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.
Apocryphos makes his introduction by handing Allen, Road and Tyki their asses and was quickly revealed to be the one to (possibly) kill General Cross. And Innocence doesn't even work on him, because he has control over any Innocence.
Eshi, the very first Level 3 Akuma to appear, played this before all other Level 3's underwent massive Badass Decay, taking down Lavi and Bookman before they can do much, locking down Miranda's time-reversal powers and then requiring Lenalee to severely injure herself and damage her Innocence to the point of being unable to walk for the next arc just to beat him.
Dragon Ball: Frieza. He's so dreaded that King Kai was terrifie' of him upon first appearance and ordered Goku to avoid fighting him no matter what. Before we even meet him, he'd already wiped out the Saiyans down to single-digits and effectively conquered the galaxy. During the Namek arc, he is responsible for the deaths of: the remaining 100 Nameks and directly kills Dende, Vegeta and Krillin single-handedly. He causes the destruction of the planet Namek itself.
Every major Dragonball villain fits this label, starting with Piccolo Daimao. Good lord, Piccolo Daimao. He was the evil counterpart of Earth's Godwho went around causing great destruction for the hell of it. It took a suicidal technique from Master Roshi's teacher just to seal him away. When Pilaf releases him, the first thing Piccolo Daimao does is give birth to sons, Hero Killers in their own right. Just one, Tambourine, manages to off Krillin, defeat Goku, and destroy Nimbus. After Goku kills his son, Piccolo Daimao challenges him, and despite his advanced age, curbstomps Goku to near death. The moment that definitely solidified Piccolo's status as a Hero Killer? When he ANNIHILATED SHENRON.
There's a good reason Future Trunks is scared of them. In the future, they are far more monstrous. They slaughtered every hero except Trunks, and proceeded to exterminate most of humanity for the fun of it before they are stopped. And he soon discovers that, due to him being from an alternate future, the versions of #17 and #18 in the main timeline are much stronger than ones he was powerless to stop in his own timeline. Oh Crap doesn't even begin to cover it.
Cell fits this trope to a T, since unlike other villains, he DID kill The Hero Goku, along with Future Trunks (twice when you count the Trunks from his own alternate timeline) and #16. And before that, he absorbed countless innocents and the aforementioned #17 and #18, just as he was programmed to do.
Mazinger Z: If you are one of the heroes, you don't want to run into Archduke Gorgon. Neither of them can fight him toe-to-toe. On the other hand, he can take either of them down easily. Not only he is terrifying and physically imposing (being a half-tiger Cyborg) but also he is too strong. Even the other villains are frightened of him, in spite of the fact that he's supposedly on their side.
UFO Robo Grendizer: In a episode of the first season, the Big Bad sent one of his right-hand men to fight Grendizer directly. Duke was scared of him, saying that the commander was all but invincible, and his Humongous Mecha was more powerful by far than Grendizer. Still Duke deployed Grendizer to fight him... and lost. And he would have been killed if Blackie - another of the Co-Dragons - hadn't backstabbed and killed his own partner in order to maintain his position.
Barthomelew Kuma from One Piece. The first time he meets the Strawhats, he nearly kills one of them. Their next encounter initiated a Wham Episode in which he curbstompsall of them. That's how dangerous he is.
Chief Warden Magellan of Impel Down fits better. He easily defeats Luffy, making clear the latter's pre-emptive strike was no more than a "lucky shot", causing Luffy to pay ten years of his life just to go through the treatment to keep him alive. (This makes Magellan a somewhat literal version of this trope.) He also one-shotted the Blackbeard Pirates, including Blackbeard, the probable main Big Bad of the series, and he proved what an Implacable Man he is when he gave chase to Luffy after he instigated a prison riot, taking out two of the new higher-tier fighters while barely being scratched, and forcing even Jinbei and Crocodile to run for it. Luffy fights Magellan again with protection from Magellan's poison, but Magellan just unleashes a poison strong enough to dissolve Impel Down itself, thus upgrading to an Advancing Wall of Doom and making escape from the prison the only viable option. The escapees barely make it out alive, thanks to Mr. 2's Heroic Sacrifice, but we later learn that Magellan was beaten off-screen seemingly by a reinforced Blackbeard and co.
