* ''FanFic/DungeonKeeperAmi'': a strange, strange example in the form of [[CrazyAwesome Mukrezar]]. Mukrezar is a goofy, slightly manic Keeper of dubious competance. He dosen't even look threatening, he's a pink-haired elf! Following [[spoiler: his resurrection]] he serves as a decidedly plucky comic releif sub-plot and foil to Ami's introverted competence. Yet despite all of this, the story has not only arguably taken a turn for the darker since his introduction, but the level of threat posed by Mukrezar has risen exponentially with his every appearence. Why? Despite dubious success in most of his ZanyScheme -ness, his [[SlasherSmile sadistic streak]] and ability to [[LaughingMad bounce back]] from crushing defeat is intimidateing to the extream.
* [[spoiler: [=ChaosGallantmon=]]] from the ''FanFic/TamersForeverSeries''. Before his appearence, the series was a lighthearted romantic comedy series. However [[spoiler: [=ChaosGallantmon=]]]'s appearance kicks the series MythArc into high gear by triggering the series first WhamEpisode. And his second appearance is the catalyst for the series DarkestHour.
** For an even more [[HeroKiller extreme]] version of this, we have [[spoiler: Daemon]]. See DarkestHour for more information.
* Saint Ayame and Saint Sarutobi from Fanfic/OnePieceParallelWorks. We first see them trying to enslave an entire island, [[spoiler: which Aria talks them out of]]. But, as the series progresses, we learn that they've done some pretty nasty stuff, ''especailly'' to Heathcliffe and Yuki-Rin. Kicking Heathcliffe out of his house and killing Molly's parents ([[spoiler: even though her parents faked their deaths]]) don't even begin to describe how they've also [[spoiler: almost brainwashed Holden by sending him to a boarding school for the children of the Tenryuubito]]. It's also implied that [[spoiler: Saint Ayame frequently beat Yuki-Rin for her love of pirates]].
* ''FanFic/GettingBackOnYourHooves'' had some sad moments in the beginning, but mostly a rather lighthearted story about Trixie trying to do exactly what the title says...Then [[BigBad Checker]] [[CorruptCorporateExecutive Monarch]] enters the picture. WordOfGod states her mannerisms were based off those of a [[TheSociopath sociopath]] if that's any indication. Example? [[spoiler:She MindRaped Rainbow Dash and convinced the Diamond Dogs not to try anything funny with her by saying she'd turn their mine into a parking lot...with them under it.]] That was only her second appearence.
* [[spoiler: Gilda]]'s initial appearance in ''[[{{Fanfic/AceCombatEquestriaChronicles}} Ace Combat: The Equestrian War]]'' ends with her breaking Medley's wings and causing a HeroicBSOD to Rainbow Dash.
** A few chapters earlier, Black Star. When he stated that only Firefly was able to fight him equaly, he wasn't making it up.
* The arrival of the Benefactor's disciples in ''Fanfic/ChallengeOfTheSuperFriendsTheEnd''.
* The ''FanFic/PonyPOVSeries'' has [[spoiler: [[EnemyWithin Loneliness]]]]; up till she showed up, the Reharmonization series was much more light hearted than the Discorded Ponies series -- it had its dark moments, yes, but nowhere near the amount of horror [[spoiler: Loneliness]] brought with her. Not only that, but only a couple of chapters later the Princess [[spoiler: Gaia]] arc started up.
** Shining Armor's arc has [[BigBad General-Admiral Makarov]]. Things take a turn for the serious when he shows up, whereas before it was all pretty light hearted [[spoiler: minus an early appearance by [[EldritchAbomination the Stalker]]]]. And then even more so once we find out [[spoiler: he's an [[HumanoidAbomination Equineoid Abomination]] who [[OnlyICanKillHim only Shining may be able to defeat]]]].
** [[spoiler: [[BiggerBad Nightmare Eclipse/Paradox]] manages to be this for ''[[BadFuture Dark World]]'', which was ''already'' DarkerAndEdgier. When the story finally turns to her, it instantly becomes a [[WhamEpisode Wham]] ''[[WhamEpisode Arc]]'' and the stakes go simply from freeing the world from Discord's tyranny, to having to save ''the entire timeline'' from being erased from existence due to her GroundhogDayLoop plan. Add in her rather frightening FauxAffablyEvil personality and the fact she has the largest body count of any villain in the fic due to having gleefully erased the entire timeline and everyone in it several ''million'' times and she's most certainly this trope.]]
** Queen Chrysalis is this for the Reharmonized timeline. Where previous villains may have been more powerful, her [[LongGame planning]], legions of {{Mooks}}, mind control powers (which see much more use than in canon), [[{{Sadist}} sadism]], and [[spoiler: knowledge of the Heart World]] make her quite possibly the greatest threat this timeline has ever faced.
