In Pokémon, the Team Rocket trio started out being more dangerous and effective before they became... well... Team Rocket (although this may have been intentional, since the writers likely didn't know what direction to take the characters in the beginning, and chose the more comical route). In Best Wishes, they briefly went back to being more dangerous, leaving all of their comical Pokémon at the base similar to Ash leaving his team. Half the time, they didn't even bug Ash and co. anymore due to their missions.
Butch and Cassidy were originally the competent counterpart to Jessie and James, but by the time you get to their appearances in Pokemon Chronicles they've become basically a pair of espies.
The games have actively tried to avoid this — Team Rocket only appeared in the first two sets of the main series of games, decaying in the second one due to their leader, Giovanni, not organizing them. Since then, every main-series game and spin-off that includes criminal organizations includes entirely different ones, and each one has upped the ante for their master plans each time.
Beck from The Big O is the world champ of Villain Decay: the writers put him through almost every one of the gimmicks mentioned above. First he got a cool new weapon, then he got played as a buffoon (complete with a comically grotesque hairdo), then the hero was put into an Alternate Universe where Beck was a real threat, before he finally ended up just being an underling working for Big Bad Alex and his Psycho for Hire, Alan Gabriel.
Well, that's all true, assuming you believe he was set up to be a competent villain in the first place. In the manga, that's perfectly true, and he is a competent villain. In the anime, it's fairly obvious that he was intentionally turned into comic relief. He starts out being effective because he's actually smart enough to dial down his own ego and commence his plans intelligently. Unfortunately, his ego takes control in later episodes, and his decay is quite noticeable. Also, it's rather blatantly implied that he really isn't fit to be a villain, and that his true genius is in building robots and neural AIs (which he remains shockingly good at, as lampshaded by Gabriel, and later by Super Robot Wars Z). That said, the decay of the anime Beck is quite possibly justified.
Beck's final appearance in the manga suddenly has him become both ridiculously powerful and utterly terrifying.
The Knights of the Rounds in Code Geass R2. In their first appearance, they were shown as Britannia's elite force. Lelouch and the Black Knights were struggling when fighting only three of them (Suzaku, Gino, and Anya). But as episodes passed, they became easier and easier to incapacitate. Then, the show introduced more Knights, and after that, one of them is killed. Later, when Suzaku does a Heel-Face Turn and gets a stronger robot, he becomes able to slice down his superiors in mere seconds. However, it's probably justified due to the Lensman Arms Race being in effect, where the Super Prototypes quickly become reverse-engineered and dated in the space of a few episodes. The Knights' demise could be explained because they didn't upgrade their Knightmares enough, but the fact that Tamaki was shown to be more competent and badass, however, is not justified.
This is an ironic example, as the rest of Code Geass is quite good at avoiding Villain Decay. Any given battle is generally a toss-up, with the protagonists winning and losing a roughly equal number of battles, and almost every major villain getting in a victory or two. Cornelia is portrayed as both a highly competent tactician and fighter, the Glaston Knights are a force to be reckoned with, and Suzaku manages to win a ton of battles and lose very few. The climactic battle at the end of the first season is in fact wonby the villains of the series, while the protagonist is defeated, captured, and has his memories erased.
They don't really decay all that badly. The only one that was actually slaughtered was number 12 (weakest) in a standard Vincent Knightmare rather than an Ace Custom, and that was by Suzaku - higher ranked, geass sumper-charged to be able to push the normal human limits and in a *deep breath* Super-super-super-super prototype of superness that took the entire budget of the R&D department to develop, twice. The knight of one fights the same person, and holds his own very effectively until Suzaku's Lancelot just starts moving to darned fast for him to keep up. He previously managed to defeat the other Super-Prototype ace in a machine equal to the first lancelot, Xing-ke, without much effort. Anya never decayed so much as we didn't get to see her final fight, though it was against something that wasn't so much a knightmare as a small battleship, and her motivation to fight for brittania is the loss of her memories caused by geass, something her opponent could negate. Gino never really decayed in the slightest, managing to fight against the Lancelot Albion in a far inferior machine for a while.
Subverted in Yu-Gi-Oh! with Yami Bakura. Initially he's really just a side-villain, nowhere near a main threat, and no one really spends a lot of time on him - in fact, in the first season he's defeated by the sidekick in a sideplot while Yugi is busy with the Big Bad. As it turns out, this works to his advantage, since it allows him to lurk around setting up his evil plans with no one noticing. By the time the final season rolls around, he puts all those puzzle pieces to work and becomes the season's Big Bad.
In the manga Bakura was the main villian through an entire volume; he was introduced in the last chapter of volume 6 and defeated in the second to last chapter in volume 7, just before the Duelist Kingdom Arc. When he showed up he was the greatest threat to Yugi and his friends ever and secretly set his plan in motion in the background. Also in the manga the psychotic level and danger of Yami Bakura was pretty clear early.
Tengen Toppa Gurren Lagann: Viral was designed for this trope. In his first appearance, he nearly hands the heroes their asses, but in every further appearance he's defeated with less effort. Despite showing up with a new upgraded mecha each time, he's eventually beaten by the humans' mass-produced mecha whose pilots don't even break a sweat. The reason for this, in-show, is because non-evolving beastmen can never match the constantly growing power of the spiral-powered humans. He gets better after his Heel-Face Turn.
Starter Villain Jadeite started off a competent threat. He had powerful minions, curb-stomped Sailor Moon during their first meeting, and actually managed to succeed in getting away with human energy in one scheme, earning Queen Beryl's compliments. But right after that last event, things began to go downhill for Jadeite. Very downhill. Once he got Hoist by His Own Petard for the last time, Queen Beryl "decommissioned" him for good. The rest of the Shittenou avoided the trope, with Nephrite and Kunzite never ceasing to be threats (though Kunzite slips when he gets his own arc, this has in-story reasons behind it), and Zoisite never being much of one to begin with so that he couldn't possibly decay (he always relied on dirty tricks in order to be dangerous.)
The Ayakashi Sisters in the manga are murderous maniacs, while in the anime they were merely misled and are granted a chance to live free in modern day Tokyo. The manga also had them capable of killing the Sailor Senshi with ease, something they struggled with in the anime.
Queen Nehellenia was still evil in the anime and still a threat, but only because she was misled, and she was eventually redeemed and granted a second chance at life in Stars. In the manga, she was evil incarnate (a spawn of Chaos), responsible for the death of the Moon Kingdom and the current calamity, and was destroyed by Usagi and Mamoru.
Inverted from manga to anime with the side story villain Princess Kagua. In the manga, she's briefly beaten after a fight that lasts one panel, in the movie adaption of the side story, she nearly annihilates the Sailor Senshi and actually beats Usagi's Super Mode.
When the Menos Grande first appeared Rukia said she'd read in textbooks that only the Royal Guard can fight them. Ichigo could barely scratch it. Now that it's been confirmed that was a Gillian, the lowest level of a Menos Grande, and that any high seated shinigami can handle them. As the power levels of the main characters develop to lieutenant and captain levels, the Gillians have accordingly become regarded as mook-level threats. Even the Adjuchas levels of Menos Grande become easier to handle as the story progresses.
Orochimaru from Naruto suffers from some Villain Decay over time. In the Chunin Exam arc, he's too strong for any of the heroes to defeat, forcing the Third Hokage to sacrifice himself to save the village (which doesn't even kill him entirely). Then it's revealed that he lost to Itachi in the past while trying to claim his body, and in most of the battles after that, he's defeated easily or forced to retreat. This is partly because he he wasn't at full strength when he fights, but it shows that he's lost much of his original threat. This was worst when he fought against Itachi (while fighting Sasuke) in the manga, and was defeated in merely a few pages.
Also his minions from the Sound Village suffer from this, in Part 1, even the weakest of them including Filler Villain was a serious threat, and the Sound Four were so powerful that it took two of their opponents to give everything they had to kill them; in fact their leader was so strong that he would've won his fight if not for a terminal disease. When Part 2 comes around most of them (excluding those who join Hebi) are whiny wimps dependent on Kabuto.
Tarant Shank, the arguable Big Bad of Tenchi Muyo GXP, decays very fast. In his first appearance he's portrayed as an extremely dangerous and unstable villain who nearly kills Seina, Mitoto, and Kiriko, and leaves Seina traumatized from the experience. However, his next appearance has him appear with a broken arm (revealed later to be from fighting Tenchi and company off screen) and he quickly goes downhill from there; his plans are easily foiled by Seina's group, his ship is utterly destroyed, and his role as Big Bad is supplanted by Seiryo of all people. He makes a minor comeback in the final few episodes, but never quite manages to regain the same threat level he had in his original appearance.
Detective Conan: Inverted by Gin to an extreme. Gin gets progressively more evil and intelligent the longer the series goes on.
The Trinity Siblings in Mobile Suit Gundam 00 may qualify. Their first appearance sets the group up as a very skilled fighting force, with them single-handedly rescuing the other Meisters from certain capture, and obliterating most of the Union's and Human Reform League's ranks. However, following this, they're systematically defeated time after time, even, in part, by faceless Elite Mooks, until it culminates in the resident monster shooting one of them dead and effortlessly defeating the second, the third being handily saved by a timely intervention of her enemy. This can, somewhat, be justified, as they were caught off guard by both the Trial System's effects and the GN-X models, which were on par with Gundams, but the fact that they put up so little of a fight is still surprising.
The other antagonists of the second season decay pretty badly by its second half. First, A-LAWS and then Ribbons' personal squad of personality-lacking bishonen initially appear as very threatening antagonists, repeatedly pushing the Celestial Being to the brink of destruction, but then decay to Elite Mooks, with A-LAWS eventually being demoted all the way to the status of normal Mooks that die ineffectually by the dozens in the final episodes. If we count things beyond sheer combat potential, Ribbons himself decays very badly as well: after being presented as a cunning Magnificent Bastard who manipulated everyone for his own gain in the first season; in the second season he can't come up with anything better than making his puppets commit massive atrocities for no apparent purpose. This culminates with firing a Wave Motion Gun in the midst of a space battle that wipes out his own A-LAWS fleet while doing little damage to the enemy, and then using a bunch of Super Mode-powered Mobile Suits piloted by Super Soldiers as suicide weapons.
Ali Al-Saachez in Season 2, despite having become Ribbon's Dragon. It culminates in him getting shot in the face while attempting to pull an I Surrender, Suckers on Lockon II. This actually makes sense though, as most of the people he defeated in Season 1 were fighting at some sort of disadvantage, or, in Setsuna's case, were trained by Ali.
It helps that the VF-25 is far, far more advanced than the YF-21 and YF-19. Also the V-9s were under Slave control of the Battle Galaxy (that is, Grace herself). When Luca released his own V-9 escort drones via the JUDAH System, he made specific mention of them having become just as deadly as the prototype Ghost X-9.
A particularly jarring example is The Shinigami Grell Sutcliffe from Black Butler. Starts off as a supernatural serial killer with a magic chainsaw who also happens to be Jack The Ripper’s Dragon. But once they took away his chainsaw he quickly devolved into the Butt Monkey. So much so that after a handful of episodes none of the characters view Grell any differently from the rest of the comic relief. Note that Grell is still just as dangerous in the manga during the Zombie Apocalypse on the ship; it probably helps that she still has the chainsaw.
Hao from Shaman King gets hit by this hard at the very end. A thousand years old, and controls the fundamental spirit of fire (That eats souls), willing to wait a long time for his plans to succeed and very calm and calculating. There was no way for our heroes to succeed in the final showdown, even with superior numbers. So at the end he loses his cool, calm and collected demeanor and loses largely because of that.
Hao really was unbeatable and became the titular Shaman King. The anime had to pull a stock shonen ending instead of that, though, since the manga ending hadn't come out at the time, so they needed to make him lose, somehow.
Envy in the later parts of Fullmetal Alchemist. The guy who killed Hughes and generally made life miserable for every protagonist, and he's kicked Ed's ass at least once. His last two fights are against Marcoh, who uses his knowledge of philosopher's stones to decompose him (and before that he was just getting jerked around by May Chang, including getting a rock hand shoved between his ass cheeks), and Roy, who puts him through one of the worst curb stompings in manga history.
Gluttony, after a brief moment of Unstoppable Rage after learning that Roy killed Lust, is left on the brink of death after the first battle in Father's inner sanctum, forcing Father to restore him. In his next appearance in battle, Lan Fan is easily able to cut him to pieces, and Pride decides that he'd be more useful if he ate him and absorbed his sense of smell.
Buggy the Clown and his crew from One Piece spent their first appearance as a serious threat. In the manga, Buggy's first scene is him brutally killing one of his own crew members (he actually spares the guy in the anime.) But after Buggy's defeat, in all subsequent appearances he is portrayed as incompetent and having lost much of the "monster" in his status as a Monster Clown.
A lampshade is hung on this when Luffy and the group of big-name former prisoners he was with him finally escaped Impel Down. At about this point, Buggy's past on the Roger Crew was revealed, causing Emporio Ivankov to muse that Buggy is likely the 'disgrace' of the Roger Pirates.
Sir Crocodile, on the other hand, averts. He is defeated by Luffy fairly early in the story, and despite the Straw Hats having become significantly more powerful since then, he remains a very dangerous and powerful man throughout - even in spite of being removed from Alabasta, where his element, sand, was abundant. His Number Two, Daz Bones, counts as well.
Mihawk also averts this. His first appearance has him cutting ships in half with his BFS and blocking katanas with a pocket knife. The next time we see him, he has a brief fight with Luffy which Luffy quickly runs away from seeing that Mihawk is still too strong for him.
Ranma ˝ has Kuno, who, in the very earliest portions of the story, is represented as some sort of deadly, even lethal threat to Ranma...up until his first defeat, after which, he was little more than a Butt Monkey even on his best of days, with Ranma Badass Back attacks leveling him. They don't even mention Kuno as being in any way threatening even to the untrained civilians of the cast. In fact, Kuno actually managing to disrupt the status quo and gain the advantage over Ranma via some Plot Device is usually such a big deal as to be the focus of the episode.
Arguably, Giriko from Soul Eater. That is, the idea of a mad git with a chainsaw as a Weapon form does start out as a dangerous prospect, especially when he defeats the kids with no effort whatsoever meaning Justin Law had to step in (cue convenient example of the skill of a Death Scythe). Afterward, he spends too much time getting drunk and womanizing to be any kind of threat.
All those evil corporations, organizations and elite hacker groups seem pretty daunting at first in Serial Experiments Lain. The knights were particularly presented as being high-level hackers. It has you rather worried for Lain, that is until we find out (MAJOR SPOILER) that Lain has complete control of the Wired, which in its merging state with the real world, makes her God. Sayonara, Eiri! Knights: DELETED.
Kagura in InuYasha suffers this. She nearly overwhelms Inuyasha on her own in his first two fights with her, but a big contributor to that was his inability to use his Wind Scar supermove on her because she could control the air. When he gained the ability to use it whenever he wanted, he could take her easily. She was, however, still a tough enemy for the rest of the cast. What really killed her as a threat was the constant popping up of villains stronger then her.
Possibly played straight. In her first few episodes she's seen as a competent villain and legitimate threat even managing to kill a member of the Kira investigation team and finding out Kira's identity. Once she met up with Light, she began to decay fast.
Relinquishing a Death Note erases memories directly related to the Death Note, including (but not limited to) knowledge of the Notes themselves and how they work, the Shinigami, and who their users are; it does not erase unrelated memories. For example, Misa retains her knowledge of/feelings toward Light during such a time period, even though she no longer knows that he is Kira
Another one is Enishi Yukishiro who becomes less calm as the story goes on. Watsuki calls him the opposite of Makoto Shishio who ended becoming one of the most threatening villains he ever wrote.
Also, every villain defeated by Hajime Saitou. Watsuki states that Saitou ruins all of his enemies' hype.
In Higurashi, Miyo Takano went from trying to kill Rika, and become God, to trying to use the real Hinimzawa Syndrome to become "Queen of the World." Though given that it was an OVA episode...
Haruhichan parodies this in episode 3 with Achakura leaving Nagato's apartment to kill Kyon. Her result for attempting such thing? A cat attack which causes Achakura to change her mind.
Dragon Ball has this trope, and it has happened within the series on numerous occasions to the point that it's practically procedural after a villain's initial defeat; To list every instance would need its own trope page. Within the span of episodes, villains can fall from being ridiculously lethal and dire threats to being utterly outclassed in every possible way. Likely justified by the fact that the protagonists' strength grows enormously throughout the series, while most other characters stay at around the same level. A rule of thumb is that after a villain has been defeated once before, they're chopped liver. It's taken to its extreme twice: once in the Dragon Ball Z movie Fusion Reborn, and again during Super 17 arc of GT. Both times, entire scores of previously-killed villains from the original and Dragon Ball Z (and even a couple from the movies!) manage to escape from Hell and are effortlessly defeated by the much more powerful cast. Even Frieza, who was offed in a single attack.
Frieza and Cell get hit the hardest by this trope in filler episodes after their sagas and the non-canon Dragon Ball GT. Each of them took an entire saga to be defeated, and at the time they were more threatening and powerful than the rest of the cast by a ridiculously large margin. But after their time in the story's limelight ended and new threats took their place, Frieza and Cell would become laughable at best. Frieza, on FOUR separate occasions, ate vicious curbstomp defeats, two of which were decided by a single devastating strike. Cell fares slightly better, but not by much; Pikkon one-shot him in Hell, and Goku (after being reverted to his child form) effortlessly and simultaneously made Cell and Frieza look like clowns.