On that note, Crocodile was pretty much this in his early appearances, As he defeated Luffy Twice before finally going down.
The Three Admirals also play this role, especially to the Straw Hats. They are regarded as the Marines's "Greatest Force", and forgoodreason. Their appearances are often marked by expressions of shock and unrelenting terror on the faces of every pirate unfortunate enough to bump into one of them, and said appearances are also marked by mind boggling displays of sheer power, and at least one No-Holds-Barred Beatdown. Plus, Admiral Akainu actually broke the taboo of "no major characters dying out of flashbacks" by killing Ace.
This even extends into Part 3, where he kills Kakoyin and Joesph (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 Abdul (for real this time), and proceeding to kill the team's mascot, the loveable stand-weilding dog Iggy of all things.
In Part 5 Diavolo turns up,and justifies his name by killing Buccellati,Abbachio,Narancia and almost killing Polnareff.
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.
Neart the end of Jojo 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, 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 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. Scar's Evil Counterpart, Kimblee could also count. While he doesn't freak everyone out to the degree that the Homunculi do, his reputation alone has everyone on edge, until an entire base occupied by a Badass Army is plotting ways to try and kill him. It doesn't work.
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.
She doesn't start out this way, but by the end of Victory Gundam, Katejina Loos is pushing into this territory, killing four members of the Shrike Team, and The Hero's best friend, and coming very, very close to putting Uso himself out of commission.
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.
There is a mini-arc in Naruto that introduces the Akatsuki organization by sending two of it's members to Konoha. A few fights ensue, and they toss many of the most powerful main characters around like rag dolls, even leaving the legendary Kakashi Hatake with psychological damage for some time. They continue their rampage until Jiraiya (who is so powerful that the only reason he isn't Hokage is that he doesn't want to be) shows up and chases them off.
Orochimaru is introduced this way as well, but we aren't shown the full extent of his power until much later (and he was just screwing around with two rookies when he was introduced).
Bleach: Aizen has summarily destroyed, annihilated, curb stomped and effortlessly flicked away just about every hero he's gone toe to toe against. Even when it looks like someone has finally hurt him, it's just an extremely cruel trick.
When the heroes are preparing to invade the Soul Society to rescue a friend, they are told to just turn back if they are unfortunate enough to run into Gin. As the place is crawling with reapers (being their headquarters), they find this advice to be particularly unsettling.
As of the series' final arc, we have The Vandenreich. So far, they've conquered Hueco Mundo, imprisoned Harribel, killed Head Captain Yamamoto and his Vice-Captain, Sasakibe, injured Byakuya to the extent that he may never regain his former abilities, stolen at least five Captain Class Bankai, and broken Ichigo's Tensa Zangetsu. Not to mention the several thousand no name Soul Reapers they killed in a few minutes.
Anytime Battle Royale's Kazuo Kiriyama shows up, everyone you are rooting for is going to die.
Pretty much any incarnation of Astro Boy has Pluto, whose entire function is to destroy the most powerful robots on Earth.
Fate Averruncus in Mahou Sensei Negima!. Not during his first appearance, though he did have everyone completely outclassed then. No, it's actually when he spears Negi through the chest, curbstomps Setsuna while his minions take out the next two best fighters and ends the battle by separating the entirety of Ala Alba across the entire magical world. And then it turns out that Fate is about six tiers of power above the entire main cast.
Even worse, when they finally manage to kill him, it results there are more of him around
The Kill SatChachamaru used on him that supposedly killed him? He was barely singed by it, but luckily for the heroes he appears to be helping them for now
The Huckebeinfamily from Magical Record Lyrical Nanoha Force starts this way, first by trashing the new main character(despite the fact that his Heroic Resolve allows him to properly use his newfound powers) and let him live only because his attacker "runs out of ammo". They then send one of their craziest members to slay one of the main characters who is previously stated as being one of the most powerful mages of the cast capable to go toe to toe with the freakin title protagonist and only barely fails. Also, before and after both mentioned events they're named and referred to by a lot of characters as very dangerous and threatening people. They cemented their status as this when their apparent leader impaled Hayate through the chestIn the Back and Curb StompsVita and Erio without even Reacting.
