Can happen (on a smaller scale) in tabletop RPGs in general if the Game Master grows too fond of some of his or her own self-created NPCs. This is one of the primary reasons why the GMPC (a.k.a. DMPC) concept enjoys a mixed reputation at best among many groups.
In Bleak World the Princess class is an obvious favorite of the creator, to the point they seem to show up in every single backstory of every other class. While fans' views on the Princesses are mixed, it is clear that the author has nothing but love for his girls and presents them in a way that screams Incorruptible Pure Pureness. Issue has also been taken with whether or not magical princesses belong in a Horror/Action RPG.
In an enforced example, the Buck Rogers content that TSR put out in the late 1980s; because her family owned the license to the character, CEO Lorraine Williams pushed the company to focus on Rogers to the detriment of everything else, which resulted in fan-favorites like Spelljammer getting screwed as well as costing the company a huge amount of money due to forcing them to buy the Buck Rogers game rights at an inflated price in a clear conflict of interest.
In Cartoon Action Hour, a game meant to mimic the feel of an 80's Saturday morning cartoon, there's an optional set of rules where characters can earn "cool" points and trade them in toward perks that happen as a result of things on the production side of the show. One such thing is a writer taking this kind of interest in the player's character, although the actual bonus it confers is minor.
The supplement Black Circle introduced "a major new force in the Weird West: the Cackler". Not only is he a creator's pet, but all that was ever revealed about him is that he's supposedly the most evil Big Bad ever to walk the world. Heck, the authors even told you not to use him in the game because they had huge plans for him. Actual quote: "What you should not do is bring the Cackler into your game yet. He’s coming, and you’ll know it when he does. We’re not ready to say exactly when, but his presence will change the Weird West forever." Pinnacle later did a complete 180 on this, saying that it's highly unlikely that The Cackler will ever turn up in a Deadlands rulebook. Shane Lacy Hensley said that perhaps he'd tell the Cackler's story in some other media (probably because the story wouldn't have much to do with a posse), but they never again brought him up in any supplements.
To a lesser extent, the Four Servitors were Creator's pets as well, given how much was written about them and how far the authors went to make sure they were overpowered and unkillable. A few scenarios even involved the heroes unwittingly making those characters even more powerful (sometimes without them even knowing it)! By the time that Deadlands Reloaded came around, the authors generally relegated them to the background and commissioned a comic series where each of their backstories (which again, had little to do with a posse) could be properly explored.
2nd Edition's Complete Book of Elves caused some fan backlash over the text praising elves extensively, giving them superior knowledge of almost everything, moral sanction for everything (even for the grey elves, who keep slaves of other "lesser" elven races who are always inexplicably happy in slavery), the text outright insulting other races, and ignoring elven hypocrisy - this last one most evident in the sea elves, who go on great hunts that can depopulate a region of sharks but are given a pass on disrupting the ecosystem Because They're Elves, so it's okay. Somehow. For some reason. The author eventually apologized for the Blade Dancer, a fighter/mage kit that gives lots of mechanical bonuses, with drawbacks that amount to "things a Player Character would do anyway" (such as a tendency to being a badass loner who had spent years of Training from Hell before the campaign began, a tendency to rush off into an adventure if it seems that another elf is threatened, and focusing their skill on a single type of weapon), with implicit permission to derail the story by getting to determine for themselves whether a situation is even covered by their code of conduct. Torment: Tides of Numenera's Kickstarter even made one of its stretch goal rewards a videotaped apologyfrom the book's author.