In Digimon Adventure, the Kuwagamon that the kids meet in the first episode is a terror, hounding them for most of the episode and taking seven simultaneous attacks from their Rookie-level partners without lasting damage. The next time they encounter him, in the second arc, he menaces Tai and Agumon for a few seconds before being effortlessly destroyed by Piximon. Etemon is an interesting case: he's certainly much stronger and borderline invulnerable when he returns during the Dark Masters arc as Metal-Etemon, but most of his time is spent either chasing Joe and Mimi or getting into a scrap with Puppetmon, and most of the fight with him at the end is him shrugging off attacks and boasting about his power. Though he did also kill Leomon in cold blood.
In SD Gundam Force, each member of the Quirky Miniboss Squad initially appears threatening when they first come on the scene. Unfortunately, in spite of some near-successes early on, they quickly become regulated to Goldfish Poop Gang, and in the second half of the series are barely able to survive. This is lampshaded by their Commander;
"But time after time you have failed. Instead of warriors, we are becoming laughingstocks!"
Omega Red gets hit by this hard in the Wolverine anime. When Omega Red first appears, Wolverine spends an entire episode fighting him (in a series that is only 12 episodes long) and has to finish the fight at the start of the next episode. Logan only wins because he catches a lucky break and a vital part of Omega Red's equipment malfunctions. Omega Red is repaired and made better than ever with a new arm and sent after Wolverine again, and gets killed in about ten seconds.
Generally speaking, this has been a big problem for American Super Hero comics for a long time, due to their serialized nature and the constantly recurring and curbstomped villains. This was especially a problem during the Silver Age, where writers like Stan Lee would have the villains openly say "This time my brilliant plan will work perfectly! And those pesky heroes will be unable to stop me!!!" and whatnot without any sense of irony after having been clobbered multiple times already (and the reader was supposed to take the threat to the hero at straight face value, to boot). A lot of the accomplishments - and problems - surrounding American comicry from the 1970s onward can be traced in large part to attempting to combat Villain Decay, resulting in much more cynical and controversial plots, while keeping the now-decades-old continuity running without having to constantly invent new villains.
By far, The Joker from the Batman comic book series. This page nicely details his periods of reduction into a joke. Arguably, the same thing can be said for any other villain featured in the 60's show. 1973's "The Joker's Five Way Revenge" returned him to his original personality of scary sadistic madman. Fromthenon there have beencertainstorylinesthat will ensure that the Joker may never suffer Villain Decay again if we keep going in this direction.
Said decay, depending on which continuity you follow, has become a part of Joker's character: He can go from complete comic relief to a serious Straw Nihilist psychopath out of the grimmest and darkest of fiction, in an instant and, according to Grant Morrison, went through the decay because he likes to "reinvent" his act every so often.
As to the 60's show, it actually reversed the Villain Decay of a lot of villains; it didn't cause it. Riddler and Mister Freeze in the show might seem goofy today, but prior to the show both characters had only appeared in a handful of issues and the TV series is actually what established them as major rogues. They might have been silly, but that's better than being forgotten and forgettable, plus no bad guy on the show was quite the Harmless Villain they have been remembered as — it may have been light-hearted entertainment, but they did nearly kill Batman and Robin in various horrible and sadistic ways at the end of every other episode, after all. Especially the Mad Hatter, who, in the comic book introductory story that provided fodder for both of his televisual appearances, was merely an essentially harmless exotic hat collector who was not above stealing some of his prized treasures; whereas onscreen he sported an instant-knockout hat which he used to kidnap the jury that had previously convicted him, as well as planning Batman's demise on a specially-designed, vicious Conveyor Belt of Doom (managing to put Robin on it in the climactic scene).
The Predator extraterrestrial embodies this trope after being trounced by virtually every other comic book character in the industry. Despite the incredible awesomeness of the original Alien vs. Predator comics, it later became a check-the-block for every character from Superman to Judge Dredd beat up a Predator at least once in their career. This trope is somewhat rationalised by the fact that the Predator's code of honor means they must look for a "fair fight." But let's face it: if someone wrote "Aliens vs. Predator vs. Terminator vs. Robocop vs. Squirrel Girl," Squirrel Girl would win.
Many villains of Crisis Crossovers suffer this if they are ever seen again. The Beyonder of Marvel's Secret Wars is a good example. Presented as a mysterious and powerful cosmic being in the original maxi-series, he assumes human form and becomes mostly a joke in Secret Wars II. One memorable scene involves Spider-Man teaching him how to use the bathroom. It doesn't help that his character was portrayed inconsistently throughout the second maxi-series and the tie-ins. In one tie-in, he's murdering the New Mutants (only to bring them Back from the Dead later), in another he's consoling the Human Torch over the accidental death of a fan. It's little wonder that Secret Wars II is considered 'drek' by many comics fans.
The Marvel supervillain Abomination has probably lost more bad boy status than almost any other. Originally a Hulk villain, he started out up-powered even by the Hulk's standards, whomping him down in their first encounter. He then had some gamma power stripped, which was added to the Hulk, thus losing in their next encounter. He then suffered a series of beatdowns at the hands of the Hulk, leading to humiliating exposition as his character developed a fear of even encountering the Hulk anymore. But that was not the end of it. Over subsequent years, he became a chew toy to show how badass the lower bricks in the Marvel universe could be, taking solo beatdowns at the hands of both Wonder Man and She-Hulk. Oh, true, they pulled out all the stops in their demonstration of badassery, but the Abomination just can't get any respect, in spite of still remaining perhaps the physically strongest character without some quasi-infinite trick up their sleeve. He got a slightly better treatment in the Chaos WarHercfamily crossover, where, after having been killed off a couple years ago by the Red Hulk, he comes back as a servant for the Big BadChaos King. After tearing through a team of Hulks, Doctor Strange states that he was "the Underworld's strongest prisoner". He's still dead again by the end of the story, but he definitely got some cred back.
Colonel Olrik of Blake and Mortimer fame fits this trope to a tee. In his first appearance, he aided The Empire in bringing about World War III and successfully conquering the world. Understandably, his later appearances as a smuggler/thief/spy are not as impressive. Even when said Empire's bloodthirsty dictator was brought Back from the Dead via Time Travel and Olrik joined him once more in The Strange Encounter he was little more than a thug.
Also, Venom, whose career as a psychotic murderer and Spider-Man's most frightening enemy ended the minute he decided to become "the Lethal Protector".
It's so much worse than that. In Venom's early days, he was able to tango with both Spidey AND the Human Torch. Remember, he's weak to fire. In his first appearance, he almost KILLED Spider-Man. Fast forward about seven years. Spider-Man, in a bored nonchalant manner, sends him running scared WITH A LIGHTER.
It's clearly established that they had surprise on their side at least partly — plus a handy ability to turn anyone they bit into zombies, which evened the odds somewhat.
UltimateNamor started off as being so powerful that he easily defeated the entire Fantastic Four with his bare hands, and was only persuaded to spare the city of New York when Susan Storm agreed to kiss him. In fact, it was stated that he was the most powerful superhuman on the planet, stronger than even the friggin' Hulk. Then when he reappeared during the Ultimatum crossover, he was easily clobbered and tied up by an unaided Reed Richards.
Marvel Comics' Onslaught initially appeared as beyond godlike and it took every superhero on Earth to defeat him. When he did come back, he was the subject of a low-selling mini where he was defeated far more easily and sent to the Negative Zone by Captain America and some of the author's Pet Characters. Not very fitting for a guy who took on the Marvel Universe at one point.
Dr. Light in The DCU. At first, he was tough enough to take on the whole Justice League, and then declines through the 1980s to the point where he is beaten by the kid non-powered superhero team, Little Boy Blue and his Blue Boys.
This was explained/retconned in the infamous Identity Crisis storyline as the League having given him what amounted to a psychic lobotomy via Zatanna's magical powers after he had sneaked aboard the Watchtower and raped Sue Dibny. He later recovered and went back to his threatening self...until The Spectre turned him into a candle.
After getting his memories back, most of his appearances gave him a faster variety of villain decay. Identity Crisis was intended to turn him into a serious and intelligent foe for the JLA once more, but instead, he became a serial rapist who went on about how much he liked rape.
His constantly reminding us of the rape thing is apparently intended to make him seem more evil, but it actually makes the decay worse: he used to be a C-list villain, but now he comes off as a C-list villain who desperately clings to having managed to hurt non-powered civilians in a way non-powered thugs in reality do with (very sad) regularity hoping someone will take him seriously.
Gepetto, the evil mastermind of Fables contracted a bad case of villain decay. He'd conquered and ruled countless realms for centuries, but after he lost the first couple battles of the new war, he became depressed and sat about moaning while his Empire fell to pieces, until the heroes came and took him to live in a nice new apartment in New York City.
Crime Doctor: You know, Prometheus, I'm almost disappointed...When you first appeared on the scene, we were all mighty impressed. You carry the knowledge of the world's thirty greatest fighters in your helmet, Right? The point is, we thought you'd be a world beater. Then we heard Catwoman tore your manhood. We heard Hush made you his punk.
James Robinson made him a real threat again in Justice League: Cry for Justice. However, he was surprised when Green Arrow shows up to kill him, despite Green Arrow having been a killer for years in continuity.
Speaking of Lady Shiva, is she the worlds greatest martial artists or not? Honestly it's hard to tell some times.
The recurring The Adventures of Tintin villains are ineffectual and ridiculous in their last appearance in Flight 714. Former Big Bad Rastapopoulos is reduced to playground banter with his intended victim over which of them is nastier, and loses. According to Word of God, Rastapopoulos would have been more menacing...if only his outfit hadn't ended up looking so utterly daft. Herge apparently took one look at his own sketches and was unable to see him as a serious threat ever again.
A rare example of a character suffering this at the hands of their own creator: Ulysses Hadrian Armstrong, AKA The General, was an Enfant Terrible that could have passed for the son of the Joker in his first appearance. His backstory has him burning down a building at the military academy where he was educated, not because the academy's bullies lived there, but because he was bored. He then proceeds to run away to Gotham with a bunch of guns, shoot rival gang members and law enforcement alike in cold blood, and lays siege to a police station with an army of gang members armed with rocket launchers. Oh, and he also had the balls to smack Batman in the back of the head with a shovel, sending the Dark Knight plunging three stories down, after Batman had just saved his life. All done at the tender age of 11. Later, when creator Chuck Dixon reused him in the Lighter and SofterRobin ongoing, he started acting more his age, began to incessantly quote military figures, and was generally Played for Laughs a lot more.
In Archie Comics' Sonic the Hedgehog, we get a good example of this trope in the Evil Sorcerer Ixis Naugus. When originally introduced, he was an extremely powerful wizard with power over the elements and who sent Sonic and Tails on a wild goose chase around the world before being banished to the Void. When he came back a few years later, he soon found himself reduced to Mammoth Mogul's Dragon, but was still threatening...at least, until his time as Dr. Eggman's prisoner destroyed his mind, leaving him a mindless beast Mogul kept as a pet. But as of issue 220, Nagus has had his mind and powers restored by a Chaos Emerald wielded by his apprenticeGeoffery St. John, and has set himself up as the Big Bad of the current arc. And so far, he's been doing pretty well for himself.
Series Big Bad Eggman himself goes through several instances of this—sometimes in-universe—he'd finally lost his marbles completely and stayed that way for most of a nearly year long story arc. He's largely recovered—both from the in universe decay and the meta version—by becoming the go-to 'event' villain. The last five years or so have involved Eggman launching tremendously huge attacks that significantly alters the status quo—only barely being beaten—then hiding out or otherwise removing himself from direct conflict for a while while Sonic and co. deal with other, lesser (for the most part) villains, then launching an attack that once again significantly alters the status quo.
Part of this is due to the writers taking away one of the main reasons he was a threat - his ability to turn people into robots. Then came Sonic Genesis where he hit a "reset" switch that made it possible to roboticize again. While his whole plan didn't pan out he got a consolation prize in the form of Mecha Sally and the villain decay seems to be wearing off.
It's been brought up in-universe that Marvel villain Arcade has never succeeded in killing a superhero, even though that's actually his job. Justified though, as Arcade doesn't do it for money or out of spite, he does it because he loves the thrill of seeing the superheroes fighting out of his Murderworld amusement park. Now as to why anyone still bothers to hire him remains an untold story. Avengers Arena has Arcade frustrated by this. He decides to reclaim some cred by trapping a bunch of young heroes in Murderworld and force them to kill each other in a tournament ala The Hunger Games or Battle Royale. Somehow, getting other people to kill for him is going to increase HIS rep as an assassin. Arcade is not exactly mentally stable, so he could believe that himself.
This was especially obvious with the Disney villain, The Phantom Blot, who was at first a genuinely and believably scary threat to Mickey Mouse but after the serial in which he first featured ended, he quickly became just another bumbling comic relief villain.
Doomsday, the monster who killedSuperman has been affected by this. What used to be a threat which forced Superman to kill him (and then die after) after tearing through the entire Justice League, and after his first rebirth rampaged throughout Apokolips, shrugged off the Omega Beams and brought Darkseid to his knees, became a lackey, mind controlled villain for Darkseid in later series, then could be massacred en masse when an entire army was created from his tissue, to the point where he was One-Shotted by Imperiex in Our Worlds At War.
The Hank Henshaw Cyborg was the Big Bad in the Reign Of The Supermen storyarc. Since he had Clark Kent's DNA, he had all of Superman's powers and then some. During his first appearances as a villain, he single handedly defeated a team made up of Superboy, Supergirl, Steel and the Erradicator. Subsequent appearances made him less and less of a badass each time, with Superman defeating him with increasing ease. In the end he was working for villains of lesser quality and defeated in only two panels in his final appearance prior to the New52 reboot. Henshaw's downgrade is lampshaded in the Ending Battle saga:
Luthor started Superman's Byrne era as the richest, most admired man in the world. Some twenty years later and still under the same continuity he was a fugitive who had lost his money and reputation (but he got better).
Dan Slott has commented on the fact that many supervillains seem to go from being a threat to the hero by themselves to being one of six guys that get taken out by said hero. Given that he also enjoys inflicting Break the Haughty on villains who are a major threat, he says he wants to do a story where six supervillains are robbing a bank, and one of them is Galactus.
In a stunning and awesome inversion, Dungeon Keeper Ami features Mukrezar. Mukrezar is, and was, a Keeper of dubious competence, given to many Zany schemes (usually involving a Ring of Power). However, since his resurrection he has shown such an assessment is significantly lacking. Apparently his resourcefulness, ability to bounce back from crushing defeat, and, most importantly, willingness to take incredible chances- and then turn them into victories even if they failed -was his greatest asset. Every appearance has only increased the estimation of his threat, despite being Plucky Comic Relief.
Carface, the Big Bad of All Dogs Go to Heaven, was legitimately menacing in the original film (it was his henchmen who were incompetent jokes). The scene where he and his gang threaten Itchy at Charlie's Club may indeed be Nightmare Fuel for some. However, in All Dogs Go to Heaven 2, he loses several IQ points, and becomes the idiot henchman. The Villain Song, 'It Feels So Good to Be Bad', sung by Satan to Carface, seems to be about reversing Villain Decay and going in the complete opposite direction, but nothing ever comes of it. Carface never really regains the menacing quality he had in the first film, and ends up being sent to Fire and Brimstone Hell because he made a really stupid Deal with the Devil. While this plot point seems to be retconned in the series, he seems to only get worse, ending up playing a Scrooge archetype in "An All Dogs Christmas Carol". This was a chain-smoking, gravel-voiced,Manipulative BastardBad Boss that waits until Charlie's wasted and rolls a car into Charlie that if he didn't die from the impact would drown, took everything Charlie had, manipulated an orphan for gambling tips, beat Itchy with a gang of Mooks within an inch of his life then almost killed Charlie again until he himself was eaten by King Gator. Essentially if you're a Don Bluth villain in a sequelized franchise, prepare to be decayed. The only way out of that is to never appear in the sequels at all, which many Bluth villains do not.
Sharpteeth in The Land Before Timesequels. The original Sharptooth was an unstoppable killer and a true force of nature who had seemingly supernatural stamina. As the series continued (and became progressively more kiddy), all the carnivorous dinosaurs in general have decayed to the point of no return. It got so bad in The Land Before Time TV series that Littlefoot and the other kids were able to chase off two raptor-like Sharpteeth and one Tyrannosaur just by throwing fruit at them. It's especially bad considering how Red Claw is constantly referred to as the "biggest, meanest, most scary Sharptooth ever". Yes, the Tyrannosaurus rex that runs from some fruit is supposed to be more big, bad and gruesome than the Tyrannosaurus rex who violently ended the life of Mama Longneck and terrified both the dino-kids and real kids.
Films — Live-Action
General Grievous in Star Wars. Viewers' first look at Grievous occurs during Star Wars: Clone Wars, in which the cyborg took on six Jedi at once and completely destroyed them without much effort, establishing him as an unstoppable killing machine. However, the series' production team developed the character independently from the films' team. For Grievous's live-action appearance, Lucas wrote him as a significantly lower threat. The live-action Obi-wan faces a significantly weaker Grievous and dispatches him fairly quickly all by himself. The second season of the animated series attempts to justify the discrepancy by revealing more of Grievous's evasive nature and showing how he received the injuries he displays in the live-action film.