Asura from Soul Eater is his first appearance curbstomps Black Star and Death the Kid with a couple of flicks.
No heroes actually die in either example. The only named character to die so far was killed by Gellal.
The current arc may well be the one to take Fairy TailDarker and Edgier. The heroes have been getting beaten soundly throughout, including MAKAROV, at the hands of Hades and his guild. Natsu just barely pulled out a win against a Fire Godslayer—think Dragonslayer, but, y'know, GODS—and collapsed almost immediately afterward. For the most part, the Fairy Tail mages have been losing even in 2 on 1 fights against Black Grimoire mages.
Update to the above. The Black Grimoire is losing ground, though at least one example was due to tugging heartstrings rather than actually winning a fight. The villain had set up a Taking You with Me for both Juvia AND Grey. The fight between Leo and Capricorn was won by good luck & timing. And now Cana is getting an upgrade. But the Black Grimoire Guildmaster, the one who took out Makarov, is still standing and his most powerful subordinate is on the loose... Still no casualties on the good guy side, but Charle had a vision of everyone crying before this arc started...
You know why no one have died yet? It's thanks to that huge tree, because it's giving a divine protection to the members of the guild. Yes, that tree, that right now has fallen, because one of the enemies has took it down. Also said enemy took down MIRAJANE and nearly defeated ERZA
In Chapter 253 he takes this to a whole nother level by destorying Tenrou Island and Fairy Tail members on it.
Puella Magi Madoka Magica's Walpurgisnacht is an example. It's existence is apparently well-known among magical girls and it's powerful enough to destroy an entire city. In every timeline that Madoka and Homura have faced her in, they've either ended up dying, or killing it at the cost of Madoka becoming a witch.
Treesea in Yumekui Merry curbstomps virtually everyone who faces her.
In Blassreiter, the first time Xargin shows up, he effortlessly wipes the floor with Joseph and a recently-amalgamized Malek, even going so far as to put the latter into a coma which takes most of the series for him to recover from. The second time he gets in a fight, he walks straight into enemy headquarters, once again effortlessly wiping out everything in his way, including a newly powered-up Joseph, who had gotten so strong that he had an army of 30,000 demoniaks terrified of him.
BlackWarGreymon in Digimon Adventure 02. He is an Evil Knockoff of one of the strongest Mons in the original series, and he is introduced when no one in the main cast has unlocked enough true potential, so the first few episodes after his introduction had him throwing around the Digidestined pretty easily, and he remains a major threat until his eventual Heroic Sacrifice. Even when they get get their DNA digivolve forms, BlackWarGreymon still beats them, and is only stopped by Azulongmon.
It says a lot that the only hero that ever outmatches him is MagnaAngemon, who was powerful enough to curbstomp the final Big Bad (baring the Bigger Bad) of the original series.
The Big Bads in Sailor Moon often embody this. The first, Queen Metelia, killed all the Sailor Senshi in her attack on the Silver Millennium and was only stopped by the Heroic Sacrifice by Queen Serenity. Death Phantom, in the future, when he recruited the Black Moon clan, launched an attack that the future Sailor Senshi weren't able to stop. By the end of the series, we have Sailor Galaxia in both versions, she kills everyone except Sailor Moon (Chibi Chibi surviving in the manga, the Sailor Starlights surviving in the anime). Sailor Cosmos' statements about the future say that Sailor Chaos becomes this.
Trude from Otogi Juushi Akazukin. Her first appearance consisted of her Curb Stomping all three Musketeers and turning almost everyone in the group into dolls. When they finally break free of the curse, they go for a rematch, where she still No Sells everything they throw at her, prompting the group to teleport away in desperation. The next time they fight her, it's with the help of an Obi Wan who is sufficiently skilled to...bypass the block on said teleportation spell so they can run away again, and it's only with the help of a Mid SeasonSuper Mode that they're able to actually drive her off. Even with the Super Mode Trude is still dangerous, as it only gives them a weapon she can't No Sell, but is still vulnerable to her petrifying all of her opponents.