3rd Edition and 3.5 (essentially, 3rd Edition Revised) have Spellcasters, an outgrowth of the Linear Warriors, Quadratic Wizards aspect that had been part of the game since earlier editions. One of the core rules' main designers was not subtle in his love for wizards, and hearsay claims he thought fighters needed to be nerfed (despite being one of the least optimal classes to take). More generally, spellcasting in 3.x is a versatile pool of abilities for almost any niche, and new spells in every book expand caster options more than anyone else. Wizards, clerics, and druids especially benefit - wizards just have to find or research a spell and spend a pittance to add it to their spellbooks, while clerics and druids automatically have access to all spells on their classes' spell lists. In the latter case this led to the infamous "CoDzilla" (Cleric or Druid + Godzilla) whose abilities and versatile spell selection make them very easy to twink out - though wizards had more spells written for them in absolute terms. These classes have spells that let them be stealthier than your rogue note Unlocking an average lock has a DC of 25, so a 3rd-level Rogue with a dexterity of 20 (the highest the stat can be at this level), max ranks in Open Lock, and masterwork tools only has a 45% of succeeding, while a 3rd level wizard can just cast knock, which not only opens the most complex and/or magical of locks, it also can remove a deadbolt or anything else keeping said door from opening; the rogue needs to make both Hide and Move Silently checks to be stealthy (which is weird, as no class that has one lacks the other), meanwhile a wizard can just cast invisibility as a 2nd level spell. What makes this especially irritating is that to be even slightly effective in this requires a rogue to have a natural 18 in Dexterity (or spend 18 points in a point-buy), be a Dex-boosting race, and invest a lot of money and resources into just this one job. A wizard just needs 12 Intelligence (or 4 points on a point-buy) to cast knock or invisibility. You can even get to the world-shattering power of 9th level spells (assuming standard wealth-by-level to get stat-boosting items that everyone was going to get anyway) with a 13/5 in a point buy! That's assuming a non-Int boosting race, too;if you choose a race with a bonus to Intelligence, like some of the Elven and Gnomish races, then you get by with an 11/3 in a point buy. And unless the GM changes the default character generation rules, you are guaranteed to get at least one 11, and can put it in any stat, as you are entitled to a reroll if you ever get stats that low., better at combat than the fighternote Who cares if you're a sickly bookworm when you can turn into a T-Rex whenever you want? Several monsters also have abilities that the players aren't supposed to get, or at levels far earlier than when non-mages get them., and have easy access to wands and scrolls to get spells they wouldn't normally get. Said designer also hated the sorcerer class (despite its extreme mechanical similarity to the wizard) and openly sabotaged it in every way he could.
Dungeons & Dragons 5th Edition has the Wizard class, a class very heavily focused on the developers. Wizard's get the most amount of spells of any class, get several subclasses that are all, at worst, okay, and often end up being among the best builds in the edition. That itself wouldn't be enough to be a Creator's Pet, but where it becomes clear it is loved by the developers is how often the class gets new stuff; just about every single module, book, or Unearthed Arcana gives a bunch of new spells, magic items meant for them, or new subclasses that blow the others away by comparison. Worse, a number of the subclasses are designed in such a way that they come across as trying to take the main features of other classes to give to the Wizard, like the unreleased Theurgist Wizard being almost a straight up better Cleric, or the (also) unreleased Lore Master Wizard, which would have destroyed Sorcerers from being viable. In comparison, the community tends to be vocal about the blatant favoritism on display and often request other new UA that isn't Wizard focused, to the point that many potential subclasses or features from UA have never been officially released due to the community being frustrated by the focus on Wizards. Even people who like the class have agreed that it is blatant how much other classes are given less focus and new content by comparison to the Wizard, especially the Sorcerer.
Dragonlance has the Kender, the setting's Hobbits, which both novels and sourcebooks hold up as the purest and most innocent of all races, waxing rhapsodic about how cute and adorable they are despite being — or because of being — impatient thrill-seekers whose limited sense of fear and insatiable curiosity make them both prone to charging recklessly into danger and compulsive kleptomaniacs. Other official statements include talking about how people who don't like kender don't understand them at best, are pure evil at worst, and are painted as jerkasses in the medium, how "a crying kender is one of the saddest things in the world", and how Krynn would lose something irreplaceable and priceless if kender vanished. Most of the fandom, especially the greater D&D fandom, despises them. Complaints range from their lore making them obnoxiously perfect and/or nonsensical (why are people treated as being bad for disliking the presence of compulsive thieves?), to getting annoyed at the kenders' status as hypocrites (they get absolutely furious when people call them thievesnote Kender were first introduced back in the AD&D days, and due to the way AD&D works, Halflings (and thus Kender, who used Halfling stats with minor changes), are, almost universally, what later editions refer to as Rogues. You know what that class is called in AD&D? Thief., but obsessively pick pockets and locks in search of fun new things to look at), and the fact that, at the actual tabletop, a race whose fluff calls for them to be played as a combination of Chaotic Stupid and "that jerkass rogue who's always swiping the party's stuff" in order to be played accurately inevitably turns out to be disruptive and frustrating. It does not help that while a kender character blithely goes about his business with innocent unawareness of how his/her behavior comes across to others, that same character's player sitting across the table with a smug grin after pickpocketing some magic item or piece of loot from the party absolutely knows.