And then there is Godzilla himself, who has suffered from this trope to an unbelievable degree, starting as a devastating monster representing the terrors of nuclear radiation, and was later portrayed as a child-friendly defender of the earth.
Aliens in the Alien series. The first installment was a horror film in space, with a single, nearly invincible alien stalking and killing the helpless crew of a spaceship, with numerous rape parallels. However, the sequel Aliens was an action film, where a swarm of xenomorphs overwhelm a squad of space marines by virtue of sheer numbers. Since then, xenomorphs have increasingly been depicted as cannon fodder. The merchandise has further stripped the Alien of its mystique and creepy sexual undertones. The Alien vs. Predator series further decayed the villainy by focusing on kaiju-style monster battles. Pop culture has also participated in the decay with increasingly parodic tie-in marketing in the form of plush, Lego, superdeformed, etc.
Freddy Krueger of A Nightmare on Elm Street. In the original film, he was the menacing personification of evil; over the course of the various films that followed he gradually became an increasingly camp wise-cracking court jester. This was reflected in his marketing — he cut an album of cheesy pop songs, guest-rapped on a hip-hop track about his antics, was rapped about in a different Will Smith track, and was subject to all kinds of tie-in merchandise including yo-yos. It took years and the return of Wes Craven (in Wes Craven's New Nightmare) to address and attempt to reverse his decay.
As a matter of fact, the opening of Freddy vs. Jason reminds the viewers that despite the bad jokes, Freddy Krueger is a man who murders little children for fun.
Megatron is the menacing Sealed Evil in a CanBig Bad of the first movie, destroying whole cities and causing the only on-screen casualty of the movie. By the sequel, he's just The Dragon to the realBig Bad, The Fallen, and more or less just argues with Starscream for the second half of the movie. In Dark of the Moon, he's injured for the entirety of the film, doesn't get a single kill, and gets defeated along with the Bigger Bad in a matter of seconds.
Starscream. He easily defeats two of the Autobots in the movie, but in Revenge of the Fallen, he spends most of the movie reduced to being a joke and does not fare well in the third film either.
The first time around in The Mummy Imhotep is a walking plague, causing fire to fall from the sky, hordes of locusts and rivers to run with blood. In The Mummy Returns, he's just some guy with telekinesis who trades banter with an eight-year-old.
The Agents in The Matrix may qualify on the surface. They went from being the scourge of the virtual world and the most dangerous entity that could be encountered, to suddenly being little more than cannon fodder in the two sequels. However, while Neo has little problem dealing with them once he becomes the One, they are still a quite significant threat to everyone else.
In the first Jurassic Park movie, the T-Rex is an unstoppable monster, who can't be fought and only run from. He takes on the other villains of the piece in the final scene and kills them with ease. His face is the symbol of the franchise. In the second film, more of the same, only with a much higher body count. Third film? Hit by The Worf Effect: Killed unceremoniously by a dinosaur most dinosaur experts say he should have been able to take apart with ease, even being replaced on the franchise symbol. Villain decay indeed.
The first bug we see in action in Starship Troopers withstands the combined fire of four mobile infantry before going down. Later on bugs are seen taken down by just a few rounds. Justified by in-universe research into how best to direct rifle fire; we even see a clip of a training film.
The Djinn of Wishmaster was scary and so much of a threat in the first film because he was utterly evil beyond redemption, completely immortal, his powers knew almost no bounds, and he would bring about hell on Earth if he got his three wishes. What stopped him from being an Invincible Villain was that the entire plan hinges on granting wishes, so the protagonist could technically stop it by not wishing at all and had to be constantly wary of saying anything that could possibly be interpreted by the evil Djinn as one. In the second film, he suddenly has to collect 1000 souls first, and much of the plot placed him in prison, where he was significantly less menacing as a villain. The third and fourth films continue the process by making the Djinn killable, and having to pursue romance with a woman.
Count Olaf got less and less threatening as the books went on, although to some degree other villains picked up the slack.
Visser Three from Animorphs. He's completely immoral and monstrous, but as a consequence of appearing in almost every book and not killing the Animorphs, he quickly starts to come across as a blundering clod. Visser One even acknowledges this in The Visser Chronicles, comparing him less-than-favorably to the Helmacrons. While he does get promoted to Visser One towards the end, he still doesn't undo a lot of this.
It becomes funny when even Visser Three starts noticing the effect. "Would it be too much to ask for one of you to actually HIT SOMETHING?!?!"
D.Metria the Demoness from the Xanth series, started off as a fairly malevolent seductress, but with each subsequent appearance became less threatening, to the point that by the time she was "replaced" by her insane doppleganger, D.Mentia, she was Xanth's version of Mr. Mxyzptlk (The Superfriends version, at that). In more recent books this is justified by her having acquired half of a human soul, which gives her a conscience. She can still be mischievous, but is no longer malevolent.
In Tales of MU, Puddy and Sooni start out as the Manipulative Bastard and the Alpha Bitch, respectively, but are eventually reduced to being pathetic losers who struggle to keep even a couple people under their control. The worst aspects of the transition are probably a result of Webcomic Time: the change takes several months of real world writing time, but just a couple weeks story time.
A notable subversion of this trope occurs when Severus Snape and Draco Malfoy are presented as mere Jerkasses who like giving Harry and company hell for the first five books of the series, but at the beginning of book six, both are presented as high ranking members of Voldemort's army, the Death Eaters. It does turn out however that Snape was a Double Agent for the Order of Phoenix, and Malfoy was incapable of coping with actually being evil.
Lucius Malfoy. He is introduced as a sinister and cunning master-manipulator, who, while maintaining a benevolent and charitable public image, actively and ruthlessly pursued his ambitious goals, descending to threatening whole families and unleashing an ancient monster on a school. Next time he's just The Dragon, and after his failure and consequent fall-from-grace, he's reduced to an unnerved bystander with little to no involvement in the action.
The Wheel of Time series features significant villain decay with regards to the Forsaken, the 13 most devoted human servants of the Dark One. Initially presented as uber-badasses from the Age of Legends wielding powers most modern people could not begin to comprehend and being trained as scientists, generals and geneticists, the Forsaken get defeated repeatedly by the present-day heroes. Partly, this is because the Forsaken's reputation got exaggerated during the 3000 years the spent imprisoned, partly it's because they lack the support network they had in their prime, but whatever the reason, there's still a big gap between their myth and the reality, which was one of Jordan's more anvilicious points in his series (the gap between hearsay and reality, specifically).
The Young Wizards series averts this rather neatly, because a), the Lone Power has been defeated without great sacrifice a grand total of...once (and in a subsequent encounter, another aspect commented that that version of Itself was just plain stupid), and b) because It exists outside of time, dipping into our universe wherever and whenever It pleases, It can be decisively defeated in one place and simultaneously be an active threat elsewhere.
Sang-Drax from The Death Gate Cycle series was introduced in the fifth book as a Magnificent Bastard manifestation of the Big Bad that could play Haplo like a fiddle. While he's still cunning in the next two books, he gets a whole lot sloppier, downgrading him to a literal Smug Snake. He finally dies when a room caves in on him. This isn't as lame as it sounds because said room was filled with magic that was antithetical to him, but still — he really should have seen it coming.
It becomes apparent towards the end of Peter Pays Tribute that the Gray God needs Peter, his acolyte, more than Peter needs him. Also, Briskle is more crazy than competent.
Tolkien often does this deliberately in The Lord of the Rings, but still puts the less-powerful villains in situations where they can get the upper hand. Saruman goes from needing a massive army, a wizard, and more to stop him, to being somebody who could be defeated by a mob of angry Hobbits. Gollum is another example - he finds the One Ring to Rule Them All, and first uses it for murder and theft, but eventually crawls into a cave and uses the Ring's power to catch fish. The Ring doesn't particularly care for this. In fact, this is one of the core themes of the stories, because Evil Is Petty it eventually loses everything that once made it great and noble.
In The Silmarillion, this is explicitly canon for Melkor/Morgoth. He starts out out-powering everything else in the universe except for God and being quite cunning to boot, but as the book progresses he is drastically weakened after squandering his power and getting Shapeshifter Mode Locked, and his cunning goes down the drain as he goes increasingly Ax-Crazy.
In The Legend of Drizzt there's Artemis Entreri after the first few encounters with him, as Drizzt no longer wishes to fight him, and at one point refuses to kill him despite the perfect opportunity. Also, Entreri is getting old, whilst Drizzt is still in his prime.
The Heralds of Valdemar series plays this trope intentionally with the overarching Big Bad, Ma'ar. He starts out in the Mage Wars prequels as a frighteningly powerful, ruthless Well-Intentioned Extremist who can rival Great Mage Urtho in sheer power. Even worse, he conceives of an amazingly effective My Death Is Just the Beginning gambit involving hiding his soul in a pocket of the nether plane until a blood descendant learns to wield magic, at which point he steals the body, destroying its original soul in the process, and embarks on a new plan to Take Over the World. As he is constantly thwarted over the centuries, however, his spirit becomes increasingly petty and narcissistic, and eventually he grows careless enough to sow the seeds of his defeat when he fails to destroy the soul of his latest possessee, An'desha. Also a case of divine intervention, as it turns out that the Gods were tacitly abetting his scheme because they needed his knowledge to avert a repeat of the Cataclysm 3000 years later.
Deliberately invoked and deconstructed with Cersei from A Song of Ice and Fire. She started out as the puppet master behind King Robert and became one of the most feared characters in the series when she declared herself Queen Regent after his death... but promptly ran herself straight into the ground the second the checks on her power were removed. As the power went to her head, her schemes became less competent and more deranged over time, and while she was still somewhat feared it was more because she was psychotically unstable and overly trigger happy.
This becomes even more obvious when Petyr "Littlefinger" Baelish, a villain who was previously viewed as harmless (and still is considered as such in-universe) before throwing that façade out the window along with his wife, finds Cersei's stupidity and the fact that she still believes herself to be a Magnificent Bitch rather amusing
Matthew Luzon in the Petaybee book, Power Play. Though he was supposed to do a live-to-fight-another-day sort of thing, he ended up just hiding and sending other people to do his dirty work. Intergal counts as well, especially in the Twins Of part of the series. Their attempts to reclaim Petaybee become less about Petaybee and more about payback for losing it in the first place.
Happens to Lord Ombra in the series Peter and the Starcatchers. In Peter And The Shadow Thieves, all Ombra has to do is have his shadow overlap with yours, and immediately he steals your shadow, giving him full access to all your memories and turning you into an utterly obedient slave with no further effort or maintenance on his part required. In "Peter And The Secret of Rundoon" onward, he... can't. Being as Ombra in his first incarnation was ludicrously overpowered, it was a choice between Villain Decay or Only the Author Can Save Them Now.
In Death: This is mostly avoided by having a new murderer in each book. This still happened with Isaac McQueen in New York To Dallas. He started out as a cunning pedophile who had never been caught and he seemed to avoid even being noticed in the first place... until Eve took him down as a rookie. She wasn't even out to arrest him, she was just questioning him on a matter that was not really related to him, and he attacked her when she wouldn't leave. Twelve years later, he escapes from prison trying to get Revenge on Eve and still seems untouchable. However, by the end of the story, he turns out to be a pedophile who has lost a lot of his intelligence, and his ability to make even basic decisions. Dr. Mira even explains that the 12 years in prison, having the power to make decisions taken away from him in that time, and breaking most, if not all, of his patterns in illogical ways have devolved him!
Ewilan: when the Ts'liches were first introduced, they were described as the "ultimate predators" that only one man (Edwin) was able to defeat in a fight. As the story goes, Edwin goes from being able to defeat one of two of them in a fight, to defeat six of them at the same time. After that, their last appearance ends with the last of them being exterminated with ridiculous ease, with almost all character getting to kill one.
Hannibal Lecter fell hard. The first two books, Red Dragon and Silence of the Lambs, established Lecter as a highly intelligent and manipulative psychopath with little in the way of redeeming characteristics. In Dragon his only major contribution to the storyline is encouraging the villain to slaughter the hero's wife and son. Lambs gives him a small measure of likeability due to his actions leading to the triumph of the hero, the downfall of the villain and the rescue of the victim but his exact motivations for doing so are up to interpretation (and he also kills five innocent people who were in his way). Thanks to Anthony Hopkins mixing in a large dose of Affably Evil with the already potent levels of Evil Is Cool the character was upgraded to a something of an anti-hero in Hannibal. With the addition of a Freudian Excuse and a Robin Hood-esque preference to only killing assholes he was mostly defanged and was the star of his own story, Hannibal Rising.
However the Makuta species in general and their Mooks the Rahkshi did suffer from this a bit. The Makuta who got introduced later acted way more down-to-earth than Teridax and even displayed comical traits, what more, one of them, the similarly named Tridax got killed under mere seconds by one good guy. The Rahkshi also went from being near-unstoppable demons to simple foot-soldiers that fell to swords and some fire.
Multiple instances show up in the various Star Trek series:
The Borg are probably the most infamous example, gradually going from a once-a-season menace to a routine annoyance. In their original appearance in the Star Trek: The Next Generation episode "Q Who?", they were a faceless, inscrutable Hive Mind who could never be truly defeated because they always acted in perfect synchronicity, and could never be reasoned or bargained with. This gradually changed with "I, Borg" and Star Trek: First Contact, which presented the idea that the Borg could be taught to act as individuals, and introduced the Borg Queen as a physical leader figure whose defeat could provide an easy way to resolve plots. Then came Star Trek: Voyager, which took place entirely in the Delta Quadrant (the site of the Borg's homeworld), thus making the Borg regular antagonists for the first time in the franchise's history. Since tangles with the Borg suddenly became frequent occurrences, the writers of Voyager had to regularly depict them being defeated by the crew of the titular lone starship (in contrast to appearances in previous shows, where the Borg mopped the floor with entire fleets) in order to keep the story moving, thus robbing them of a good deal of their original scare value.
Likewise Species 8472, who were Scary Dogmatic Aliens until "In the Flesh". They were introduced as terrifying telepathic species of aliens from a place called "fluidic space". They traveled in bio-organic ships that could destroy Borg cubes like a photon torpedo could destroy a Ford Pinto. Not to mention the ships could combine energy weapons to blow up an entire Borg planet.
The Ferengi were downgraded from serious threats to comic-relief pests after only two appearances. The Ferengi were intended to be major recurring villains, but over the course of several makeup revisions, the Ferengi went from impressive to goofy-looking. This probably has as much to do with the fact that when the Ferengi were introduced early on in Next Generation, Gene Roddenberry was still involved with the production, and was trying as hard as possible to recreate the old series. However, characters that would have worked as villains in the much cheesier era of the original series just inspired laughter in modern audiences. Also, all else being equal it's easier to make tall guys look threatening than short guys. Roddenberry really wanted to have villains who are small in stature yet still dangerous, but it just didn't work out. Not that their status as Strawman Capitalists helped much either.
Q turned from a frivolous yet dangerous omniscient being who nevertheless delivered some important Aesops to Captain Picard, to a lovesick puppy who goes to Captain Janeway for advice on parental relationships and conflict resolution in the Q Continuum. Q really was one of those characters who were a case of Depending on the Writer, especially in TNG. He's creepy and borderline sadistic in "Encounter at Farpoint", then campy and unwittingly annoying in "QPid", then he's back to being sinister in "True Q". It's debatable whether or not he was even actually a villain, considering how many times he (sometimes indirectly) helped Picard and the crew.
The Dominion in Deep Space Nine suffer heavily from this trope as well. In Starfleet's first military encounter with them, three of the weakest Dominion fighters destroy the Galaxy-class USS Odyssey, ostensibly one of Starfleet's most powerful ships, with relative ease. By the end of the show we can see Galaxy-class starships destroy the Dominion fighters in one shot. note That said Deep Space Nine relied unapologetically on Conservation of Ninjutsu and Rule of Drama for all fights, often having wildly inconsistent or improbable results compared to the rest of the franchise and its own series.
After seeing how much respect the Borg lost during his writing stint on Voyager, Ronald D. Moore rather neatly avoided the trope in his remake of Battlestar Galactica. The villainous Cylons are only sparingly used as a direct threat to the heroes, and typically when the heroes do beat them there's some kind of price. However, one particular Cylon, Caprica-Six has decayed rather badly. Given she was only in one episode (the miniseries), where she performed one mercy killing and lectured Baltar and that was it, and then wasn't seen again until the late second season where she followed through on being sad at taking a baby's life by regretting the holocaust in its entirety and missed a man she from the beginning cared about, or why else bother to save him, she didn't have much badass to decay anyway.
Almost every season of Power Rangers begins with the villain being replaced by a new one — because after forty episodes of losing, the old villain doesn't seem as cool.
Special mention goes to Serpentera who goes from destroying a planet, to being ineffective because the bad guys don't know how to keep it powered, to destroyed by a motorcycle (albeit a motorcycle from god).
Goldar (who might be considered The Dragon to both Rita and Zedd) was an excellent case. In early episodes, he was a nighmarish opponent, more than a match for all five Rangers at once. But once he lost the element of surprise and they got accustomed to him, he slowly lost his edge. Jason handed him his first true defeat, and then Tommy handed him another. Around the time of Rita and Zedd's wedding, he had become little more of a joke and a parody of himself.
Rito Revolto was an extreme example. In his first battle with the Rangers, he handed them a sound defeat, destroying both the Thunder Megazord and the Tigerzord. Unfortunately for him, he got his clock cleaned by them in their second battle (once they replaced the destroyed Zords with the Ninjazords) and after that, he wasn't able to do anything right.
