In order to add more drama to a series which has been, up until that moment, lighthearted and comedic in nature, a villain (or antagonist of some sort) is introduced and is portrayed as a genuine threat, in contrast to the bumbling and comedic villains the heroes have faced before. The villain's arrival is sudden, and is usually either a totally new character, or a character that has undergone a particularly dramatic FaceHeel Turn.
This villain's arrival is heralded by a sudden downturn in humor (although if the villain is Laughably Evil, expect it to turn into Black Comedy), showing the audience this one means business. In other words, a catalyst for a drastic change in mood toward the dark and dramatic. Some characters may take the story to darker territory without being villainous in the slightest, but this trope is reserved for evil creatures. Grimdark shows and stories that take place in a Crapsack World are much less susceptible to this kind of influence, but even they have exceptions if this villain shifts the story from Black Comedy to being more serious. This isn't only about the character themself, it is also about their relation to the story.
A Knight of Cerebus may be a blatant Hero Killer who poses an actual threat, or even an Invincible Villain. They also tend to be the trigger of The Worf Effect against the hero. They may be the Arch-Enemy as they might have a strong personal tie to the hero or heroes of the work. They are usually distinguished from the other villains as perhaps being worse, stronger, or more personally tied to the protagonists. Sometimes this type of character can be a Complete Monster if they have no redeeming traits and do terrible things by the standards of history in an initially more lighthearted series. Often this character's arrival is a Gut Punch.
Their opposite is the Fun Personified, who may not be able to change the tone as effectively, but still works to prevent things from getting too dark. See also Dead Serious and Not-So-Harmless Villain, when a villain starts as ridiculous but, because of some change from within, ends up becoming this. Compare Shoo Out the Clowns, when the darker tone coincides with decreased screentime for comedic characters.
Remember, Cerebus Syndrome is a prerequisite for this trope. The Knight of Cerebus heralds a long-term tone shift in an episodic work and cannot exist within a single installment. An increase in danger as a conflict escalates is present in almost any story with a villain, and is not automatically notable. Can overlap with Vile Villain, Saccharine Show when the setting itself contrasts heavily with the character's grimness (and that trope may be what you want for a single work).
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- Reflets d'Acide starts out as a light-hearted Heroic Fantasy parody with no villains beside monsters met by the heroes. Then, we get Alia-Aenor, who double-subverts this trope; when first seen, she appears as an intelligent, humongous black dragon. A few minutes later, she turns out to be a female, whose human form, much to the narrator's dismay, is a beautiful girl with a sweet, girly attitude... then she kills a bunch of thieves trying to rob her with a single spell.
- Shuffle Quest was a generally light-hearted RPG podcast, until a Mad Max-style tyrant named The Ginormous decapitated fan favorite Terok Nor.
- The Adventure Zone has Kravitz, the Grim Reaper, who informs the party that they have all died multiple times. He's rather annoyed about this, but they're incredibly confused, because they absolutely do not remember the incidents he's talking about. It's the first sign that the party aren't being told something very important about the quest that they're on.
- Adventures in Odyssey is largely a realistic show about realistic small-town problems, frequently more comedy than drama. It had a few forays into serious territory early on, whether it was an adaptation of a Bible story or a robbery... but when Dr. Blackgaard showed up, things changed.
- The Cult of Apophis in Yu-Gi-Oh! East Academy presents a very real and significant danger to the heroes; their appearance marks the darkening tone of the RP.
- Doc Scratch somehow got the players of Destroy the Godmodder interested in actually roleplaying. The more he did, the more people began roleplaying.
- Warhammer 40,000 effectively has two in its history.
- The first to appear were the four Chaos Gods. Before they showed up, humanity was more or less kicking ass and taking names. Then the Chaos Gods make their presence known by corrupting Horus, turning half of humanity to their side, nearly wiping out all of Terra, having their new servant Horus cripple The Emperor and indirectly causing the imperium of man to become the Crapsack World it is today. However, this was only in the Horus Heresy, and it only really applied to humanity. After their first defeat, humanity began gaining ground against the forces of Chaos until they downgraded to The Usual Adversaries. Something that The Eldar ALREADY saw them as.
- Then came the Tyranids. In their very first attack, they crippled the most well respected Space Marines in the galaxy. In their second, it took both the Eldar AND Humanity working together just to gain a Pyrrhic Victory over them. And their third assault STILL hasn't been stopped, despite the sacrifice of billions upon billions of humans. Every time a Hive Fleet has appeared, the races of the galaxy have had to cross the Godzilla Threshold in order to stop it. EVERY Hive Fleet is treated as an apocalyptic event for the galaxy and these are just the scouting fleets, the main force isn't even here yet. While every other race has spent millennia fighting over one galaxy, the Tyranids are implied to have consumed multiple ones. This is just their next meal. The Chaos Gods are scared of the Tyranids, because they have the power to completely block out the Warp. And the Tyranids are implied to be running away from something EVEN WORSE.
- Makuta from BIONICLE started out menacing enough, but his constant defeats and failures gradually robbed him of his credibility. After the original head of the Story Team left, the character of Makuta underwent a serious retcon, which resulted in him turning into a calculating mastermind who had planned his victory from the start, turned out to be the Greater-Scope Villain behind a lot of former villains, and by the end of the story arc, ended up winning.