Elminster from Forgotten Realms. Think about what would happen if Gandalf were the main character of Lord of the Rings, and the story consisted of him beating up anything that is a bother and boning the goddess of magic whose boobs are totally big and rad to the max. Part of the problem is that Mystra, in turn, comes across as Ed Greenwood's pet goddess, so much more powerful than any of the other gods (the introduction to Shadows of Doom comes out and says so) that she has to deposit parts of her own godly power in her mortal Chosen. Elminster becomes her Chosen at the end of The Making of a Mage. His problem is, in part, that while all the other famous Forgotten Realms characters may be people your player characters can look up to, emulate, and maybe one day catch up with... unless you can somehow talk your Dungeon Master into allowing your character to enjoy a similarly "friendly" relationship with Mystra, El will always be Better Than You because the Realms' own canon says so. (And during fourth edition, that option didn't even exist anymore because, well, Mystra was dead. Thankfully, this was later undone, along with almost all of the hated changes to the setting in 4E.)
Exalted. In a game whose premise is "Near-perfect demigods tragically cursed to bring about their own downfall," the Infernals stand out as being the writer's darlings. Compare the nature of thei charms, the amount of printed material and their prevalence in the Meta Plot to the Abyssals and Fair Folk, the other two playable "antagonist" splats who had both been around since first edition.
A long-standing complaint of Legend of the Five Rings fans, though the Creator's Pet in question changes depending on the creator. Notable pets included:
Toturi. His faction gets the story prize for the first arc despite another faction winning the tournament.
The Crane Clan, who get to avoid The Corruption running rampant during the second arc, to the point that a Bloodspeaker from a Crane family was printed as unaligned when every other faction got an alignment.
The Phoenix Clan, who after becoming thoroughly corrupted and nearly destroyed, were given such a massive power boost that they completely dominated the tournament circuit for over a year. A number of powerhouse cards even received additional effects just before going to printing. The fact that the lead developer at the time was a longtime Phoenix fan had nothing to do with this, of course.
Daigotsu killed two emperors, effortlessly showed up The Dreaded Iuchiban, blackmailed and betrayed the Empress, and then rewrote the setting's cosmology to his liking. Twice. To the Spider Clan, he's a hero. To everyone else, he's a tiresome character that just won't go away.
One of the main line writers during the first edition of the RPG was open about his infatuation with the Scorpion Clan's Dragon Lady Bayushi Kachiko. Cue the Scorpion Clan getting talked up as way more clever, better informed, and smarter than anyone else in Rokugan during that time. The Metaplot overall was less kind to the Scorpions when in other writers' hands, given the Foregone Conclusion of the first edition being set before the first story arc of the card game, but Bayushi Kachiko would remain important and influential for a long time.
The RPG brings us the Kolat, a Captain Ersatz of the Illuminati who are a Greater-Scope Villain to the players, quietly working to undermine the entire empire. So far, so good — it's a great way to bring a party together across clan lines. When they became big players in the card game, however, they became a Generic Doomsday Villain with no coherent goal or method, and which were implicitly beyond the players' ability to oppose, since it was rare to have story tournaments involving them more than once every five years or so. Even then, it was rare for these setbacks to stick — the Unicorn, previously riddled with Kolat agents, won a tournament to purge the Kolat from their ranks... a victory which the story team said caused sweeping changes, but which never had a single line of story or flavor text acknowledging the Unicorn's vengeance on their most hated foes.
In the Living Death campaign, the character of Jason Lindaman was supposed to be a super-intelligent, Crazy-Prepared investigator who was taken out by the enemy before the PCs arrived. Because the PCs' only real interaction with him was after something heinous that took all four to six of them to handle had physically or emotionally crippled him, many players considered him a joke and/or incompetent, to the point that they wished for his death.
A single character who qualifies is JaceBeleren. In each story expansion, he's given more weight and his importance is emphasised. He's gone from being just one of the several new planeswalkers in the Lorwyn/Shadowmoor block to being so important as to being the primary focus of the Shadows Over Innistrad block, despite not doing much more than steal Tamiyo's notesnote As in, there is literally a a card in this set that is her journal. (another planeswalker who was introduced in the original Innistrad block) and steal the spotlight.