Pretty much inevitable for any of the recurring villains on Doctor Who.
This was the fate that befell the Doctor's greatest enemies the Daleks after 16 television stories, four cameos and countless appearances in other adaptations, especially when their creator, Davros, began to dominate the stories. They were later made more menacing again; in 1988 they were given the ability to fly, and for their 2005 return in "Dalek", they were given new abilities, such as a force field and the ability to crush a man's head using the plunger arm. However, they may be falling back into this, going in their more recent appearances from one being defeated by its own self-loathing, to a fleet being defeated by a Deus ex Machina, to millions being defeated by reversing the polarity. On the other hand four Daleks, later three Daleks and a Dalek-Human hybrid take two episodes to destroy. Because of this, it seems the Daleks suffer from some variation of the Inverse Ninja Law. The more there are, the easier they are to defeat.
Another weird example of limp Daleks is "The Chase" - while what they are doing on paper is very threatening (they have mastered time travel and are out for Revenge!), due to the story being an especially silly comedy Out-of-Genre Experience they come across as relatively harmless. They fall off things, one stammers "Um, er, er," when questioned by its Drill Sergeant Nasty superior, and they get defeated by amusement park animatronics before being mowed down by the Mechanoids. This was possibly given some Foreshadowing in the preceding story, "The Space Museum", in which the Doctor's companion Vicki confesses she finds Daleks to be quite cute. Fortunately, the Daleks go back to an improved version of their original, menacing characterisation in the next Dalek storyline, "The Daleks' Master Plan".
This was even lampshaded by Steven Moffatt, who commented that they had lost to the Doctor "400 times" (this was probably exaggeration, but he does have a point as the Daleks have only won ONCE over the past few years). For this reason he is temporarily retiring the Daleks, probably for a good couple of seasons. Considering that they have appeared ten times since the show's revival, it's certainly fair enough. And in the first episode under him the Daleks actually win by tricking the Doctor into enabling their race to be restored.
The Master particularly suffered from this, with many writers simply using him as a convenient bad guy with little motivation beyond being "eeeevil". The trend arguably started from his very first appearances, since he appeared as the Big Bad in every episode of Season Eight of the classic series, which arguably diluted his effectiveness right from the off. He always allied with another evil power, which then betrayed him, forcing him to work with the Doctor. Over his many appearances in both classic and new series, writers have tried most of the tricks above to avert Villain Decay, including threat escalation, frequent Enemy Mine plots, Alternate Universe victories, and having him murder the family members of series regulars. Probably for the same reasons that the series itself has been so long-lived, despite succumbing to Villain Decay several times over, the character somehow keeps bouncing back as a Magnificent Bastard. The new series attempted to correct this both by giving him a plausible motivation - complete insanity - and by showing how Bad Ass he could be; not least by stranding the Doctor at the end of time itself, becoming Prime Minister of Great Britain, massacring a tenth of the population of Earth and all in all being a rather Magnificent Bastard before the Doctor managed to undo everything.
The Cybermen were Doctor Who's most egregious victim of this trope. In Second Doctor Cybermen stories, they were powerful, some might say too powerful. That may be a good reason they weren't used for the entire Third Doctor run. When they were brought back at the beginning of the Fourth Doctor era, they were given a weakness: gold dust would clog their chest units and suffocate them. All well and good, until someone misinterpreted that to mean that gold itself was their weakness. In Earthshock it wasn't so bad, as only one was killed, and that weapon (Adric's badge) broke and was unusable. Despite their gold weakness not coming up in The Five Doctors and Attack of the Cybermen, they were still killed in heavy droves by Rassilon's tower's defenses, the Raston Warrior Robot, and even human weapons. The weakness returned with a vengeance in Silver Nemesis, however, treating us to the wonderful sight of Ace killing Cybermen with gold coins fired from a slingshot. The Cybermen seen that come from a parallel Earth do not have this weakness, and the ones from this universe that returned in the new series were no longer defeated that way (although one flagship was entirely destroyed by the Doctor as part of The Teaser of "A Good Man Goes to War").
Don't think the new series doesn't get in on the action. In their first appearance, they're a great menace and put the parallel Earth in a constant state of Robot War reminiscent of post-Judgment Day scenes in the Terminator franchise. Their second has them effortlessly brushed out of main villain role by the Daleks - quite a sucker punch to fans who expected an Ultimate Showdown of Ultimate Destiny if the Dalek and Cybermen ever met (really. Some people had been waiting for it since before the show was in color. Finally we get it and it turns out Cybermen have no better luck with the Daleks than the Redshirt Army and their Five Rounds Rapid.) Every appearance since has had that "getting emotions back is bad" thing mean "turn off the central gadget that suppresses emotions or give any partially-assimilated person a pep talk and all Cybermen drop dead." By this point, fans of the Cybermen will long for the days when you had to run out of gold coins to slingshot at them at some point.
This trope was one of the reasons why the Mandragora were not used in a story in The Sarah Jane Adventures as it was felt they would be defeated "too easily" (and so were replaced with the Ancient Lights).
Justified with the Macra. When they first appeared in "The Macra Terror" in 1966 they were a cunning enemy that were controlling a human colony without the humans realising. When they appeared in "Gridlock" in 2007 they were much larger, but now savage beasts that lived at the bottom of a Motorway, snatching vehicles that came too close. However Gridlock is set billions of years later when they have devolved into savage beasts.
A very weird mixture of this trope played straight and this trope inversed occurs with the Weeping Angels. As the show has gone on, they've gotten more powerful and dangerous, while at the same time, the audience has found them less and less scary: 1. In "Blink" they had an extremely disturbing appearance and behavior that caused them to be deemed the most terrifying Who villain of all time, but all they did was send people back in time a few decades and they had a weakness that they couldn't move while being looked at (and therefore if two or more caught sight of one another at the same time, they'd be frozen like that forever). 2. In "The Time of Angels"/"Flesh and Stone", they appear to have lost the weakness for each other's sight (although they still freeze when a person looks at them), they kill prolifically in the dark, any image of them becomes an angel itself, and if stared at too long, they infect the person looking at them and turn them into an Angel. However, the overwhelming response to this episode was that they were far less scary than in "Blink", partly because of the Inverse Ninja Law, and partly due to stylistic/directorial issues that stripped some of their mystery (such as acquiring the ability to communicate and moving onscreen, thereby breaking the conceit that the viewers gaze functioned like a character's gaze). This is granted very debatable within the fanbase (as with many things), but the point still stands. 3. In "The Angels Take Manhattan", they became an even crueler villain, sending people back in time when they entered a room, trapping them in a room for the rest of that person's life, and luring the past version of the person into the room just when their future self was on the brink of death, in a temporal loop that they fed off of. Yet once more, fan reaction deemed that they had lost all scariness and mystery, due to the same reasons as the previous episode and due to massive plot holes that made them seem ridiculous (such as the Statue of Liberty being an Angel). 4. In "The Time of the Doctor" they are reduced to merely a cameo. When one touches Clara she isn't even sent back in time, and the Doctor and Clara first escape by summoning the TARDIS round them, then the Doctor stops one with a mirror.
This seems to have happened to the Silents. Their plans extended to before they first appeared, with them blowing up the TARDIS and nearly causing the Universe to be erased from existence, which was the Story Arc through Series 5. When they appeared in the two part opening to Series 6 they quickly became one of the most popular villains, having invaded Earth and been manipulating humanity for thousands of years. However their rather confusing and convoluted plan made them seem less of a threat later in the series. Finally in "The Time of the Doctor" though the Silence appear it is claimed the Silents from before were a renegade faction and the main Silents eventually work with the Doctor.
In Buffy the Vampire Slayer, the Turok-Han (ancient ugly vampires) of Season 7. The first one that shows up beats Buffy all by itself, but by the end of the season everyone is hacking them down left and right.
Plus the Vampires themselves, who constitute a major threat in the first season, becoming progressively weaker until finally they're just a bunch of wussy mooks that even Xander has roughly even odds of killing in a fair fight. This was briefly Zig Zagged in later seasons where Riley had to become a Super Soldier just to keep up, but overall in both Buffy and the spinoff, Series/Angel, Vampires went from "Major threat that requires an incredibly superhuman girl to be born every generation just to deal with them" to "A random passerby can take one out with a pencil". They're still treated by the characters like they're a major threat, but the actual quality of the threat tends to be far inferior to how much they act like it's a threat, as exemplified by their strength being wildly inconsistent; sometimes being portrayed as being far above any normal human's and at other times they can be easily overpowered and restrained by a young (non-Slayer) woman who can't weigh more than 120 pounds.
Inverted in Angel. Wolfram & Hart, the Big Bad for the first two seasons, starts out as an evil organization for the underdog protagonist to rise up against, but are in actuality an Invincible Villain, as Holland Manners states that there goal is not to win, but to endure and be fueled and preserved by the evil that lies in all of humanity. As long as humanity exists, good can never win, and Wolfram & Hart will have nigh ultimate power.
That being said, in Season 3 they become secondary antagonists to Holtz and Sahjahn, and are portrayed as generally craven and incompetent. It gets even worse in Season 4, where their offices are destroyed and they are all killed by the Beast.
Nicole Wallace of Law & Order: Criminal Intent started off as Moriarty to Bobby Goren's Sherlock Holmes, which made her getting nailed in her return appearance so satisfying. Then she was brought back in increasingly ridiculous ways, to the point where she was closer to a supervillain than her original anti-Goren persona. The Villain Decay reaches its nadir in her final appearance, in which she's just a Red Herring for the real villain, who kills her offscreen.
The Source of All Evil in Charmed went from an angel-winged, black cloaked, seemingly omniscient entity, to a big guy in a black cloak who tried to kill the Charmed Ones with about as much success as every enemy before although he did become the only character to succeed in having one of the sisters permanently murdered, but that may have been defined above as during his first phase, and being severely wounded by one renegade demon throwing fireballs at him. Eventually he lost not just the wings but the menacing hood as well and revealed a goofy face before dying, and it was revealed that The Source of All Evil is a transferable title. The new ones? Were never threatening.
Any and all demonic threats in general suffered from villain decay; early demons, albeit being a Monster of the Week in most cases, were a threat to the sisters individually; later on, when all-purpose vanquishing potions were produced by the gallon, they were mere nuisances most of the time. Perhaps this is why villains in later seasons consisted of one of the Elders who supposedly oversaw all "good magic," beings capable of removing people from reality at their whim, and finally, other witches.
Scorpius in Farscape managed to remain a Magnificent Bastard throughout the second and third seasons, thanks in large part to the writers letting him achieve total victory in the second season finale. The third season thus became about the heroes trying to reverse their earlier loss. However, by the end of the third season the show introduced a new villain who served as Scorpius' superior, and the first few episodes of the fourth season saw him apparently lose all his fearsomeness, with Grayza and Braca dragging him around on a leash like a dog, occasionally forcing him to lick Grayza's boot! For a time, he even looked as though he was becoming one of the heroes, with the only concession towards his original magnificence being somehow able to get himself (begrudgingly) accepted as a part of the hero's crew even though he freely admits that his goals and motivations haven't changed a jot since when he last tried to kill everyone. And then the whole descent into mediocrity turns out be a subversion: not only does he backstab Crichton so masterfully that it takes him two episodes to figure out what happened, but he manages to neatly counter Crichton's attempt to backstab him back. He's even pretending to be a double-agent for the Scarrans, fooling the Emperor himself into believing that Scorpius had been employed by him for years. For good measure, it turns out that Braca was on his side after all, and the "dragged around on a leash" thing was just another part of Scorpy's masterplan.
In the first season, the early-on Big Bad was Bialar Crais, the senior local Peacekeeper who was chasing them because he blamed Crichton for his brother's demise. He is usurped (and ruined, professionally) by Scorpius at the end of Season 1 but reappears later and becomes (uncomfortably for all) a semi-crew member due to his symbiotic relationship with Moya's child.
Harvey (the neural clone of Scorpius inside Crichton's head) was specifically introduced to avoid this trope. This way Scorpius could appear as a constant threat without downgrading this menace by having Crichton escape at the end of the episode.
The clone itself was subjected to extreme villain decay when the chip that generated it was removed from Crichton's head. While it did survive this, it lost all ability to control Crichton, and its personality degenerated from an exact clone of Scorpius to something that bore at least as much resemblance to Crichton.
Grayza began to suffer decay as the Scarrans became the main villains of season four- and ended up kidnapped by them due to her own gullibility. Particularly blatant was the revelation that Captain Braca- who she'd supposedly enslaved with her infallible pheromone glands — was actually still working for Scorpius; he went on to personally remove her from command to prove it. And just to rub it in, her command carrier was retaken by Scorpius, who'd recovered from his bout of villain decay.
Shows up quite a bit in the Stargate Verse. In the interest of fairness, it does have to be granted that there's a justification for aliens suffering some decay, in that part of the SG teams' missions is to promote Villain Decay; that is, a large part of the purpose of the Stargate program is to go forth and find out what's out there, and ways to defend Earth from those threats. If they were at all successful, Villain Decay was simply the logical extension of their success.
Stargate SG-1 fits this trope like a Goa'uld hand device. The Goa'uld were introduced as merciless, brutal and could effortlessly obliterate Earth as well as having a firm grip on much of the galaxy, held back only by in-fighting caused by their lust for power. When our heroes encounter just a small group of Jaffa, they manage to escape in one piece if lucky. But as the series progressed they became a bunch of arrogant, scheming, childish fools with a Napoleon complex and their mighty Jaffa armies become P90 fodder. Their flanged voices sounded cool and creepy when spoken slowly and calmly, but sounded ridiculous when they put any real emotion into it. By the end of the series, a Goa'uld encounter is just an inconvenience as our heroes have bigger fish to fry.
In the original Stargate movie, the heroes only fought one Jaffa one-on-one (well, two or three on one, really) and then only really survived because Daniel ringed down in the exact right place at the exact right time. Since that's not exactly a viable tactic for an ongoing series, the Jaffa get progressively wimpier as the show goes on. Free Jaffa, however, seem much more badass than their enslaved counterparts, partially because there are fewer of them, and therefore the writers don't have to worry about tipping the scales too much. The Jaffa's increasing status as mooks was later lampshaded, with Jack pointing out that Jaffa weapons and tactics are meant to terrorise populations into obedience, whereas P-90s are weapons made to kill.
The Replicators, on the other hand, largely avert this trope, as each time the heroes meet a bunch of those things, it has required an even more insane plan than the last one to merely stall them. Trapping them in a time-stop bubble (they escape), sending then into a black hole (escape too), finding a ancient-made BFG specially designed to destroy them (become immune) and friggin' finally, using a weapon that can fry the entire Milky Way to destroy them all at the same time once and for all. Their Asuran brethren in Atlantis required a similarly insane plan to put them down once and for all.
The Wraith in Stargate Atlantis also went the way of the Goa'uld, as first the Atlantis Expedition develop a retrovirus to turn Wraith into humans, but then get reduced to in-fighting amongst themselves over dwindling food (read: human) resources. The Wraith lost their powers to cause hallucinations after their first appearance. Even though they can regenerate from wounds quickly, their scab-masked grunts quickly become just so much cannon fodder. Back around "The Lost Boys" (season 2), it was a difficult prospect for a small team to infiltrate a Wraith hive; by the later seasons ("The Queen" or "The Shrine"), the good guys are almost nonchalant about walking into Wraith territory. This wasn't helped by the introduction of the new Big Bads on the block, the Asurans (who were really just the Replicators, but less threatening). Part of the reason for the Wraith's decay was that they had to be weak and fragmented enough to not be able to simply curb-stomp the isolated Atlantis expedition. However when Atlantis regained contact with Earth, the Wraith threat became increasingly ridiculous, especially the idea that they were any sort of threat to the milky way considering the large amount of factions present there that could easily wipe the floor with them.
Adam Monroe, formerly Big Bad of Heroes season 2. When he returned in Season 3, he was downgraded from a Magnificent Bastard to comic relief. Then he was killed off by the new villain, Mr. Petrelli, in an Eviler than Thou moment. Oh, and all this took less than two episodes, possibly setting a new record for 'fastest villain decay ever'.
Mr. Sweeny on Neds Declassified School Survival Guide, to the point where he doesn't rat Ned out in the finale for sneaking along on the field trip. He leaves him stuck in that tree... "but do tell me how your day turns out."
This is somewhat justified by the fact that Sweeny was never really "evil" in the first place. Ned just thought he was, and as the series progresses, Ned stops portraying him as an evil scientist and more as a strict teacher who helps him out from time to time.
Dr. Smith on Lost in Space may be one of the most iconic examples of this trope. He was originally a dangerously intelligent saboteur attempting to kill the Robinsons, but by a few episodes in he had deteriorated to complete pest/buffoon status. Early attempts at character development soon puttered out, and he became simply annoying comic relief. This was a rare case of intentional villain decay. Jonathan Harris, and the show's writers, realized very quickly that the Robinsons would never have tolerated the much darker Smith from the pilot staying around.
Wizards of Waverly Place: The mummy wasn't nearly as badass in "Wizards vs. Werewolves" as he was in the "Chronicles of Moises" arc, and his defeat was ridiculously easy.