In Eyeshield Twenty One, Hakushuu Dinosaurs centre Rikiya Gaou gains this reputation over the course of the Kantou Tournament, crippling quarterbacks and linemen alike. Forget beating the Dinosaurs—most of the cast spend that arc trying to figure out how to beat Gaou. During the Youth World Cup, the freakishly strong and utterly terrifying Donald "Mr. Don" Obermann takes on this role.
The titular Zeiram in Iria: Zeiram The Animation. While Iria spends all of the movies hunting it down to destroy them, the animated prequel shows how it killed and absorbed her older brother and mentor, Glen, who had a been a great bounty hunter of his own right.
A rare example on the side of the hero; Alucard from Hellsing. He is more or less this trope to a T, complete with literally everyone in the series of being terrified of him (with very few exceptions), and he wipes out almost every villain with no effort. And he is possibly the definition of the Horrifying Hero; try not to crap your pants when he mercilessly rips apart the innocent police team in OVA III (albeit for a reasonother thangetting his jollieseven though he ended up with them anyway), then parades around their corpses. Literally everything about him is perfect for this trope, save for the fact that he's on the side of the good guys (although calling them the 'good guys' is probably going a little too far).
When Masher came along in the fourtieth episode of Kirby: Right Back at Ya!, this guy proved to be so powerful that Kirby had LOTS of trouble with him. Not even Meta Knight could help Kirby against this thing, and not to mention the guy had jets in its feet that allowed it to fly and spin to repeatedly strike at the pink spud. And if that wasn't enough, Knuckle Joe tag-teamed his attacks with Masher, causing Kirby to be pummeled until he couldn't even stand anymore. Just when it looked like it was the end for Kirby, Joe told Kirby to suck up his Smash Punch, and it took Joe's betrayal against Masher and later the teamup of Kirby and Joe to destroy this guy. Then again, there's a reason why Masher was called one of Nightmare's most powerful Demon Beasts/monsters, after all.
And then in the sixty-fifth episode, Masher comes back and is EVEN MORE POWERFUL - now overpowered to the point of being nearly impossible to defeat even by the combined efforts of Kirby and Knuckle Joe thanks to its now almost-impenetrable armor. It took combining Joe's father's locket with a gauntlet created by King Dedede and Escargoon (who built it to make their own Demon Beasts/monsters) to penetrate Masher's armor and destroy it again, this time for good.
In the manga, He isn't defeated at all. The heroes simply cannot reach such power in such short a lapse of time and let him win in the end. He ends up giving up on his plans when he matures through the experience of becoming THE Shaman King. It is also said that absolutely everyone who can see ghosts is good deep down.
Before the beginning of Kaze no Stigma Kazuma was one of these for a while. For a good comparison, think angry Hei of Darker Than Black, but with wind powers and freelance. He reverts back to this personality at the end of the anime, and is more than willing to kill Ayano when she gets in the way of his revenge, and proves it at the climax. He goes around killing relative innocents for information for a few days immediately prior to this.
Attack on Titan has the titular titans. Almost every single fight against them is a Curb-Stomp Battle with humans on the receiving end (and in a series where Anyone Can Die, the body count rises quickly,) and the only thing that can reliably fend off titans is another titan.
And even among the titans there's the Female Titan, who has the highest body count in the series, utterly destroying much of the Survey Corps single-handedly in her debut, killing almost all of Levi's squad and even defeating Eren in his titan form.
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.
The nine Mass Production Evangelions in End of Evangelion managed to kill Asuka and Unit 02.
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.
The Joker is the only person allowed to touch one of the Bat Clan and have it stick for any length of time. It took 20 years to bring Jason Todd back from the dead, and 30 years to de-cripple Barbara. That's got to be some kind of record in the Death Is Cheap DC realm. Heck, even the DC Villains are scared of him.
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. (Now granted, even untrained Kryptonians are pretty dangerous. But remember that, in his first appearance, Doomsday beat not only Superman but also a couple dozen assorted heroes along the way, without ever really stopping to rest.)