Many fans feel Blue in general sometimes gets preferential treatment from the game designers. When Snapcaster Mage was printed, he was all the daily articles would talk about for several days in a row, and later articles would regularly bring up how great he was. Later, Snapcaster Mage's impact on the game was so profound that the next few expansion sets required several graveyard-hosers to counteract his power.
While there was little question before, since the Hour of Devastation set Nicol Bolas is unarguably this. He's an odd duck in that, unlike Jace, he's not technically a modern-day character (he was originally one of the five Elder Dragons from Legends, but like his fellow Elders was completely devoid of personality or story relevance) but was reinvented as one to provide the Magic universe with a new Big Bad after they killed off Yawgmoth. Since that time Bolas has been on a trip, from defeating classic evil planeswalker Leshrac to effortlessly one-upping his would-be The Starscream Tezzeret. But what made Bolas fall squarely into this was Devastation, where not only was a plane ruled by him shown as being so perfect even the heroes had to acknowledge it (although their admiration turns to horror and disgust the more they learn and he later destroys it for fun) but in the climax, he easily defeats allfivemembersofthe Gatewatch (including the other Creator's Pet Jace). The resulting Curbstomp Battle is seen mostly from his perspective and played almost entirely for laughs. It's as if the only solution the Magic creative team could come up with for the constant accusations of the Gatewatch team getting too much focus was to make them suffer a humiliating defeat at the hands (claws?) of an even more egregious example. Thankfully, in War of the Spark he finally gets his comeuppance, being stripped of his planeswalker spark and sealed away for good.
Poison counters, and the abilities that produce them, are personal favorites of Marc Rosewater, and so they keep making their way back into the game despite being unpopular with fans as a whole. He's also made sure to use his influence to keep Leeches, the only real way of actually removing them ever printed, from ever seeing the light of day again.
The Old World of Darkness setting has Samuel Haight. The writers just kept giving him more and more rule-breaking, crossover powersnote Sam was a ghoul, a skin dancer, a true mage who didn't suffer paradox, and the owner of a sword that let him steal even more powers and kept instructing GMs to ensure he lives for another adventure before they finally clued into the massive backlash he had accumulated and killed him off. (Then, in the afterlife, he was turned into a (still sentient) ashtray.)
Some oWoD writers had a bad habit of doing this with any named NPC. Writer Dean Shomshak elaborates:
That's the other thing: WW hires people who want to tell stories. But, the only characters they have to tell stories about are the NPCs. So, they tell stories about the NPCs. Gods, I wanted to smack some of my fellow writers upside the head on some Vampire projects when they burbled on about the cool things they'd have Hardestadt do, or whoever. What were the PCs supposed to do?
This quote sums up why this trope happens in tabletop RPGs — it's easy for writers of "official" game fiction to forget that even the most interesting NPCs they're writing about are only the supporting cast meant to help player characters said writers will never even hear about shine. It's the main fallacy of writing for a game line with an evolving metaplot. You can't necessarily predict what player characters will do in a given adventure to push the plot in your desired direction, so you rely on NPCs to do the heavy lifting for you; of course, since they have all the agency, the party tends to play second fiddle. Shadowrun seems to have better luck at this sort of thing, as they've had more practice and they tend to leave their plot advancements in sourcebooks and leave their adventures as standalone, rather than mixing metaplot into adventures where the party may not want to play by the script.
Both old and new World of Darkness can get into crippling "arms races" between creators and their pets. Every line has them, and the line developers will often slip into treating entire character types as their pets.
"Pet armies" in wargaming are a major source of debate. In any period there are armies that people argue are over-powerful in many sets of rules;
In Ancients - Imperial Romans
Medieval games - 100 Years War English
The Character Tiers of Renaissance games seem to be as follows: English Civil War armies > Other European armies > Everyone else.
Napoleonics - the French or the British.
WW2. The Germans. One oft-repeated joke in many clubs is about a player who fields more Tigers than Hitler managed.
The American Civil War is a whole other can of worms, because players sometimes take the politics into the game.
US armies in US rule sets.
In Warhammer Fantasy the Blackorc Warboss Grimgor Ironhide is hated by a large part of the fandom for replacing a black orc considered to be better thought through; the writers liked him enough to let him defeat Archaon, another badass character, and thus save the world by being badass.
