence and tries to destroy them all. He manages to erase the entire 2k3 world, including the likes of April and Casey!
The immortal martial arts master Chase Young from Xiaolin Showdown isn't so much a hero killer, than a Hero "Defeater". Anyone he defeats becomes another one of his big cat minions (tigers, lions, panthers, etc.). He does this temporarily to Omi, who is forced into his service as a cat, until he and his team defeat him and restore everything to normal.
Kilobot from Cubix spends his time feeding on the energy of other robots and copying their EPUs as ordered by Dr. K, growing stronger with each energy he feeds on and each EPU he copies. He eventually grows so powerful that Robixcorb decides that a stronger Cubix is the only hope to save Bubbletown from such a dangerous threat. Even so, in the second-to-last episode, Kilobot ACTUALLY manages to destroy Cubix at the end. Of course, it was just a temporary destruction, and Cubix pulled himself back together afterwards, and even then he needs to recruit help from Dr. K. and Kolossal to defeat this creep for good.
Toadborg from the animated series of Bucky O'Hare basically got introduced like this, curb-stomping and capturing the entire crew of the Righteous Indignation with little effort. While he did suffer some Villain Decay as the show went on, he remained the biggest recurring threat to Bucky and the others, usually having to be tricked and escaped from rather than beaten.