Also in the Marvel Universe is Galactus. He eats planets. If you're a non-Terran Marvel character, there's a pretty good chance he's eaten your homeworld (hey, he's gotta eat worlds that matter, but he can't eat the one that really matters).
Unfortunately for Galactus, he's been subject to The Worf Effect many, many times. A competent writer can still make him come off as threatening, though. For example, Annihilation had him beaten when he was ambushed by two cosmic beings just as strong as he was. When he recovered, he was not happy, and was pretty much what guaranteed the heroes their victory.
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 super-heroes 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 of Empowered, 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 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.
Shuma-Gorath. Doctor Strange was forced to kill his mentor, the Ancient One, to prevent Shuma-Gorath from entering our world. In a later story, Strange had to kill himself to prevent Shuma-Gorath's rebirth. Fortunately, Strange got better. Unfortunately, so did Shuma-Gorath.
Durge in the Star Wars Clone Wars comics and the television show. The most prolific slaugtherer of Jedi prior to Greivous' 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.
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.
The Terminator in the first movie. The only reason the Terminators in the sequels don't count is because the good guys had one on their side.
Even when they have one on their side, enemy Terminators are usually powerful enough that they can defeat, if not outright destroy, the humans' Terminator allies. This is especially prevalent in The Sarah Connor Chronicles, where the series makes it clear that Cameron is actually the underdog in most fight scenes involving her and other Terminators.
Not to mention the T-800 always tells John to run in both the 2nd and 3rd film from the T-1000 and T-X. Its obvious Skynet won't send trash assassins against its biggest nemesis John Connor.
Syndrome from The Incredibles builds a robot to kill off superheroes so that he can be the only one left to defeat it and be seen as a hero himself.
Godzilla falls under this fairly often, occasionally even in incarnations where he's the protagonist. Increasingly outlandish weaponry such as specialized airships/submarines, Humongous Mecha, and in one instance a superweapon that opens black holes are made to stop him, with little success.
In one movie, we have three other monsters from a prophecy that were supposed to rise to fight him, and he kills them all.
Mechagodzilla also qualifies, easily beating Godzilla in his first appearance along with the other monster that was supposed to stop him, King Seesar. Even with the two working together, Godzilla has to pull out an odd Chekhov's Gun to win.
Destoroyah, a monster born from the one weapon that killed Godzilla in the past. Destoroyah actually kills Godzilla Jr. and nearly kills Godzilla himself.
KingGhidorah, Godzilla's Arch-Enemy, is often played this way, with his mere arrival being enough to send most people into panic. In most of his appearances it takes Godzilla plus an assortment of other monsters to bring him down.
The final assassin in Bodyguards and Assassins is a highly-skilled martial artist who mows down pretty much every bodyguard trying to take him on. Eventually, one of the last few manages to get a pistol, after which Reality Ensues.
The Dark Knight has a more played 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 several more deaths, including Dent's own death.
And the Balrog if you're unlucky enough to meet one underground. Gothmog from The Silmarillion is a prime example.
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.
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.
The Eldest Brother Gruff also has these trappings. Hitmen for the Summer fae, each one defeated by Harry swears his elder brother will come after him, just like in the fairy tale. The Eldest one is maybe five feet tall, if you squint. He also managed to dropkick an elder demon with magical might, and wears the stoles of three powerful wizards he's slain in battle. That's when Harry realizes he might very well be screwed. Fortunately, Eldest Gruff turns out to be, at worst, a Punch Clock Villain, and Harry manages to talk him down without having to fight... by asking fora donut.
Jared Kincaid, an ostensibly non-magical humanoid something that has been in the business of assassinating supernaturals for centuries. Probably a good thing he is currently on a permanent bodyguard duty for the above-mentioned Archive. In Ghost Story, he is revealed to be the one who was hired, by Harry, to snipe Harry in the end of Changes.
Though he hasn't actually killed an heroes on-page yet, Cowl might also qualify. He packs a magical punch stronger than Ebenezar (who's a Senior Council member and the Blackstaff), is a necromancer even the other necromancers are scared of, doesn't seem terribly worried about being hit by other wizards' death curses, and shrugs off having a car flipped over on top of him like it's nothing. Also a prime candidate for being on the Black Council, though whether he's its leader or just a member isn't established.