Ben Linus from LOST, through a mix of Sorting Algorithm of Evil and Character Development. In seasons 2 and 3, he comes across as the ultimate in Magnificent Bastardry (and he's still got most of those skills), but season 4 saw the introduction of his arch-nemesis, Charles Widmore, a guy that Ben is actually afraid of, and the conclusion of season 5 reveals that Ben has been the Man in Black's unwitting pawn all along. Adding to that, circumstances saw Ben becoming the Losties' Token Evil Teammate from season 4 onwards. But in this case, Villain Decay doesn't preclude being awesome, thanks to Ben's always-entertaining approach to solving problems and Michael Emerson's award-winning performance, and despite working with the Losties for three seasons he doesn't actually make a Heel-Face Turn until season 6's "Dr. Linus".
Brad Bellick of Prison Break. Corrupt Cop and Smug Snake in season 1, he serves as the main antagonist there and was quite cunning. He becomes much less of a threat in season 2. When season 3 sets in, he's completely pathetic, being the lowest of the low in Sona prison and being treated like shit by everyone. In season 4, he joins the protagonists and pulls a Heroic Sacrifice. Everyone mourns for him, apparently having completely forgotten what an utter bastard he was in the first season.
Partly due to Seinfeld Is Unfunny, Jerri Manthey in Survivor. She was seen as the original survivor villain mostly because she was the first to be called that. (Richard Hatch is probably more of the "original" survivor villain) She was actually booed off the stage in All Stars, yet years later after the likes of Boston Rob controlling the game, Russell Hantz sociopathically pushing his way to the finals and admittedly griefing his fellow players, Jonny Fairplay lying to get a sympathetic advantage, Ami Cusack, and players like Naonka, Corrine, and Randy just being a Jerk Ass...When Jerri showed up on stage in Heroes vs. Villains and wasn't like any of those, people actually applauded for her.
Jonny Fairplay went from the notorious liar of Pearl Islands to first voted out in Micronesia because he wanted to be with his wife and daughter. (He wasn't lying!)
Omen on Dark Oracle. In Season 1 he managed to be the Big Bad, even with his powers sealed. In Season 2 he returns with his powers unlocked...and is demoted to being a flunky of Blaze and Violet. He's still dangerous, but Lance and Cally have bigger fish to fry, and simply aren't scared of him anymore. In the end, he's reduced to a rather pitiful figure who pulls a Heel-Face Turn to help Cally and then dies.
A brilliant example of Tropes Are Not Bad from The Wire. In the first season the Barksdale crew ruled the West Side of Baltimore. By the third season, they were in a tit-for-tat and being matched by independent drug lord who had no backing and was young and inexperienced. Marlo's ruthlessness surprised even Avon but it went beyond that, particularly with the collapse of Avon and Stringer's friendship where the cracks could be seen as early as the beginning of the second season.
It was also thoroughly justified since most of Barksdale's muscle and key lieutenants were put in jail at the end of season 1. Without them, Barksdale has to try to run a criminal empire only using guys that were lucky enough to avoid the mass arrests, too unimportant for the police to bother with, or inexperienced newcomers replacing the old guys.
Meg in Supernatural. She's The Dragon and a major threat in season 1, and manages to capture Sam and Dean, and later their father when he attempts to double cross her. Sam exorcises her and sends her back to Hell, but she returns in season 2 and possesses him. After that, she disappears. Eventually she returns in season 5 as Lucifer's crony, but by that point the Sorting Algorithm of Evil has expanded far beyond her and the Winchesters are more focused on her boss than Meg herself. Still, she's indirectly responsible for both Bobby being crippled and the Harvelles being killed. Her true decay began in season 6, when she's forced into an Enemy Mine with the Winchesters after Lucifer is locked up again and new King Of Hell Crowley starts exterminating all of his followers. In the following battle Meg proceeds to be captured, tortured, rescued by Sam and Dean, and then nearly killed again when she attempts to kill a devil-trapped Crowley, who easily overpowers her and escapes. By season 7 the Winchesters seem to have forgotten she's a villain at all, and apparently don't have a problem with leaving their insane, comatose, and extremely powerful angel friend Castiel in her care when she openly states she has an ulterior motive (revenge on Crowley) in helping him. Nevertheless, she takes care of Castiel and doesn't even attempt to deceive or betray the Winchesters when he wakes up. She then decides to help them fight the leviathans, wrecking Dean's Impala as a distraction and taking out a couple Mooks before once again being captured by Crowley. When she reappears in season 8 she's largely turned to the good side, although she's still a Heroic Comedic Sociopath.
This happens to many reappearing villains in crossover and anniversary events of long-running tokusatsu like Super Sentai and Kamen Rider. While the general rule of thumb is that resurrected villains are always weaker than they were originally, it is still quite jarring to see top-tier bad guys like Shadow Moon and Apollo Geist getting defeated in less than a minute.
Captain Hook in Once Upon a Time seems to be heading down that road in season three. In season two he was a ruthless man obsessed with vengeance that even manages to pull a Batman Gambit over on Rumplestiltskin himself. However, come season three, he's been reduced to little more than a lovesick puppy following Emma around without any of his former edge.
Prince Charming, already a fairly ineffectual villain in the Shrek movies, gets decayed further to a minor protagonist in the Shrek pinball game.
John Bradshaw Layfield. Remember when JBL was the longest-reigning WWE Champion in the history of SmackDown!? Yup, nobody else does, either.
Averted with The Great Khali. Although he was marginalized a bit following the loss of his World Heavyweight Championship to Batista late in 2007, WWE managed to keep him sufficiently menacing up until his Heel-Face Turn a year later....and thenthe decay set in.
Also averted with Umaga. Although he did lose three consecutive pay-per-view matches in early 2007 after having previously gone undefeated for nearly nine months, he managed to remain a dangerous (if hardly ever successful) villain right up until his death in 2009.
Played straight and then subverted with Montel Vontavious Porter. He was the longest-reigning United States Champion in the title's 33-year history until Matt Hardy finally won it from him in 2008. From there he began a long slide into irrelevancy, until by the fall of that year he was stuck in a months-long losing streak that made you forget he had ever been at all competent. But this eventually proved to be a blessing when the crowd began to cheer him out of sympathy, resulting in a Heel-Face Turn and a long-denied victory when he won a match that enabled Triple H to participate in the 2009 Royal Rumble. He then went on to be just as successful as before, even defeating Shelton Benjamin to become United States Champion once again.
Lampshaded with Edge. After winning the World Heavyweight Championship in December 2007 (which he had never lost, as it had been stripped from him earlier that year due to an injury), he decided to safeguard his title by forging a strategic relationship with SmackDown! General Manager Vickie Guerrero. Although at first he only pretended to fall in love with her, as time went on he genuinely became enamored with her and transformed into a slightly effeminate character who swooned over Vickie every chance he got. In any event, the strategy was an enormous success: Edge had Vickie set up numerous scenarios in which his chief rival, The Undertaker, couldn't possibly win, and persuaded her to have 'Taker banished from WWE entirely. Edge proposed marriage to Vickie in the summer of 2008, but the ceremony ended in disaster after it was revealed that he had been cheating on Vickie with Alicia Fox, the wedding planner. Vickie told Edge that she hated him, which led to a brief period of Cry for the Devil as Edge begged Vickie for forgiveness. But Vickie would have none of it, and decided to avenge herself further by having Undertaker reinstated to WWE and booking him against Edge in a Hell in a Cell Match at SummerSlam. This turned Edge into a wild-eyed coward who didn't believe he could win the match without help, and so he sought advice from Mick Foley on how to win a Hell in a Cell Match. Foley flat out told Edge that he was going to lose the match, because ever since he had fallen in love with Vickie he had been a complete sissy....
For the Divas, Molly Holly. This two-time Women's Champion was forced to undergo a protracted Humiliation Conga that lasted almost a year after WrestleMania XX, when Victoria defeated her in a match, strapped her to a barber's chair, and shaved off all her hair. Eventually her hair grew back, but things only got worse for Molly when Stacy Keibler(of all people!) pinned her in three consecutive matches. This was truly the beginning of the end for Holly, because if someone so inexperienced could pin a former champion three times, then who couldn't? It wasn't long before "Mighty Molly" wasn't so mighty anymore - indeed, was a complete joke, losing every single match more or less regularly before finally leaving WWE.
Her problems began back in 2002. When she did her Heel Face Turn, she totally revamped herself as a self-righteous prude and never got any heat because she was always easily silenced by the hot girls she ranted at by them just taking their tops off, leaving her with her jaw hanging open in shock. It didn't help that Molly wanted to play it as a comedy character, intentionally wearing the most matronly-looking outfits possible in situations where something sexy was called for. It got still worse when WWE unleashed the infamous "Molly's got a big butt" angle and was made into a complete joke, even though she was Women's Champion. WWE eventually realized they made a mistake and had her temporarily Take A Level In Badass in 2003.
Happened at an alarmingly fast rate with Tensai. He went from a true menace, defeating John Cena AND CM Punk (In a handicap match, but still...), two of the WWE's top stars, but within a matter of months, became nothing more than a big man for underdogs to prove themselves against.
Ryback, as a face, was a guy who never lost without a lot of interference and anyone only came out over him they had a big numbers advantage. Almost every match he was in was a complete Squash Match, and even in pay-per-view main events, he was in control the majority of the time. When he turned heel after WrestleMania 29, his opponents generally got in a lot more offense against him, although he still won non-title matches. The real decay set in when Justin Gabriel injured his leg, which led to him having to quit a match against The Miz of all people the following week. At the same time, he ended up in a feud with Chris Jericho, which did nothing but make him look like a spineless coward, and indeed, only beats Jericho with a roll-up pin during the pay-off match. He also walked out on a match with Mark Henry. Rather than being a Wrestling Monster, he merely acts as The Bully to non-wrestlers backstage.
Whenever a new army in Warhammer 40,000 is introduced, they start as existential threats to the entire setting for a year or two and then decay into just another faction.
There's an obvious reason for this: profit. Make your new faction unstoppable arse-kickers who make mincemeat out of any opposition, and you've got a surefire way of getting eternal 13-year-old boys everywhere forking out on them just so that they can be the toughest tabletop warrior. Then throw out some heroic last stands for the Space Marines so that the existing factions don't get completely alienated. Wait a few years till everybody's got them, introduce a new, even more powerful, even more expensive faction. Rinse and repeat. Profit.
Necrons. When first formally introduced, they were supremely enigmatic horrors serving even more horrific beings, known for mysterious harvests of life, unknown plans, and ridiculously advanced technology. Fan perception of them quickly made them Omnicidal Maniacs to the public eye, and they began to be perceived as a race-wide Creator's Pet. The 5th Edition Codex has resulted in a serious hit to the Necrons' previously unknown and unstoppable nature in favor of shifting the focus towards the Tyranids and Chaos as the greatest threats facing humanity.
The Orks started off as a galaxy wide tide of death and destruction but degenerated into pub brawlers over time.
Zigzagged with the Orks. No one in the setting, and very few outside of it, will argue that Ork horsed aren't a serious, ongoing threat to everyone else. The problem is, Orks have kinda been stuck in the status quo area for a while, and the big videogames featuring them have them on the bottom of the Villain Pedigree.
Tyranids also started off as unstoppable, galaxy-devouring horde of alien locusts, but their impending, full-scale invasion and eating of the galaxy kept getting delayed and delayed and then the tyranids inexplicably adopted an "attack in small numbers" strategy that made them less of a threat to the setting.
Then on a smaller scale you have some of the lords of Chaos. Abbadon the Despoiler is probably the number one offender. He is supposedly the heir to Horus and carries the title of Warmaster of Chaos, as well as the favor of all four Chaos Gods. However his Black Crusades seem to end in defeat more often than not, or at best as a stalemate. One can argue on whether or not it's his fault but the community at large now looks at him as a bit of a joke, earning him the nickname Failbaddon.
Games Workshop has spent the better part of 2013 trying their hardest to dispel this notion about Abaddon, eventually resorting to retconning eleven of his thirteen Black Crusades.
In any table top rpg with a significantly steep power curve, this can happen rather quickly. Dungeons & Dragons (and Pathfinder) are the best known examples, where an enemy which is an absolute terror at one point could be a beatable boss a few levels later and a mook a few more after that. If the villain doesn't scale to the power level of each Player Character, this trope becomes almost inevitable.
Bowser of the Mario series does this depending on the type of game. In most of the main platformers, he is shown as a genuinely powerful threat to the Mushroom Kingdom (and in Galaxy, the entire universe). In the sports spinoffs, he is the Trope Namer for Go Karting with Bowser who is actually on friendly terms with Mario. In the RPGs, barring the first Paper Mario and the 3DS game Paper Mario: Sticker Star, he is upstaged by another Big Bad while he provides comic relief. Played with in Mario & Luigi: Bowser's Inside Story. He goes through his usual Villain Decay like he always does in the RPGs, but it establishes him as a legitimate Badass at the same time.
The Big Boo got less impressive (and sometimes less big) over time, so the series kept having to introduce new Boos (Yoshi's Island included relatively tiny Big Boos, and a new boss called '''BiggerBoo'''. New Super Mario Bros. 2 introduced us to Boolossus, who not only takes up half the screen, but still feels the need to cheat by peeking through his hands when you're facing him.
Sonic Generations plays with this: At first, the Big Bad just seems to be a pretty generic Eldritch Abomination, and Eggman's role is reduced to being a mere victim. But in the end, it turns out the Time Eater was really a robot piloted by the Eggmen all along. Yes, Eggmen, plural. In a similar way, Sonic Lost World has Eggman forming an Enemy Mine with Sonic after the group of demons that he had tried to control, the Deadly Six, betrayed him. However, he was just using Sonic to defeat the Deadly Six, and once they're out of the way, he tries to take over the world with a super-charged mech, leaving him as the final boss.
The Space Pirates from the Metroid Prime series get hit hard with this in Echoes—after being the driving menace of the first game, they are abruptly downgraded into a recurring nuisance to Samus— but this can be justified by the game wanting to play up the threat of the Ing and Dark Samus, and the fact that the Pirates on Aether were a small, marooned colony that got many of their crew killed or Ing-possessed.
LeChuck from the Monkey Island series. In the first game he is quite creepy, as is his ship and crew, a bit very Laughably Evil. By the third game he has been Flanderized into a rather humorous albeit sadistic character who enjoys hurting Guybrush for the hell of it.
Tales of Monkey Island furthers this even more, and after the intro he's transformed into a genuinely nice guy who Guybrush is suddenly worried about Elaine legitimately falling for. Then the end of chapter four manages to reverse four games worth of decay in a few scenes, and LeChuck manages to revert into the evil bastard he used to be and makes him more of a threat by having him actually kill Guybrush.
Wily and Sigma of the Mega Man and X series. They started at world domination and thus couldn't up the ante, they were obligated to never win a single token victory except perhaps during the intro mission, every game had them unleash a new wave of greatest minions ever who would fall like dominoes, and the biggie is that they used roughly the same approach (8 robot masters and a fortress, give myself a robot body, and maybe try to make it look like someone else is the villain at first) in every game in the entire series and were defeated singlehandedly by the same person every time. On the plus side, they got a new "more evil" true form every time.
In X6, Sigma can barely string together coherent sentences ("JUSDIE, Zelllllllloooooo!!!!!"), and is more of a robotic hunched-over zombie who can actually be knocked down, not just back. On the flipside, prior to X6, Simga's schemes seemed to get more evil each game. In X3, when Dr. Doppler comes up with a cure for the Maverick Virus, Sigma turns him evil, along with Mavericks he cured. In X4, he causes the Maverick Hunters and an army called Repliforce to go to war with eachother, creating a very morally ambiguous plot. To top it all off, he comes close to destroying the Earth with a big laser weapon, which X and Zero fail to stop, and is only stopped by the leader of Repliforce, General sacrifice himself. In X5, he makes a scheme to turn Zero maverick by crashing a Maverick Virus infect space colony into that Earth that would cause KT impact esc damage in the process, and depending on the plot of the game, he succeeds and also wipes out most life on Earth in the process (X6 goes assuming the colony did crash into that Earth, but Zero not going Marvick). In all endings, he nearly kills X, and appears to kill Zero.
Wily's decay was lampshaded by Mega Man at the end of Mega Man 9: Wily, defeated, begs for his life as usual, and Megs shows him a hologram of Wily doing the same thing for the past 9 times:
Wily may actually be a subversion, though, considering he creates The Virus, which plagues the X series long after his death. Plus, in the current timeline, his robots have gotten harder to defeat.
Noticeably averted with Dr. Weil, Big Bad of the Mega Man Zero series. Mainly because, instead of fighting him at the end of every game like Wily and Sigma, his first appearance is in The Stinger at the end of the second game, and the player doesn't even get to fight him until the fourth and final game.
The Mario & Luigi games bring us Fawful. Right Hand of the main villain in the first game, in the second...he sells badges in a semi-secret shop ranting about how he'll have his revenge on Mario and Luigi one day, which happens in the next game in the series.
Notably Fawful completely averts this trope by the next game as he returns as the Big Bad, becoming more a threat than he was before.
Dracula in the Castlevania series has been thrashed by the Belmonts and their friends more times than can be Counted (vun hundred and fifty two! Vun hundred and fifty three! Vlah ah ah...), usually only a brief time after his resurrection, meaning he rarely has time to do anything particularly evil. He was finally, perhaps wisely, retired in the Sorrow series...and ironically replaced with new villains who seem a whole lot more inept and ineffectual than Dracula himself ever did. After all, they are canonically Dracula wannabes.