** Chrysalis' MadScientist minion, Kabuto, is this for the Wedding Arc's [[HeroOfAnotherStory side story focusing on Misfit Actual]]. Their story is actually has ''less'' action than the Mane Six's due to them acting covertly for the most part, and the fights they do have they tend to win fairly easily, with no real bad will towards their opponents, who are only trying to capture them. Then they go up against Kabuto, a FauxAffablyEvil MadDoctor who casually talks about performing monstrous experiments on his own kind and innocent ponies, [[spoiler: and is also an incredibly dangerous opponent due to experimenting on himself.]]
* In ''FanFic/TheDresdenFillies'', the story takes a darker turn when [[spoiler:Trixie and the Nightmare kidnap Spike.]]
* In ''FanFic/MyLittleDenarians'', the Denarians. Discord may be more powerful, but the Denarians are more ''evil''.
* Fanfic/ShardsOfMemory, the story takes a much more foreboding turn when Chaos makes his entrance. [[spoiler:Killing Cosmos]] was just the start of the havoc his introduction caused.
* The [[Fanfic/PonyPOVSeriesChaosVerse Chaos Verse]] (an official spinoff of the ''POV Series'' above) is pretty lighthearted until the BigBad [[spoiler: [[EnemyWithout Nightmare Phobia]]]] shows up and nearly manipulates [[spoiler: Fluttershy]] into [[DrivenToSuicide killing herself]].
* Umi Tsuda in [[http://www.fanfiction.net/s/4004316/1/Windows-of-the-Soul Windows of the Soul]]. The first time she shows up, she [[spoiler:tries to kill Shizuru for killing her fiance]], and the parts in which she's involved are some of the darker parts of the fic.
* ''FanFic/EarthAndSky'': A mild example, all things considered, but Diamond Tiara's entrance into the story (followed shortly after by the Flim-Flams) not only brought in actual antagonists, but also an overall darker tone to the story.
** Subverted with [[spoiler: Chrysalis]]. At first it looked like she was going to be a Knight, but instead was [[VillainDecay reduced to]] an IneffectualSympatheticVillain, whose one major act of villainy [[spoiler: (framing Rarity being a changeling)]] was negated within hours.
* ''FanFic/ThePowersOfHarmony'': While it was fairly obvious from the start that the story was ultimately going to go in a dark direction, it was pretty easy to ignore, given the story's mostly light tone for most of Act 1. Then, just after the Guards' DarkSecret and backstory are revealed at the beginning of Act 2, [[BigBad Cetus]] shows up, and everything goes straight to Hell.
* ''[[FanFic/StarWarsParanormalitiesTrilogy Star Wars: Paranormalities]]'': While Maesterus is one of Zolph Vaelor's most dreaded foes throughout Episode I (Zolph is defeated by him three times, and [[YouKilledMyFather he killed his father]]), he's a charismatic AntiVillain at best, as he prefers not to kill his enemies. The [[TheLegionsOfHell Forceless Collective]] and their generals, the [[SealedEvilInACan Archfiends]], are not so kind in comparison. They are perfectly willing to kill and [[DemonicPossession possess]] other beings without second thought (and for elaboration, [[AndIMustScream the victims are consciously aware of it]], and they don't get control of their bodies back unless they serve willingly). While they seem to have ambiguous morals in their debut appearances since they don't talk (usually) and are [[EldritchAbomination pretty bizarre to begin with]], [[spoiler: this gets jossed near the end of Episode I when they are revealed to be capable of sadism and are also willing to resort to [[DespairEventHorizon psychological warfare.]]]]
* ''Fanfic/ShadowsOfGiants'': Godzilla. [[TheDreaded Big]] [[AssKickingEqualsAuthority time]]. While he's been confirmed by WordOfGod as an AntiHero, he's treated with the same degree of seriousness as he was in the 1954 film.
* In ''Fanfic/PerfectionIsOverrated''
** Hitomi Kirihara is the first [[ParodySue SUE]] with hostile intentions toward the [[MaiHime Himes]], and abuses her MindControl powers to rob and murder people. Over 20 people end up dead as a result of her actions, and this turn of events causes Nao to agree to cooperate with the Himes in spite of being reluctant to do so.
** [[spoiler:Bachiko and Meiko]] present more of a threat to the Himes than most of the [=SUEs=], [[spoiler:setting them against each other and playing into the canon antagonists' plans]]. By the end of Chapter 19, [[spoiler:Yukariko and Ishigami are dead, Mai barely avoided causing Yuuichi's death while surviving an attack from Shiho, Natsuki had to protect Nao from Shizuru (and then Shizuru from Nao), and Bachiko and Meiko are finished off by a {{Brainwashed}} Mikoto]].