The Skinwalker, too. It beats the crap out of Harry several times, and it did actually kill Kirby.
As of Cold Days, Maeve who kills Lily. She also inflicts immeasurable damage to the humanity of Mab and may have led to the death of Molly's humanity.
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 almost kills Hermione in Order of the Phoenix and is, according to Word of God, the person who slew Lupin in Deathly Hallows.
In the Sherlock Holmes stories; Moriarty, and Holmes isn't the only one he seriously threatened.
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.
Spike, when he first showed up, with his history of killing Slayers. In his first battle with Buffy, Buffy survived only because her mother (of all people) intervened with a fire axe and caught Spike off guard as he was about to deliver the killing blow. Sadly, there's a reason Badass Decay was once called Spikeification.
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.
Almost. 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").
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 impressive 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.
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.
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.
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
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 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.
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.
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 and Harbinger in Mass Effect 2. From the first scene, you know they're bad news when they kill your character from the first game. Granted, s/he is revived by a secret project, but it really sets the tone. 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.
Mass Effect 3 gives us Kai Leng, who had previously made a name for himself in the Mass Effect novels. For those who didn't read the novels, though, Kai Leng needs to be given a chance to show how deadly he is.
Your first meeting with him, he has the salarian councilor at hand-blast point. If you're new to the series, he kills the Councilor and there's nothing you can do.
If Major Kirrahe is available, but not Thane, he will save the Councilor, but Kai Leng will kill him, even though Kirrahe is a lifetime veteran of the elite Special Tasks Group.
And if Thane Krios is alive, he also stops Kai Leng from killing the Councilor, and they begin to fight. Thane, while slowed down from his Kepral's Syndrome, is widely known to be the best assassin in the galaxy and a master of hand-to-hand combat. Kai Leng impales him through the stomach with a sword, though Thane did make Leng have to work for it.
Then Kai Leng escapes from you, and, when you try to chase him, he disables your hovercar by jumping onto it and stabbing its control systems. While deflecting your bullets with a field from his hand.
Later, on Thessia, you get to actually fight him, but when you get the upper hand, he calls in his gunship to blast the supports of the building you're fighting in. He walks calmly along the central path, the one stable part of the building, while you try to avoid falling into an abyss of death. He steals the data you came to collect, and follows it up by sending a taunting email to your private terminal.
In the Sanctuary mission, he will kill former party member Miranda Lawson, a powerful biotic, if you don't warn her that he's alive and active.
Final Fantasy VII: Sephiroth. 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.
Lavos from ''Chrono Trigger''kills the main character, Crono, at least until you find the "Chrono Trigger" to bring him back
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.
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
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.
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, inspires similar fear amongst the trolls, though his kill count is considerably less than Jack's with only two trolls in this timeline and the entire team save Aradiabot in a branch timeline.
Grimbark!Jade tries to be this, but it never works out. Meanwhile, Aranea is well on their way to achieving this status.
Lord English decides that killing main characters just isn't enough, and goes on to kill the author of Homestuck. Then for an encore, he kills an entire dreambubble's worth of already dead ghosts (including the alternate dead John that Vriska dated and at least one dead Dave,) making them Deader Than Dead. Meanwhile his past self, Caliborn succeeds in organizing the death of his split personality, Calliope, allowing him full control of his body.
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 kills Roy (who had killed him earlier, though Xykon regenerated since he's a lich) without any real trouble. Later, he curb-stomps ''soul-spliced Vaarsuvius despite being outclassed by figuring out how to destroy his soul-splice.
Lord Dragos from The Beast Legion falls invokes fear in everyone he comes across.
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 super hero 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, which routinely kill one out of every four parahumans that fights them. Behemoth in particular is known as the "Herokiller," for it's exceptionally high body count-while Leviathan can do the most geographic damage, and Simurgh is arguably the Big Bad of the entire setting, Behemoth is exceptional at removing humanity's defenders whilst simultaneously irradiating the area.
The Slaughterhouse Nine also qualify, as they kill or inflict a Fate Worse Than Death on several characters.
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.
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.