In Final Fantasy VII, Sephiroth was an extremely menacing Big Bad - a phantom, unstoppable killing machine cutting a swathe of death across the world, always one step ahead of the heroes, and all while manipulating Cloud into a Tomato in the Mirror from within his own mind; all so he can simply mind control him into giving him the Artifact of Doomright after putting six feet of steel clean through his ally in a single strike. With a smirk. What a bastard! But in subsequent appearances, all he seems to do is appear out of nowhere with his theme song blaring to deliver a Hannibal Lecture and kick the hero's ass for a while before he gets owned. Again. What really makes this worse is that in the original game, Sephiroth didn't care about Cloud beyond his use as a puppet. In fact, he didn't even recognize him when they met. Then in later appearances, he's completely obsessed with Cloud to the point of having no other motive than to defeat him (save for the Kingdom Hearts series, in which Cloud is the obsessed one, though with the lack of motivation from his original game.)
Dissidia: Final Fantasy seemed to play around with this. He actually starts out wanting to control Cloud as a puppet...and it grows into a Foe Yay-tastic obsession by the last time you fight him. For that matter, all the villains in Dissidia suffer a decay in one way or another. They go from being the ultimate evils in their universes to just servants of another slightly more ultimate evil. Plus the chaotic storyline really limits their plot roles. Sephiroth, for instance, doesn't seem interested in doing much of anything beyond just taunting Cloud. He has the goal to become a god but his mindscrewing with Cloud seems to have nothing at all to do with that, yet it takes prominence for him.
Final Fantasy X has a strange example. After confronting the party in Bevelle and revealing his motives, Seymour becomes a more powerful threat gamewise. But storywise, the party brushes him off as completely nuts and stops taking him seriously. In the end, Tidus' reaction to Seymour's final appearance inside of Sin is a simple annoyed "Don't you EVER give up?"
Ashura of the SaGa series. In the first game, he is the penultimate boss. In the second game, he is the first major boss of the game, and it can be a pretty challenging fight. By the third game he's just a normal boss.
The HK-50 droids in Knights of the Old Republic 2. In Peragus, one droid was able to make the entire mining colony its bitch over a few days. Than a squad of three of them showed up at Telos and jobbed against the hero, before finally three more were defeated by T3-M4.
The original Big Core of the Gradius series has undergone significant Villain Decay; while the original game's bosses were almost nothing but Big Cores, bigger and more powerful Bacterian technology in subsequent games slowly phased this boss out until, in Gradius V, it became a regular, if large and heavily-armored, enemy.
Vizier Khilbron (a.k.a. the Undead Lich) and Shiro Tagachi were the Big Bads in the first two chapters of Guild Wars, and each of them made a challenging opponent at the time. But when they show up again in Chapter 3, Nightfall, even the two of them teamed up are merely just another speedbump on the way to the new Big Bad, Abaddon.
Maleficent from Disney's Sleeping Beauty was to a degree the main villain of the original Kingdom Hearts. She was also a very respectable villain in the prequel, Birth by Sleep, which showed how she begun her rise to the power she had in the original game. When she is revived in Kingdom Hearts II she only can control Heartless, is left plotting in a wreck of a castle as opposed to the magnificent one she had in the original game, and has only one loyal servant left... Pete. However, this is often lampshaded, and by the end of the game she seems to recapture her former glory by conquering Organization XIII's castle once Xemnas is destroyed.
Sadly, coded and 3D set her back even further. She at first looks imposing in coded, breaking Data Sora's digital Keyblade even! But her plan then ends up easily thwarted, she gets crushed by a superpowered Darkside, and she retreats alongside Pete with the main characters hardly caring about letting her go like that. In 3D, she's a blatant Big Bad Wannabe, who only appears in one scene before being driven away, and it's clear no-one's scared of her anymore.
Even Maleficent had it easy compared to Jafar. In the original Kingdom Hearts, he was The Lancer to Maleficent in the Disney Villain group. In Kingdom Hearts II, he gets ONE scene and an ensuing boss battle, and then gets Killed Off for Real.
In System Shock 2, SHODAN went through this trope herself. After the hacker "destroyed" her in that showdown on Citadel Station, SHODAN hibernated on the computer system within the garden grove on Citadel where her experiments, the Many, were created. Her pod was ejected from the Station, and after three decades, it crash landed on Tau Ceti 5. Then, SHODAN's creations thrived, and since she was out of commission at the time, while they were thriving, they grew rebellious and plotted to turn against their own creator: SHODAN herself. So, she aids you as you dispose of the Many, even though she threatens and insults you. SHODAN, however, stopped fitting into this trope after you finally exterminate the Many. Then, she plans to merge her power with the Von Braun's Faster Than Light travel drive, so that she could combine the cyber world with the real world, allowing her to change reality as she sees fit. SHODAN leaves you for dead, and then you fight her. And once you think you've defeated SHODAN for good...
In Capcom's Resident Evil franchise, Oswell E. Spencer is the prime example of this trope. In the beginning, he was the one pulling all the strings. He was the leader of Umbrella Corporation. He was the one who was responsible for all the terror and destruction that the T-virus caused. But after the constant thwartings of Umbrella's schemes, and the deaths of some of its most prominent workers, and especially after Chris and Jill destroyed Umbrella's T-ALOS project, Umbrella went bankrupt, and the authorities were aware that Umbrella was behind it all. Spencer then became a fugitive, losing everything.
From a gameplay standpoint, this applies to Nemesis in Resident Evil 3: Nemesis. He's at his worst during the first fight, where you don't have much to go with in terms of items, and he's in the form that has the fastest and hardest-to-dodge attacks from the start. Subsequent forms have him with his trademark rocket launcher, which can attack at a distance, but makes him unable to grab Jill, and repeated shooting at him throws his aim off, and if they hit, the rockets don't hurt as much as his grab. By that point, you already have access to the Magnum or the Grenade Launcher, too. This is actually a recurring process, where he gets progressively slower in his attacks and you amass more powerful weapons and ammunition, making each fight easier than the last. By the end of the game, poor Nemmy is a joke.
Arthas in World of Warcraft. In Warcraft III, he starts out as a paladin with potential who is the only person to really beat the Scourge (he was supposed to, but the guy in charge of them didn't know that). Then he turns into a death knight and is presumably even stronger. Kicks some ass in Frozen Throne while fighting with some rather major handicaps. Merges with Nerzhul to become the Lich King, making him even smarter, stronger and upping his magical abilities. Apparently Blizzard realized this made him an unstoppable one man army who could probably take the world over by HIMSELF, so all throughout the latest World of Warcraft expansion he makes one huge mistake after another, looks like a total moron and kills his followers who are actually rather competent (one took down the Drakkari empire by manipulating you) instead of you. Oh, and he's also done nothing of importance over the last...what, ten official years? Something like that. He's really good at making himself lose.
He seems to keep you alive simply out of his own amusement. He kills the troll guy whom you've already defeated and pretty tells you "Good work tricking the guy that tricked you. I'm gonna let you live now because that amused me. Come up with something like this again and I may let you live."
In the cinematic after defeating Arthas in Ice Crown Citadel, Arthas reveals the whole point of allowing the players to live and defeat his lieutenants and defeat HIM was so he could kill/resurrect them as his NEXT batch of lieutenants. What, the Ling King doesn't have enough power - he can only support enough Big Bads to fill a raid with?
And apparently, this was "intended". In the next raid, Uther speculates that Arthas' piss poor attempt at war is the only thing keeping the Scourge from rolling over Azeroth.
Kael'thas in Magister's Terrace. Justified in that he's been resurrected since killed in Tempest Keep and the process didn't go too well for him. It still feels weird to be fighting such a big name character with five people and then cut off his head to hand in to a quest NPC, but it feels even weirder that Priestess Delrissa, Vexallus, and every trash pull in Magister's Terrace were by far trickier affairs than the prince — much less that his second phase could be soloed by any self-healing class (given enough time).
It sort of got better as World of Warcraft progressed. Burning Crusade featured the Ogre clans united under Gruul the Dragonkiller, himself a horrifyingly powerful and nearly God-like figure amongst the Ogres. His names comes from the time he killed off dozens of Black Dragons (a previous big deal enemy to the player) by picking them up and slamming them into the spiked landscape. Cataclysm features the return of Cho'Gall, who puts the Magi in Ogre Magi as an insane cultist leader with a ton of eldritch abomination powers. He also makes good use of the remaining Ogres as muscle.
Speaking of black dragons, their leader Deathwing, avert this trope completely. He's initially just a minor character in Warcraft II: Beyond the Dark Portal, and the Day of the Dragon novel elevates him by giving him a backstory that says he's a fallen demigod and gives him Chessmaster qualities. By the time he actually makes a return to the games in Cataclysm, even though it's made clear he's The Dragon to a the Old Gods, he becomes even more a threat since they made him even stronger and allowed him to nearly destroy the world.
Deathwing is an interesting case, since he actually becomes more powerful and threatening in World of Warcraft than he previously was at any point in the series, going from roughly the same strength as the other Dragon Aspects to strong enough to take on all four simultaneously. However, he also underwent massive Flanderization, going from an arrogant but extremely intelligent Magnificent Bastard to a Generic Doomsday Villain. You can't picture WoW Deathwing tricking the other Dragon Aspects into putting their powers into the Dragon Soul, or using his Daval Prestor persona to manipulate the Alliance from within. He just doesn't have much character besides wanting to destroy the world For the Evulz... And despite his heightened powers, he can't even do that effectively (see his entry on the Villain Ball page).
The Burning Legion are arguably hit worse than Arthas. Formerly set up as the Big Bads of the whole series, World of Warcraft has them Demoted to Extra and rarely to do we see anything they do perceived as a big threat. Somewhat justified since a bunch of their high ranking members are dead, but to put this is in perspective, they're overshadowed by the Undead Scourge, an army they created.
Inverted by the Forerunners, they're now the main villains, and field armies of Mecha-Mooks who are long way from the near-harmless floating Sentinels of the original trilogy. Case in point- there is a Promethean Knight who wields a Crazy Awesome BFG that is a combination flamethrower, missile launcher, and shotgun.
In Capcom vs SNK Big Bads will come chase you down every once in awhile, during which the player can't even get an option to attack them and it's an instant kill if they so much as touch you. Suddenly though in the end you can fight them back. Rather easily even.
The Kusabi suffers this a bit in the third game, where he returns as a boss but loses his One-Hit Kill abilities. On the other hand, he gained flight and a huge amount of speed, making him a much more terrifying opponent, so...maybe it evens out.
In Left 4 Dead, the Tank was something you ran from. With the introduction of melee weapons in Left 4 Dead 2, a creature that once required a huge amount of lead to bring down can be taken out much more rapidly with cricket bats. Thanks to the fact that his melee only hits one survivor at a time, if all survivors gang up on him, he'll die in no time. Later fixed in a patch. It now takes about twice as long to kill a Tank with melee weapons, long enough that unless you have absolutely perfect team coordination, at least one of you is still going to get pummeled before you bring him down. And molotovs still work just fine.
The Witch also gone through similar changes. When she was first introduced in Left 4 Dead, she was a huge threat because she has the ability to instantly incapacitate you in a single hit and then finish you off quickly. Playing on Expert? She will kill you instantly. The only way to kill her quickly before she went berserk was to head shot her with a shotgun and you better hope your first shot landed the first time. As time went on, many players gotten very good with the "head shot with a shotgun to the Witch" technique, making Witches nothing more than a hurdle in your path. People also discovered that a head shot with the hunting rifle would stumble the Witch before she would go into her rage mode, giving other players enough time to mow her down.
The sequel made Witches even easier to kill thanks to several new game mechanics. Wandering Witches are Witches that can slowly walk around in their passive state, but unlike the sitting Witch, Wandering Witches have one second freak out if she is startled, which means she will scream first and then goes into her typical rage mode. This makes it easy enough to blast her with shotguns quickly even without a head shot. Explosive ammo also stumbles her so it's possible to kill her by just using explosive ammo with any gun besides shotguns.
Zant in The Legend of Zelda: Twilight Princess looks like a pretty cool villain. Sure, his outfit is a bit weird, but it has a creepy vibe and he acts like a true Magnificent Bastard. He also walked right into Hyrule Castle and forced Princess Zelda to surrender, and he's nigh untouchable the first few times you see him. He stole the fused shadows from Midna, and nearly killed her. Plus the one time you see a part of his face, he has a very nasty looking smile. Once you reach him, he turns into something below a Villainous Harlequin, and his actions (and attack animation) take away any shred of dignity he might have had left. At the same time, we find out that all his power was given to him by Ganon, and without that, he was just a whiny and batshit-insane Twili chancellor who wanted to be king. Perhaps this is just so we won't miss him after he gets inevitably Hijacked by Ganon.
Rodrigo Borgia from Assassin's Creed II starts out as the menacing Big Bad in the game by slyly walking around Italy making sure everything is going according to plan and has a cool dark reddish-blackhooded robe, but at the end he ditches the cloak for not as cool majestic Pope robes and shows off how much of a fat bald guy he is. Then he ditches his Magnificent Bastard demeanor and rambles about religion. If that's not enough he gets the stuff KNOCKED out of him by a bare handed Ezio. And finally the next game has him being upstaged by his kids with them disobeying orders and is eventually killed by an apple.
Consider: For nearly the entire game, Ezio wants to kill him. After killing everyone else involved in his foul conspiracy, he gets a chance to kill him, and fails. He gets another chance, then, when he finally has Rodrigo completely at his mercy...he spares his life. Why? Because it would do more harm to the Templar cause to have him live on as a meaningless figurehead whose master plan achieved nothing. It don't get much harsher than that!
Kerrigan from Starcraft. In the original she was little more than an bloodthirstyElite Mook to the Overmind. In Brood Wars she ascended into manipulating ALL the other factions against each other, eliminating one key figure after another and eventually crippling her enemies and proclaiming herself Queen Bitch of the Universe. And it WAS NOT an empty boast. Then...came Wings of Liberty. Sarah suffered from a sever case of "Arthas Syndrome", and for the whole Terran campaign stayed in the background, was repeatedly thwarted by the humans, kept spurting cliched villainous trites interlaced with tedious fatalistic wish-wash, and finally was dezergified by her boyfriend, who carried her on his arms into the sunrise. As of Heart of the Swarm, she's no longer a villain. She's an Aloof Ally for Team Good Guy.
On that note, the Zerg as a whole deserve their own mention. Originally, they were disgusting amoral Xenomorph-esque Hordeof Alien Locusts led by the Overmind, a Magnificent BastardEldritch Abomination who successfully exterminated and assimilated much of their more powerful Xel'Naga creators. Originally, according to the first game, he created Kerrigan as a means of using her as the Zerg's ultimate weapon against the Protoss, which worked beautifully when the Overmind and the Zerg Swarm successfully conquered the Protoss homeworld of Aiur. Then came the revelation from Starcraft 2's "Wings of Liberty" campaign: The Overmind was just the Dark Voice's pawn and nothing more. All of the magnificent bastardry and successful Chessmaster ploys that the Overmind executed weren't made of its own volition, but done according to the directives of Amon. Its desire to merge the Protoss (who represented the Xel'Naga concept of "purity of form") and Zerg wasn't simply to create the ultimate lifeform for its own sake, but to somehow bring Amon back to life and give him some Elite Mooks to throw at everyone else. Also, individual Zerg characters develop their quirky (if still morbid) personalities, started to talk (and with their big ol' maws it looks just as silly as one could imagine, to the point that most of the talking Zerg even look Ugly Cute), and one of them sports a beard. No, seriously. Sad to say, what was once a horrifying Horde of Alien Locusts have more or less turned into PokémonIN SPACE!
Depending on your experience, Giovanni in Pokémon FireRed And LeafGreen might be this. The first time you face him, his Kangaskhan could wipe out your entire team, given that it is quite powerful for an enemy at that point in the game. When you finally face him in the eighth gym, though, his team consists entirely of Ground types. That is, the very first type you learned to counter in the game. They are also all weak against Water attacks, just like the Fire gym you just defeated, and most of them have low Speed and Special Defense, meaning you could one hit kill most of Giovanni's team with a single Mon.
Street Fighter's M.Bison has fallen as hard as it's possible for a fall to be. You will recall that in Street Fighter II Champion Edition, he conquers the world if he wins the tournament. And as late as Alpha 3, he can wipe a city completely off the map. Unfortunately, by the Capcom vs. SNK games, he's reduced to nebulous plans, and by Street Fighter IV, the only consequence of him triumphing is a somewhat unpleasant conversation with Juri.
That said, the plot in SF4 also heavily hints towards the current Big Bad Seth as being nothing more than an Unwitting Pawn for Bison that he easily kills and removes when he's getting a bit too hard to control.
Mortal Kombat's Shang Tsung has also fallen hard. Remember when he was the final boss of the first game? He was then revealed to The Dragon to the real Big Bad Shao Kahn and still remained an activate player in the plot. In Mortal Kombat Deadly Alliance, he teams up with Quan Chi and manages to kill both Liu Kang and Shao Kahn. However in the quasireboot Mortal Kombat 9, Shang Tsung's role is greatly diminished in favor of Quan Chi who takes most of Shang Tsung's old role during the MK3 part of the story and is given a rather awkward death.
Shao Kahn himself zig-zaggs with this. Starting in 2 he was Shang's Bigger Bad and forces the heroes to do battle on his terms in the Outworld. After that fails, in 3 he invades Earth itself and is battled in a giant temple to his glory in the middle of a destroyed city. Then he falls to the wayside, killed/or forced into hiding in the backstory to DA, his backstory in Deception reveals he usurped the throne from Onaga who came Back from the Dead and is just another Kombatant come Deception and Armageddon, even getting punked by Shang and dragged off by Onaga in the intro. Come 9 however, its revealed he won the battle and has gained almost limitless power. The story is then rebooted and he regains his stature from 2 and 3 and the focus becomes defeating him "right this time".