* There's an amusing attempt to ''invoke'' this in FanFic/ANewOrder. Nephrite proves himself to be frighteningly competent very quickly, his first appearance resulting in the first innocent deaths of the story. When he first shows up, he's accompanied by a three-headed wolf youma named Kerberos. WordOfGod is that this is because the author thought the trope was Knight of ''Cerberus'', as in the 3-headed hellhound, and wanted to invoke it.
* [[spoiler: Professor Moriarty]] in the first finale of ''Fanfic/ChildrenOfTime''. There have been a lot of monstrous beings and despicable actions to date, but ''no one'' with as thorough a vendetta against Literature/SherlockHolmes as [[spoiler:the Professor]]. Notably, within 5,000 words of his introduction, the whole world basically goes to hell.
* Denkuma in Volume I of [[http://www.fanfiction.net/s/5973337/1/Dragon-Ball-X-Volume-I-Eternal-Protectors Dragon Ball X]]. Up until then the series had a very "villain of the week" sort of style, usually ending with the acquisition of a Dragon Ball. Even Porter and Cuo, while dangerous villains, weren't overly threatening and were dealt with without much collateral damage. Cue Denkuma. [[spoiler: His debut had him absolutely destroying and humiliating the main characters in battle, and a chapter or two later it's made very clear exactly how dangerous he really is.]] The rest of the series immediately took a turn for the dark that didn't ease up until the last half of the final chapter of Volume I.
* Happens ''twice'' in ''Fanfic/CalvinAndHobbesTheSeries'' to indicate that CerebusSyndrome is kicking in harder:
** Thunderstorm and Shadow, the villains of season 3 finale "Thunderstorm", are shown to be the most competent threats the series has had thus far, actually managing to TakeOverTheWorld [[spoiler:and destroy [[DoAnythingRobot the MTM]]]]. It takes a ''lot'' to defeat them.
** The season 5 (and series) finale "Black Rain" abruptly introduces ''Franchise/{{the Slender Man|Mythos}}'' of all people, and his appearance is a haunting as always - ''especially'' when [[spoiler:he [[NeverFoundTheBody "kills"]] Thunderstorm and Shadow at the end]].
* The ''Anime/NeonGenesisEvangelion'' fanfic ''Re-Take'' initially came off as PeggySue fic where Shinji found the rest of the series after he escaped from Leliel was just a dream starts acting to prevent Asuka's fate in the actual series, leading to the two of them confessing their love... Then a second, badly injured Asuka appears that only Shinji can see that's more a {{Jerkass}} than he canon incarnation ever was, proving the first sign that everything is not going to work as perfectly at it first looks.
* ''FanFic/QueenOfAllOni'' has EvilSorcerer [[SmugSnake Lung]], Daolon Wong's BastardUnderstudy. He's only prominent in one chapter, but it's one of the darkest in the story, and his ColdBloodedTorture of [[VillainProtagonist Jade]] is what starts her SanitySlippage, which in turn is causing the story to start going down a dark path.
* As soon as [[spoiler: [[SerialKiller Beyond Birthday]]]] shows up in ''FanFic/ConstantTemptation'' things get really dark, really fast. [[UpToEleven He's somehow made even scarier than he is in canon.]]
* ''FanFic/PokemonMysteryDungeonHopeandFaithSaga'' starts getting ugly once Unod the World Destroyer appears.
* ''FanFic/SillyHatProductions'' appears at first to be just an AffectionateParody of ''VisualNovel/UminekoWhenTheyCry''. Then Lambdadelta decides that Battler isn't taking the games seriously enough, and decides to throw in some motivation by revealing that since it's just a 'replica board', she can erase it. She then declares that for every episode where Battler fails to present a good theory, she'll [[DeaderThanDead erase one of the pieces who died in that game's scenario]].
* The Vocaloid fanfiction [[https://www.fanfiction.net/s/5789061/1/Hear-my-Song Hear my Song!]] has IA. She's first introduced as an Korean IdolSinger who Miku listens to on the radio, and then as Yukari's StalkerWithACrush. And ''then'' she breaks SeeU.
* In StarWarsLightInDarkTime, Jinn Skywalker fits this trope perfectly. His appearance marks the first legitimate villain threat, as well as the start of the MythArc. Admiral Zorg ups the ante when he begins to gain a role in the story, being calm and calculating, in contrast to Jinn's brash, BloodKnight tendencies.
* Fanfic/EquestriasFirstHuman was already dramatic, with Connor's social isolation from Ponyville, and the Mane 6's own social isolation for relating to him. Then comes Hellfire, a unicorn stallion with a crazed vendetta against Princess Celestia. His first act on screen was to [[spoiler: mutilate one of his subordinates for failing a petty order.]]
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