Warhammer: The End Times feels at times like it was written by a Skaven fanboy.note It was written by Joshua Reynolds without supervision from Games Workshop. They destroy the Lizardmen, the Dwarfs, Bretonnia, Tilea, Estalia and Nuln all on their own, blow up Morslieband Nagash's Black Pyramid, and outright escape the End of the World everyone else was experiencing by sinking their lands into the Warp; this effectively makes them the only mortal faction that really won the End Times. Their God even usurped Slaanesh! This goes in defiance of their established military power. Inverted with Teclis, who the writers apparently so disliked that between undoing multiple past attempts to save the world and leaving a Doomsday Device sitting out in the open for the bad guys to take control of, he probably did more to bring about the end of the world than Archaon did.
40k's oldest Creators' Pets have always been the Space Marines themselves. As Warhammer 40,000's mascot characters, the Space Marines get the most exposure and the most updates, while Codex Creep ensures that they remain a potent force on the tabletop. In the setting's narrative, Space Marines are so awesome that they've been awarded the "moral victory" even when they failed a campaign's objectivesnote this was likely because the Space Marines and Imperial Guard were lumped together (being on the same side and all), and the IG definitely succeeded in their primary objective. For their part, the Space Marines at least achieved their secondary objectives. Roughly half of 40k's armies are some variant of guys in Power Armor, and Space Marine merchandise sells more than all the other factions combined.
The Ultramarines are the Space Marines to the Space Marines. This reached its nadir during 5th edition; thanks to letting an enormous Ultramarines fanboy do the writingnote indeed, originally the book was going to be called "Codex: Ultramarines" much like the 2nd edition, and was only changed at the last second, the Ultramarines completely dominated. They were constantly harped upon as being the best of the best, with very little of other Chapters being mentioned; when they were, it was for them to praise the Ultramarines and how much they wanted to be like them (thereby ignoring all of their own history and beliefs), or so they could otherwise be shafted into obscurity for their "deviance". The Primarch of the Ultramarines, Roboute Guilliman, was also held up as the Ultramarine to idolize, infamously being called the "spiritual liege" of every single space marine (a phrase that's used to mock the Ultramarines even now). This was minimised in subsequent codices.
The excessive focus on the Imperium is notable for actually undermining part of the game's point. 40K is supposed to have no good guys, and all of its factions would be the villains in almost any other sci-fi setting...but, because we constantly see the Imperial point of view on everything, they end up seeming more sympathetic than they otherwise might.
The Necrons have proven problematic for some fans. When the army got its proper launch during 3rd Edition, with a codex positing that the Necrons' undying C'tan masters were the prime source of all evil in the universe, secretly worshiped by a pivotal faction of the Imperium, and responsible for the rise of Chaos, many fans complained that these Terminator knock-offs had usurped Chaos as the setting's Big Bad. The 5th Edition codex attempted to rectify this by drastically reducing the C'tan's presence, specifically with a retcon describing how they were betrayed and imprisonedby the Necrons. The book also assures readers that many Necron Lords have gone insane over the eons and enjoy delusions of godhood, and points out that what little the Imperium knows about the Necrons are mostly half-truths, lies, or flat-out wrong. Naturally, some fans are now complaining about the changes.
The 5th Edition Grey Knights codex - which was incidentally written by the same author as the aforementioned Ultramarine-obsessed codex - introduced Lord Kaldor Draigo, who manages to one-up the Ultramarines through the sheer over-the-top-ness of his accomplishments, which includes but is not limited to: surviving an endless walk through Hell; banishing a Daemon Prince in his first combat action; defeating a Daemon Primarch and vandalizing its still-beating heart; killing a Bloodthirster with his bare hands before stealing its unholy axe and reforging it into a sword with the power of his mind; and rampaging through the Chaos Gods' private demesnes without consequence. As nicely as possible, most of these feats are impossible for any Grey Knight to pull off. The 7th Edition Codex was forced to cut all this down drastically. Not that the Knights themselves are spotless: one weapon essentially makes the Tau unable to deal damage, the Dreadknight is an impractically stupid-looking vehicle (essentially a giant robotic baby carrier), and fluff-wise, they cheerfully use daemonic weapons (one of the key reasons no Grey Knight has ever fallen to Chaos is because they stay well away from the "fight fire with fire" route some Inquisitors take and inevitably fall)... and of course, the most reviled incident where the Knights were fighting a nanomachine swarm, then noticed the Sisters of Battle's faith made them immune to it. So they murdered the Sisters to cover their armor in the blood of the innocent as an efficient protection, instead of leaving pious Imperial warriors alive. If you're thinking that sounds like something Chaos would do... you're entirely right.