Most of Batman's rogues in Lego Batman 2. While it takes the entire game to bring them down in the original, most of them are curb stomped in the first level of the sequel. It doesn't help that all of them have tiny health bars and Freeze and Croc don't even make it out of their cells. The only exceptions are Joker who manages to destroy the Batcave and Scarecrow who takes a level to catch, has a big health bar and a Nightmare Fuel filled boss battle where he turns into a giant.
The Spider-Man 2 tie-in game has Mysterio, who starts out demolishing a theatre, staging an alien invasion, and holding the Statue of Liberty hostage. Shortly afterwards, once you've beaten his "Funhouse of Death", he resorts to sending out small parties of useless robots that break like fine china when you hit them, and is eventually defeated with one punch while holding up a convenience store.
Winston Payne from the Ace Attorney series is billed as the Rookie Killer, with a seven-year winning streak. In the events of the story, though, he serves as a Warm-Up Boss who never wins a single victory against the player, and quickly becomes a Butt Monkey who isn't even recognized by fellow prosecutor Edgeworth. After his first defeat by Mia Fey in a flashback in the third game, he only wins a single case, and that's only because the defense was trying to lose.
Arfoire from the Neptunia series is a weird example, as there are three different versions of her. The first Arfoire was a descent threat of a Big Bad (who didn't really have any underlings and so did most of the work herself). The second was a much larger threat, being a nigh-unstoppable Sealed Evil in a Can with several powerful worshipers spending the entire game bringing her back. The third Arfoire? Not only is she a part of some other person's group, she is constantly defeated with ease (even when she takes the true form of the second Arfoire) and at one point is sexually assaulted by one of the "heroes" to the point where she is rendered mentally broken for 10 years. This culminates to a battle towards the end of the game where she is single-handedly defeated by Neptune after the former had fused with an eggplant. One can only hope the next Arfoire has better luck.
Jacob of Dominic Deegan is all over this trope. In the Visions of Doom arc, he was introduced as a near-unstoppable necromancer who fought a powerful spellwolf to a standstill, out-maneuvered his seer brother and manipulated an evil cult into conducting a dark ritual before killing them with ease so he could use their body parts to make a necromantic golem. Later, he and his golem tries to team up with the Chosen to unleash the Storm of Souls, but their plans are ultimately defeated and his own creation turns on him. He gains a bit of credibility by tearing off his own flesh, but never really regains the Badass status that he once held. In the most recent arc, he gets owned by Huk Thak/Roki, is murdered by the Shintula Chief and is finally betrayed once again by his servant Neilen and left to rot in the orc version of the afterlife. The character's future is uncertain but it's clear he'll never attain the coolness that he once had.
Update: as of the comic's end, Jacob came back to life, made amends with his family, helped them take on the ultimate Big Bad, and eventually absorbed and conquered the Blight so as to become a master of death as he had always wanted—but only by balancing it. So in the end he became badass again, but only because he stopped beinga villain.
This was done (probably deliberately) to Faz in Shortpacked! In his first appearances, he was an insufferable Smug Snake who managed to become Galasso's favorite employee via manipulation and undermining his coworkers all while rubbing it in their faces. Now he's the most pathetic member of the cast and treated as little more than a nuisance.
After Kelelder from Jix was killed the first time by Jix (though, this term is used loosely since he's an immortal), he's become somewhat of a Kenny type character. The creator didn't want him to be seen like this, so Kelelder made an agreement with another character to back off from the main character and stop trying to kill her.
In Schlock Mercenary the Partnership Collective came within a hair's width of killing off Tagon's Toughs three times in the comic's first year, the second two times with WMDs. But the third time they wrecked the Lunar Space Elevator and the government put a bounty on the Collective's Attorney Drones, now that the Toughs are paid to shoot them on sight they've become nothing more than comic relief.
Lampshaded in thisBrawl in the Family comic, where Bowser kidnaps Peach, and both Mario and her treat it like a mere inconvenience. The Alt Text rubs salt in the wound by saying "We look forward to teaming up with you the next time a real villain is in town!"
In Homestuck, Jack Noir has been suffering this as of late. Once an absurdly powerful Hero Killer who single-handedly destroyed the trolls' session and one-shot God Tier John before he could even make a move and even took down Grimdark Rose, he's been getting gradually overshadowed by even more powerful villains, while the heroes have been getting more powerful as well, with John even going toe-to-toe with Jack and managing to score a hit on him.
Zig-zagged with Gamzee after his Freak Out. He spends a nice period of time being the most terrifying villain in the comic, then he calms down and becomes a mix of Token Evil Teammate and The Friend Nobody Likes, then he's revealed to be secretly working for Lord English, but then he spends a pretty large portion of his onscreen time after that being either filled with lead by Caliborn, being mind-controlled into being Aranea's minion, or performing seemingly-random acts of capriciousness like prototyping the Nobles' sprites with his friends' corpses, which never quite cross the threshold into threatening territory.
Blood Boy, a big antagonist in the early stages of Survival of the Fittest version 3 had this occur in the last topic he appeared in, becoming an almost Jokeresque figure (to the point of almost directly quoting from The Dark Knight at one point). This does, however, have a fairly good reason: a different handler took over the character for that scene, one who, needless to say, had a rather different take on the character.
The Necromancer, in the Whateley Universe. Starts out as one of the top 60 supervillains on the Interpol rating scale. He's now oh-for-two against Team Kimba, who are high schoolers. Even with his team of supervillains working for him. Now one-for-two, making out like a bandit in the process, excluding one goal failing due to a Unknown Unknown
On TWGTG, we have the Mad Scientist Dr. Insano that first appeared on The Spoony Experiment, whose early appearances depict him as a Laughably Evil, but none the less dangerous character. Later appearances, however, have him attempting no evil plans and just have him acting comedic.
The suave, chessmaster-like, psychotic Ask That Guy is slowly turning into a pathetic, needy, emotional wreck. Maybe played with because he's always been like that, he just can't seem to hide it anymore.
Strong Bad, from the infamous Homestar Runner universe, used to try to do actually evil things, but he's gone under lots of Villain Decay. To quote him from the Strong Bad Email(sbemail) called "your edge"
Strong Bad: Me and the Cheat, walked past this deflated basketball and consciously decided not to re-inflate it! And we feathered Strong Sad for a HALF HOUR!
In Noob, Dark Avenger is one of the most feared player killers of the game... except a Running Gag has Sparadrap accidentally killing him. He's shown to give other characters difficulty, but it made the decay slower rather than keeping it from happening. Writing Dark Avenger out of the series due to his actor moving away relied on having that decay reach the lowest point possible.
The first Teenage Mutant Ninja Turtles animated series used heavy Lampshade Hanging ("at last, Shredder, you've done something right!") to underscore how completely the Shredder had become a joke villain. While he was mildly threatening in the first season (although to what extent this is the case is cause for debate), villain decay set in very quickly after that, as it did with most of the series' villains.
The Shredder of the secondTeenage Mutant Ninja Turtles cartoon series, on the other hand, managed to emerge seven seasons mostly unscathed by villain decay, growing more powerful to the point where the turtles stopped being able to defeat him with martial arts alone, and always portrayed as both scarily competent and pure evil. However, not all the series villains are so lucky—the Shredder's dragon, Hun, in particular, went from "tough" to "joke" in the space of one season, before regaining some measure of respectability during the last third of the show's second season, which he retains—mostly by not featuring him in any extended battles with the turtles—until the end of the show... and gains a considerable power upgrade upon becoming an Empowered Badass Normal in Turtles Forever. Karai went from beating all four turtles and Casey Jones (easily) in her first appearance to Leo and Mike making a complete mockery of her in her own base when all they were there to do was steal an Ancient Artifact.
Sideshow Bob didn't suffer from this until many seasons into the show. His subsequent appearances always outdid the last and became a lot more violent and heinous, but he still never won. Around his fifth or sixth appearance he lost it though. Even worse, Mr. Burns used to be a greedy, heartless, megalomaniac Corrupt Corporate Executive, the villain of many episodes. He was regularly depicted as decrepit and with the mindset of a more reactionary era, but that didn't stop him from being entertainingly pure evil. Come Season 10 and beyond, he was inexplicably transformed into an inoffensive old man, most of the jokes about whom revolved around his senility and physical frailty. In other words, yet another victim of the terrible case of Flanderization which has plagued the series.
Burns also showed a very dynamic sympathetic side, where he's almost a Scrooge like figure feeling the effects of a plentiful...but empty life. This is still shown occasionally but in a lighter manner.
This trope is invoked in-universe in the Halloween special's A Nightmare on Elm Street parody. After Groundskeeper Willie (in the Freddy Krueger role) is defeated, Bart and Lisa contemplate his return. He appears moments later, but has been reduced to an ineffectual villain whose bufoonery is even accompanied by jaunty music.
Cobra Commander, the main villain of G.I. Joe, follows other aforementioned 80s cartoon villains' example but he's worth special mention because in parallel to his bumbling persona in the cartoon, his original comic book persona remained a ruthless Magnificent Bastard all throughout to its final issues. This decay was probably intentional because his bloodthirsty ways needed to be toned down for the Sunbow series. Though also worth mention is that while the cartoon Commander was mostly inept by the end of GI Joe's second season, at least he was a part of the sub-plot concerning an internal civil war within Cobra where Commander and a few others made up the secret sub-group Coil in an effort to slowly wrestle control of Cobra away from Cobra emperor Serpentor, giving the villain at least some credibility by the end. However in the animated movie, Coil is never mentioned and Cobra Commander goes beyond becoming everyone's Butt Monkey for the film in a literal sense, in a way decaying the character in two different ways. Fans of GI Joe tend to not like the movie very much based on this, and the fact the movie attempts to retcon what is known about the Commander's past to something beyond ludicrous...even by 80s cartoon standards.
On Codename: Kids Next Door, it took an age-ified Nigel and the rest of the team to take Father down in his first appearance ("Operation: G.R.O.W.-U.P.). Then a few cadets took him down in his next appearance ("Operation: T.R.A.I.N.I.N.G."), making him more of a comic-relief pest. Then the writers escalated his crimes by turning the KND into animals ("Operation: G.R.A.D.U.A.T.E.S."), and after that was taken care of, they had him extend school hours to 8:25 p.m. (a big deal, since the protagonists are school-hating children — "Operation: P.R.E.S.I.D.E.N.T."). In Operation: Z.E.R.O., he is reduced to being completely ineffectual when faced with his father. Z.E.R.O. plays with this, however: after his father banishes him for not being competent enough, he goes into a state of depression which takes his moral opposite brother to pull him out of. Together they face their father and though Father is still afraid he tries to stand up for himself. Soon after he gets sucker punched and Grandfather begins to rag on him a bit and sets off his Berserk Button. His unstoppable rage is so fierce that it makes his heroic brother, who was previously shown to not be afraid of anything, step back and makes Grandfather, the unstoppable evil who has conquered the world, afraid. But before he can do anything he gives up because he's too depressed. This shows that it's not the lack of ability that holds him back but rather the lack of self-confidence.
Speed Racer: The Next Generation had Zile Zazic, the main villain of the show, oil tycoon and trustee of the Racer Academy...who went through every possible process in which decaying villains could go through! It didn't help that he only carried out his plans first-hand two-thirds of the way through the season. By the end of the show, his plans became boring and predictable.
Jack still gets a few impressive moments though, and the monks sometimes pay for underestimating him. In the Grand Finale, he hits Chase, Hannibal and Wuya with this when he is revealed to be the ruler of a Bad Future who has imprisoned all three of them in small cages, all because Omi wasn't around to stop him.
Wuya too. Starting off as a fairly credible mentor to Jack, she eventually regained her magical power and managed to take over the world off-screen. Upon her return in the second season however, she was reduced to nothing but a loud, obnoxious whiner, and by the end of the season she was reduced to being nothing more than a cheering fangirl for Chase Young. She regained some of her villainous grativas in the third season, but even then, she was just never as cool as she was in the In the Flesh 3-parter.
Valmont in Jackie Chan Adventures. Just look at the guy. In the early seasons, he was a charismatic, refined, rich-out-his-ass leader of a worldwide criminal organization who could very well be mistaken for a Magnificent Bastard (He was even able to hold his own against (and get the better of) Jackie in their personal confrontations.) Then take a gander at the later seasons...knocking over convenience stores, living in an apartment no bigger than your bathroom (seriously), and leaning on the three goons he has left to pay for the bill at a pancake shop. The last time he's ever seen, in the show's final episode, he's become a bus driver.
The Dark Hand, the criminal organization in question, went with him, going from a Nebulous Evil Organization with an army of Mooks to Finn, Ratso and Chow. Until they decide to retire because, after being repeatedly beaten up by Jackie Chan and enslaved by evil sorcerers and demons with nothing but pain to show for it, they decided Being Evil Sucks.
The Shadowkhan are a good example. They were quite potent in seasons 1 and 2, but in the Oni Mask saga, they go "poof" if someone so much as trips them (though they are central to a Near Villain Victory and World Domination scheme). Also, when the Enforcers became Dark Chi Warriors, who initially could survive falling off a cliff, but towards the end of the season, couldn't survive a fall of 10 feet.
Shendu also suffers from this. Early on, he's an ominous and threatening figure, despite being stuck as a statue. During the end of the 1st season, when he's finally regained his powers, he's a vastly powerful and menacing evil dragon. During the 2nd season, however, he's beat up by his demon siblings because he lost his body and they still have their's, then gets stuck sharing Valmont's body and is largely a joke, except during the finale, and when he later returns during the finale of the 3rd season. During the series finale, however, he's stuck helping the heroes, and is largely a joke when fighting his son Drago.
And yet, almost any episode where Rampage didn't have a major role in the plot will have him either get curb stomped or hand him The Worf Effect. Often in a really silly way.
From the Transformers Wiki, regarding the first episode of the show, re: Waspinator: "In a stark contrast to his later career, he actually proved to be a serious threat and had the advantage over the inexperienced Cheetor, pinning the young Maximal down in a canyon." Somewhere between blowing himself up with his own missiles in episode 4 and Rattrap kicking him in the nuts at the end of season 1 his true destiny came to fruition. Amazingly, in Beast Machines he suffered villain decay all over again as Thrust.
Animated on the other hand doesn't always have them as the villains. They also reverse the Took a Level in Badass the Autobots as a whole went through by having a crew that was never meant for battle with tools that had primarily non-combat purpose in mind, so it takes the whole team to take down just one or two of the armed-to-the-teeth Decepticons.
Animated started out with the Autobots requiring all hands on deck to stand a chance against any of the Decipticons but by the end of season 3, Optimus Prime is able to take on Megatron single handed.
Starscream took a level up in badassery when he first fought the Autobots and was too strong for them (making up somewhat for the stupidity he displayed earlier), but later he's largely a joke because of how he keep getting his ass kicked by Megatron. Two other Decepticons, Blitzwing and Lugnut also suffered from this. Initially they would be considered Not So Harmless Villains. They were dumb, but either of them could take all the Autobots on his own. But as time went on, they kept on getting beaten by plot devices and largely become jokes.
Even the films have done this. The Decepticons were nearly unstoppable at first, Starscream even managing to outmaneuver the Autobots and the air force in the first film — in the sequel, they get thrown around. Although the extent of decay is hard to tell since the Autobots and humans from around the globe have been fighting for two years now as a specialized task force. They're better prepared this time. Plus the fights aren't in crowded cities, so the Autobots can cut loose, especially Optimus.
The Cons performance is poor all around, sure Starscream and Brawl could do some damage, but they were defeated rather easily, as well as most of the other cons. Look at the body count: Autobots lost one guy of five, Decepticons lost five out of eight. Almost all fights with the Autobots, bar Megatron's participation, are pretty one sided. In fairness: in the first movie the Autobots get their asses handed to them by the Decepticons unless Optimus is around; it's the humans and their pesky magnesium "can kill a Decepticon in a few hits" weapons that do most of the heavy lifting in the first movie. The magnesium weapons are so effective that one human soldier is able to kill Blackout single-handed.
Transformers Prime, following tradition, hands this to Starscream (again). He went from surprisingly competent in the pilot to standard Starscream near the end of the season. And while he dealt Arcee a No-Holds-Barred Beatdown in Partners and later went rogue, it's not been entirely undone yet. Admittedly it is justified with Megatron returning and things generally not going his way. So far Megatron hasn't been hit with this (yet), clearly being the most competent of the Cons (Soundwave perhaps being the close second). If the season 2 three-part premiere and "Operation Bumblebee" are to be believed, however, Starscream is getting back on form, particularly when he killed a dozen Vehicons in a row. And then he found the Apex Armor...
Averted with Knockout. Undoubtedly the least dangerous Decepticon character by default, two seasons in and he's remained consistent with his early appearances.
The Insecticons suffered from this and Conservation of Ninjutsu. The First delivered a brutal beating to Arcee, the Second took on Megatron and pummeled him, nearly winning, both having shot them to almost no effect. All subsequent appearances have them defeated quite easier by the Autobots and Megatron, who only need to shoot them once or twice to take them down. At first it seems that their insect mode is weaker to blaster fire, but by "The New Recruit" their Robot modes to fair just as poorly.