- Worm was never anything close to light and soft, being about a severely bullied girl finding an escape in her life as a supervillain. The character who shows just how dark the setting is, though, is Leviathan, a gigantic, immensely powerful thing that does literally nothing but try to sink cities and countries into the ocean every year or so. More characters die in the first paragraph of his rampage than in the entire series up until that point. The sheer devastation his attack causes to Brockton Bay leaves the city a gang-ruled ruin that's vulnerable to various other superpowered threats. Prior to this, the only antagonists in the story were local heroes and street gangs.
- The Smirvlak Trilogy: The series, starting from Smirvlak's Stone normally borders on a Cerebus Rollercoaster, with the worst villain being the Vile Villain, Saccharine Show of Lorko Maeliss — who dies before the climax and never introduces any major lasting effect on the setting. The reveal of Gnekvizz as the villain, however, completely and permanently swings the moon around, causing a complete Downer Ending with the apparent protagonist killed and the tone for the rest of the series set up with him taking the seat as Big Bad.
- The Heaven Cycle starts out its first two installments as relatively lighthearted and optimistic, with plenty of comedy and snark to balance out the ventures into dark moments, with neither Chayne or Ash ever managing to dampen the setting in a way that lasted bar the reveal of Ash's rape of Tango in the backstory. Then Alice enters the scene halfway through The Touch of Heaven after having previously been introduced in Before Heaven as a creepy but mostly low-scale antagonist, and utterly destroys the lighthearted nature of the series by initiating the attack on Haven, with named characters killed in a contrast to the nature of the previous installments and a permanent change to the status quo resulting from the aftermath. After this point in the series, Cerebus Syndrome sets in, and Alice is all to blame for it.
- Atop the Fourth Wall starts off as a silly show lighter than many other Channel Awesome productions, but the emphasis of more story-based elements into the series also introduced one character who took the series into a whole new direction: Linkara's robotic counterpart, Mechakara, who was completely devoid of humor and introduced a serious Cerebus Retcon about the true origin of Linkara's Magic Gun. Further appearances don't diminish his threat either and the darker tone established with his presence sticks around through the rest of the series with the introduction of nastier arc villains like Lord Vyce and the Entity.
- Geronimon in Godzilla and His Amazing Friends. While previous enemy monsters weren't exactly lighthearted, he's the first one to be depicted as explicitly out to kill the group and have planned to do so (previous monsters were primarily predators or space monsters the group happened to come in conflict with). Word of God has stated that Geronimon's goal is to kill Godzilla's friends one by one, then kill Godzilla himself, and that he's done something in the past that Godzilla will never forgive. He also has the dangerous ability to resurrect other monsters. Not only do he and his monster army nearly beat Gomora and Godzilla to death, it takes the entirety of Monster Island's inhabitants to stop them. And on top of that, he manages to brutally kill Gomora, a recurring ally, by stabbing him to death with his feathers and escape before Godzilla can take his revenge.
- Similar to Atop the Fourth Wall, The Spoony Experiment has managed to pull this trope off. The first was with Black Lantern Spoony, who tries to kill the clone Spoony and take the show over. Unlike Spoony's other would be murderers who are dealt with in comedic ways, there is nothing humorous about him. His current arc has Sephiroth. While his exact goals are unknown, he has no comedic traits to him, and the hints we have of his plans are pure Nightmare Fuel. The Guardian, the Big Bad of the Ultima story arc, in fact states he had no plans of conquering Earth because Sephiroth's (though he doesn't refer to him by name) Evil Plan, he feels has already doomed the planet.
- The Lizzie Bennet Diaries starts off as mostly a regular rom-com, with short flashes into more serious things such as the Bennet family's finances and the disputes between Lizzie and both Charlotte and Lydia. The story gets exponentially darker when George Wickham enters their lives, ends up dating Lydia, is psychologically abusive and manipulative, almost publishes a sex tape of himself and Lydia for money without her permission, and denies the entire thing later on when Gigi, Lizzie's friend (and sister of Lizzie's love interest Darcy), confronts him about it. (Keep in mind that Gigi dated Wickham once herself, and it ultimately ended up that he was only interested in her because of her money.) He seems to completely get away with it, even though the tape is never released thanks to Darcy's intervention. The worst part about it is that it is heavily implied that Wickham did it just to get back at Lizzie for siding with Darcy instead of him over a dispute between the men that was ages ago and didn't even involve her.
- Noob is set in a MMORPG, so the early villains, while annoying and sometimes genuine threats in-game, were no longer a problem once characters logged off. Then enter Master Zen, who has a deep resentment against the protagonists and is willing to go after them in real life. A plot present in all three incarnations of the franchise included him breaking into one of the protagonist's apartment, while the webseries had him resort to kidnapping and the comic showed outright (but fortunately failed) murder attempts. Even this wasn't enough for Stupid Good Sparadrap who still thinks of Master Zen as his former Guild Master, so his own storyline has Roxana: she become the first player he genuinely hates after she kills his pets and he eventually gets her to break out of her But for Me, It Was Tuesday attitude towards the incident, which causes her to hate him back.
- Vinesauce Tomodachi Life had the Jahns and Vlinny. The streams started out like any other stream from Vinesauce, but then, it goes full on Alien Invasion when these weird guys show up.