One common joke for the Kaldor situation is "because GW wouldn't let Ward bring back Guillimannote the Primarch of the Ultramarines, essentially a demigod that's been in suspended animation for the past few millenia". In a curious inversion, Guilliman was later brought back from stasis... and the fanbase actually took it as a positive development, since he wasn't happy with what had been going on in his absence.
This accusation is sometimes leveled at Eldar as well; Eldar are purportedly a glass cannon faction with a healthy dose of Fragile Speedster. In practice, they play more like Lightning Bruisers, with powerful special rules that make their supposedly fragile units very durable, and brutal firepower. In fact, the only stretch in the game's history where Eldar weren't god-tier or close to it was throughout 5th edition, when their codex was out of date.
This accusation was eventually leveled at the Crimson Slaughter warband of Chaos Space Marines. Originally having no lore beyond a name and colour scheme, they suddenly became one of the two factions in the 6th edition starter set, then received a dedicated Codex supplement before eight of the nine Traitor Legions, which are instrumental to the setting's history. Late 7th and 8th edition even used them as the example colour scheme for most Chaos Space Marine plastic kits.
In the first year of 8th edition, the focus was almost entirely on Roboute Guilliman and his army of Primaris Marines (like Space Marines, but more so) fighting Chaos. Thanks to the time skip this involved, every alien faction apparently sat on their hands doing nothing of importance for centuries. This is most obvious in terms of models; Crafworld Eldar, Tyranids, and Necronsdid receive new rules and lore updates, but no new kits. The Primaris also tend to get sidebars talking about them in every available book: for example, Dark Eldar players got to read a sidebar about how the Primaris Marines were the new hotness in Commorragh's arenas and interesting to the local Mad Doctor faction, the Haemonculus Covens, while Chaos Marines got an entry on how much Fabius Bile wanted to get some to mess with... essentially rubbing the new models into the faces of fans who are likely not Space Marine players.
Black Library, GW's novel publishing house, have exactly the same issue with Imperial focus that GW proper does. In late April 2018, the Featured bar on the "New Titles" section of BL's website had eleven 40K works in it (twelve entries, but one appears twice because there's a special edition). Eight of those eleven novels - Gaunt's Ghosts: The Lost, Carcharodons: Outer Dark, Legacy of Dorn, Imperator: Wrath of the Omnissiah, Flayed, Dante, Mercy and Jaghatai Khan: Warhawk of Chogoris - are Imperial-focused. As a percentage, this was actually down from Black Library circa 2006-2007, which would at times release four 40K books in a given month, all of them having Imperial main characters, and three of those being Space Marines.
Champions brings us Doctor Destroyer, an Expy of Doctor Doom. His sourcebook has over a dozen pages describing a variety of his armors, but his personality is a paper-thin series of cliches. Unfortunate, since he's the Big Bad of the setting.
Dark Champions brings us his "good" equivalent, a supposedly street-level vigilante named the Harbinger of Justice, who's even more powerful than Destroyer.
from the book's author
Lord Kaldor Draigo, who manages to one-up the Ultramarines through the sheer over-the-top-ness of his accomplishments, which includes but is not limited to: surviving an endless walk through Hell; banishing a Daemon Prince in his first combat action; defeating a Daemon Primarch and vandalizing its still-beating heart; killing a Bloodthirster with his bare hands before stealing its unholy axe and reforging it into a sword with the power of his mind; and rampaging through the Chaos Gods' private demesnes without consequence. As nicely as possible, most of these feats are impossible for any Grey Knight to pull off. The 7th Edition Codex was forced to cut all this down drastically. Not that the Knights themselves are spotless: one weapon essentially makes the Tau unable to deal damage, the Dreadknight is an impractically stupid-looking vehicle (essentially a giant robotic baby carrier), and fluff-wise, they cheerfully use daemonic weapons (one of the key reasons no Grey Knight has ever fallen to Chaos is because they stay well away from the "fight fire with fire" route some Inquisitors take and inevitably fall)... and of course, the most reviled incident where the Knights were fighting a nanomachine swarm, then noticed the Sisters of Battle's faith made them immune to it. So they murdered the Sisters to cover their armor in the blood of the innocent as an efficient protection, instead of leaving pious Imperial warriors alive. If you're thinking that sounds like something Chaos would do... you're entirely right.