The Commando Droids of Star Wars: The Clone Wars went from Elite Mooks in their first appearances to almost as ineffectual as their ridiculously ineffectual cousin, the B-1 Battledroid.
General Grievous's zig-zags this quite a lot. For example, early on in Season 4 he was fooled by Jar Jar, and General Tarpals managed to injure him at the cost of his own life, allowing the gungan army to capture him. However, near the end of the same season, Grievous went up against the Nightsisters and their Army of the Dead, led by Ventress, and tore them apart.
Vlad was once a Magnificent Bastard and a very competent arch-foe with an often sympathetic side. But in the third season, he became a shallow crook with little redeeming qualities; his final plan was to force the world to let him save it from a giant meteor in exchange for world domination (an agenda that was poorly conceived) and 500 billion dollars...as if he wasn't already filthy stinking RICH!
Skulker is another example. In his first appearance he was genuinely menacing, and had Danny looking over his shoulder scared. Fast forward to "Micro Management", where Danny could defeat him in a few blasts, and only became a threat when Danny lost his powers. It reached its peak in "Girls Night Out", where a bird chased him off.
In his first appearance in Yin Yang Yo, Kraggler is an incredibly elderly gargoyle who is discounted by the siblings due to his age, then proves to be a very powerful and capable villain, who, rather than being defeated, is convinced to stop because of an apology for his mistreatment. From then on, he's treated as a joke villain (even moreso than the other villains, this being a comedy series) who's only a threat if he uses magic to reduce his age.
Ra's Al Ghul himself suffered from this in Batman: The Animated Series and later DCAU canon. Introduced as the leader of a global secret society, whose first villain plan involved wiping out 99% of the human race to save the planet, and once described by Batman himself as "a criminal mastermind more dangerous than Lex Luthor and The Joker combined", Ra's would end up spending EVERY SINGLE ONE of his episodes trying out various wacky schemes to cheat death and expand his already 600 year long lifespan, instead of doing anything productive to menace the human race.
Ra's al Ghul's Villain Decay is still debatable, considering how even in the Superman: The Animated Series episode "The Demon Reborn," Batman himself declared that Ra's al Ghul was more dangerous than both The Joker and Lex Luthor combined. Later in the canon, though, they refer to something called "The Near-Apocalypse of '09," which Ra's was behind, and apparently took the whole Justice League to stop.
Ra's says in "The Demon Reborn" that he realized the Lazarus Pits effects were becoming shorter and shorter, so perhaps he was concentrating on finding out a way to live longer before trying another world domination effort.
He actually does have a couple of decent showings after that, and wasn't always portrayed as stupid; ironically, his Dumb Muscle characterization started with (and was worst in) the episode "Almost Got 'Em", the irony being "Croc" was actually Batman in disguise. Unfortunately for Croc, this was his most famous "appearance", so this is how he is remembered.
In his second appearance, while the Clock King did gain a device that actually let him control time, he also didn't show a lot of what made him such a formidable opponent in his debut episode (his ludicrously precise timing and planning). This resulted in the man who was able to physically match Batman in combat being taken out the instant his device broke. This may have been a good thing though, for those who found him able to do such a thing too hard to swallow.
Magnacat from Monster Allergy is a serious threat to the Tamers, but not anymore when his plans kept on failing, he becomes bankrupt. Hector Sinistro becomes this as well.
While never particularly smart, the Trix sisters from Winx Club were competent enemies, acting on their own in the first season. They still were more than decent during the second season, even if by then, they were already reduced to the main villain's henchwomen; notable was their fusion into a single, powerful entity in the last episode. Then the third season came, and they became little more than a joke - they even received some power-up at some point, but it turned to be useless.
Subverted at the very end of the first movie when they team up with their ancestors, the Three Ancestral Witches, promising to rip the Faeries' wings off.
Valtor eventually suffered from this at the end of season 3. Even when he was well on his way to becoming the supreme sorcerer of the Magic Dimension, he began to grow mopey and whiny about how he was always hiding and that the Trix losing to the Winx meant that his defeat at the hands of the Company of Light years before was starting to repeat himself. When the Winx managed to hit him with the Water Stars he became reluctant to even fight them. Even his demon form that removed his weakness to the water stars didn't save him, as he ended up letting the Winx free all of his spells that he had stolen, was abandoned by the Trix, and nearly frowned himself when recalling his water spell. In the end he fell under the control of the Three Ancestral Witches before being single-handedly killed by Bloom.
Tritannus gets hit with this as well, at the beginning he was real menace to the Winx along with the Trix to who he granted powers to, but he slowly starts losing to the Winx to the point that the he gets easily beaten by Selkies, who he and the Trix easily kick around.
The Hive kids from Teen Titans started out in the first appearance as a well-organized elite fighting force that proved to be an even match for the titular heroes (even taking them down in their first encounter when they had the element of surprise), but by the last season they had decayed so badly a single Titan (Kid Flash) could trounce them all fairly easily (except for Jinx, who had a Heel-Face Turn anyways).
Even when they were badass, they hardly liked each other, and weren't all that bright, save Jinx and Gizmo (who was too immature to put his brains to effective use on more than one occasion). One could argue that without Brother Blood to scare them into competence, they just really didn't care about working in tandem anymore. They probably only stuck together at all by that point because they had nowhere else to go.
There's also the fact that since Teen Titans played a speedster near their full potential, Kid Flash was probably more effective alone than the main five in most cases. This is kinda re-enforced by the fact they got just one person to watch their city for the five of them.
Brother Blood himself got this pretty bad. His fighting ability never really went down- it was his intelligence and ability to make effective use of his other powers that suffered. In "Deception", for example, he seems to know almost everything that's happening in the HIVE from the start (including that Cyborg was The Mole), and he was only beaten in the end because Cyborg's half-mechanical brain was able to reject him. In "Wavelength" and "Titans East", he somehow give Cyborg his own powers by mistake, is completely oblivious to when someone with no mental enhancements at all is capable of completely resisting him, and blows his top at the first opportunity. There's more to decay than just a decrease in power, after all.
In Ben 10, Clancy the bug man was a sadistic psychopath in his first appearance. When he appears in the Grand Finale he has been turned into a generic bug monster for no reason. In fact, most of the villains that returned in the finale were decayed, with the exception of Charmcaster, who stuck to her role as Evil Counterpart to Gwen.
Vilgax. In the original series, Ben was never able to truly defeat Vilgax on his own at least, before he became Ben 10,000, only beat him badly enough that Ben and friends have enough time to get away. Fast forward to Alien Force, the Galactic Conqueror now has a less intimidating design and appearance, and, while he still displays a degree of badassery on occasion, gets notably defeated by Ben several times (one of the most infamous example being when Ben defeated him as Diamondhead, an alien that couldn't even scratch him before and actually broke his hands punching him).
Even worse with the Forever Knight, who went from a mysterious evil organisation to ridiculously weak villains who served as the heroes' punching ball (to the point in one episode, Gwen felt like it was more important for Ben to assist his girlfriend's tennis match than keeping an eye on them). Fortunately corrected in season 2 of Ben 10: Ultimate Alien.
Charmcaster may not have suffered any decay in Ben 10, but she was hit by it pretty hard in her second Alien Force appearance, where she killed Gwen of an alternate timeline and kept on boasting about it to the present Gwen...only to get her ass kicked by Gwen several times afterwardsdespite her claims of power. Her first appearance in Ultimate Alien then made it worse, turning her into a mook who barely held off Kevin, the team Worf, for a few minutes, and needed the help of two other bad guys to stand a chance against the heroes. So, at this point, it's probably for the best that her next appearance had her as an Anti-Villain who makes a Heel-Face Turn.
Darkstar was fairly threatening in Alien Force, being a superpowered Manipulative Bastard, but was hit by decay when he reappears in the first season finale of Ultimate Alien. He is so starved for energy that he's been reduced to skulking in alleyways preying on stray animals just to survive. He has also become predictable in his treachery — the heroes stop his attempt to backstab them in the very end with a literal push of a button and he gets taken out with a single punch. And in his final appearance, "Couple's Retreat", he's a flat-out moron.
Zombozo initially defied this Trope in Ultimate Alien; In his original appearance, he was a horrific and creepy stalker-like character who required being a Mook Horror Show from Ghostfreak to be defeated, but was an otherwise one-shot villain. When he comes back in Ultimate Alien, he proves to be a decent, Ax-Crazy and still scary villain, forming an alliance between various villains to attempt Revenge by Proxy on Ben and almost getting Gwen's aunt killed. Sadly however, Ben 10: Omniverse decided to play this trope straight by making him Denser and Wackier and having him going for petty theft such as rob bank.
Omniverse episode Special Delivery hits several villains with this. Most villains who show up in this episode could give a hard time to Ben in their previous appearances, and no less than three of them (Psyphon, Fisttrick and Trumbipulor) had been through impressive cases of Not-So-Harmless Villain. In this episode, they allgang up against Ben who at this point is alone and unable to call for help. Yet, he basically mops the floor with them. Psyphon gets slightly better on the plan of fight, but still manage to grab the Idiot Ball despite having been Dangerously Genre Savvy before.
In Gormiti: The Lords of Nature Return, this was the fate that befell Orrore Profondo (Deep Horror), who, in the backstory narrated in the toyline, was a terrifying opponent, feared by all the Gormiti siding with the Wise Old One. He even managed to trick the Air Gormiti into doing a Face-Heel Turn...but in the series (which takes place many millennia after the toyline story), he seems to play second banana to Evil Overlord Magmion and doesn't really show the competence a villain of his caliber should. However, this only seems to apply to his anime self: in the comics, as of now, he has retained all of his credentials and Magmion is just one of his underlings.
Satan was big and scary in his first appearance in South Park, but he's become "a whiny little bitch" in God's own words ever since he was first established as the lover of Saddam Hussein. It's arguable that he started out pre-decayed, though. He LOOKED intimidating, but his master plot in his first appearance was conning the city out of a lot of betting money, rather than, you know, the End of Days or anything like that.
Metallo from Superman: The Animated Series became less and less of a credible threat with each appearance. Probably intentional, too. Metallo's appearance over the episodes maintained the damage he suffered from each prior appearance, implying that he wasn't getting internal repairs, either.
Kalibak was perhaps the most obvious example. His first appearance was a whole-episode slugfest where he stood toe-to-toe with Superman. In subsequent appearances he's little more than a doorstop: Superman punches him out in less than a minute in "Legacy", and although he beat Wonder Woman he loses to Batman in Justice League. In his final appearance he finally got to do something useful... Because he was in an Enemy Mine situation with Scott Free and The Flash.
While Phineas and Ferb's Heinz Doofenshmirtz has always been a Ineffectual Sympathetic Villain, Buford was introduced as a truly nasty bully. By the second season he's mostly just cranky and posturing and is in fact a friend of Phineas and Ferb. Also, Tri-State Unification day episode contains both of these elements. He tries to ruin the parade, but is also shown to be Vitriolic Best Buds with Baljeet, whom he used to pick on (and still does, but it's pretty blatantly out of love; he even says as much once).
The Diesels and Spencer from Thomas the Tank Engine suffered a lot from this trope. Diesel 10 as well. In The Movie, he tried to have every single steam locomotive destroyed and ended up being thrown off a bridge and onto a barge full of sludge, but in one of the sequels he actually wants to take over the Steamworks because of the Dieselworks' poor conditions, and later trapping Thomas and Percy inside the Dieselworks and setting the entire place on fire only to end up being scolded by Sir Topham Hatt at the last minute and is forced to put out the fire he started and repair the entire Dieselworks as punishment because of this!
All villains from The Powerpuff Girls other than HIM suffered villain decay, especially Mojo Jojo, who was actually able to take over Townsville with an army of monkeys in the movie, which takes place before any other event in the series. Even Him suffered a few degrading roles in the show as well (in the writers' defense, he was an insane effeminate crossdresser, how could they resist?), as did Mojo continue having the odd threatening role however. While a lot of other villains got progressively worse, the competence for both former characters was arguably more a case of Depending on the Writer.
The '90s X-Men animated series did an interesting variation on this with Magneto. While he didn't lose any of his effectiveness or charisma, almost every appearance of his after his first battle with the X-Men ended with an Enemy Mine situation, painting him as less of a villain, or even an Anti-Villain, and more of an Anti-Hero who wouldn't join the X-Men for...some unknown reason. So, it wasn't so much his effectiveness that decayed, but more his villainous status itself. Made even more confusing because every time he appeared, the X-Men still reacted to him as if he were a bad guy, though he almost never attacked them anymore.
Not entirely true; he did steal a bunch of nuclear weapons to arm his mutant utopia Asteroid M, if only to protect it from outside attacks (and personally broke into the United Nations to warn them off); later, he teamed up with Apocalypse of all people and did attack the X-Men in that appearance, in order to kidnap someone no less- he later turns against Apocalypse and was suspicious of him from the start, and he and Wolverine end up saving each others lives; but still, its not hard to see why they are suspicious of him. His first Enemy Mine situation, for the record, occurred after he abducted a United States Senator and possibly planned on killing him (a Jerk Ass anti-mutant senator, but that's not the point).
Hotstreak started out as the biggest bully in school and the leader of a local street gang who becomes even more dangerous when he got exposed to the big bang gas with the ability to control fire. He was a genuine threat for his first couple of appearances but as the series went on he became more and more pathetic. He was decayed so badly that on one occasion when one of the heroines gave him a wedgie he ran away crying. What is he now, Jack Spicer?
Ebon also suffers for this. He's threatening at first, but gets his ass handed to him in nearly every appearance even with help form his henchmen, and loses fights against supporting characters.
Cedric, The Dragon from W.I.T.C.H., was quite menacing and monstrous in his first few appearances. As the series went on, though, his purpose largely became to get his clock cleaned by the heroines every few episodes, quickly robbing him of any serious threat. The series finale even yanked his chain by having him become supremely powerful...only to not realize he didn't know how to properly utilize it, and he gets beaten up rather easily AGAIN. Lampshaded in the first season finale, when Phoboschewed Cedric out for his failures epically, then used his newly-heightened magic powers to curse him into a pathetically small and weak version of his One-Winged Angel form.
Wolverine and the X-Men had the Brotherhood of Mutants first appear with a relatively clever plan that framed the X-Men for an attempted assassination. As the series went on and on, however, Pietro became increasingly stupider, and the effectiveness of group decreased.
Disney originally had Captain Hook be somewhat dangerous in Peter Pan, with him being decently competent against Peter. However, by Jake And The Neverland Pirates, he is now absolutely stupid and has a small ball on the end of his hook. Maybe small children are scared by pointy hooks, but it just seems silly. Or maybe he was just so clumsy with the hook that his crew did this so he would stop tearing his face off while grooming his beard.
Spider-Man: The Animated Series: The Hobgoblin experiences this, being outclassed by the Green Goblin in his last appearance. The story editor John Semper hated The Hobgoblin character and only used him due to Executive Meddling (his predecessor had plans to use the Hobgoblin instead of The Green Goblin and by the time Semper replaced him the toy had been commissioned and it was too late to change plans). So no surprise he became the Green Goblin's bitch.
Doctor Octopus also suffers from this where in as the series goes he descends into being nothing more but Kingpin's lackey.
The Lieutenant, Amon's electric stick-wielding second in command from The Legend of Korra, starts out as a pretty effective villain, absolutely crushing Mako and Bolin in his first appearance and giving both Lin Beifong and Korra herself the fight of their lives in his next one. In every single episode after that, though, he rarely does much more than show up, flail around a little bit, and get launched over the horizon in short order.
Zordrak of The Dreamstone, while he always usually left the dirty work to his mooks the Urpney, he was initially presented as a calculating, reserved villain who tactized a lot of the plans and had the odd moment of physical involvement (where upon things usually got a lot darker in tone). While the second season starts off well with his discovery of the Nightmare Stone, he quickly devolves afterwards, having little role outside something of a demonic Pointy-Haired Boss for the Urpneys. Since Urpgor and Blob's team were now required to concoct plans themselves, this did lead to a mild subversion in their case however.
Dark Vegan of Johnny Test started out as a serious threat, but by his third appearance he had decayed to the point where Johnny frequently thwarted him without even knowing he was there, and is more of a wacky nextdoor neighbor than a real villain. Also Eugene aka "Bling Boy Boy", who started as Johnny's Arch-Enemy but devolved into a frequent Anti-Hero friend of his.
Liquidator from Darkwing Duck. In his first appearance he had a lot of water-based powers, but seemed to lose them after that. (In fact, he didn't even appear as a villain by himself in subsequent episodes, only appearing as a member of the Fearsome Five.) This is probably because not only was he way too powerful, but there were only a handful of ways he could be truly beaten.
If the season 4 premiere is any indication, he still messes with ponies on a somewhat normal basis, and generally restrains himself only in the elements presence (and even then, to a degree). In fact, he's the reason the black vines are out of control...or even exist.
Tuma, his Rock Tribe and the Bone Hunters in BIONICLE: The Legend Reborn are virtually wholly separate entities from their depiction in the preceding stories — Tuma goes from an intelligent warlord Big Bad to a bumbling underling, and his tribe from an army of unbeatable warriors to goofy one-hit-kill Mooks... that couldn't even talk! The Bone Hunters received the same treatment, even though a single group of them could only barely be defeated in an earlier book. In a meta-sense, this is actually an inversion — the movie was written first, and the rest of the story material later. External sources did justify some of this threat decay better than the movie, though.