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 villian's arrival is sudden, and he 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. Grim Dark 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.
See also Dead Serious and Not-So-Harmless Villain, when a villain starts as ridiculous but, because of some changes, 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 in fact, Vile Villain, Saccharine Show may be what you want for a single work).
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- New World Order. They debuted in one of wrestling's most cartoony eras and proceeded to utterly annihilate every face who stood in their way. They were led by Hulk Hogan, formerly an Invincible Hero, now a cruel and vindictive heel who loved to play the numbers game. Hell, they were so dark that Sting went through a personality 180 from a cheery surfer to a brooding Crow-type just so he could effectively combat them!
- The Undertaker's Ministry of Darkness. They crucified and hanged their enemies, kidnapped women, brainwashed midcarders, made blood pacts in the middle of the ring... only infighting and The Undertaker getting injured put an end to them.
- The Nexus made their debut by savagely beating down everyone around the ring- and we mean everyone. The faces, the heels, the trainers, the announcers, the commentators... they even destroyed the ring! Wrestling/RAW ended with the seven of them standing tall, everything around them utterly annihilated. And it got worse from there.
- The Wyatt Family. With the WWE's current Lighter and Softer atmosphere, it's really jarring to see a gimmick like this one, especially considering it may be one of the most disturbing and genuinely unnerving characters in the history of the company.
"Hey, wanna see something... really scary?"
- A well-booked Wrestling Monster usually qualifies, since they will steamroll face after face until someone puts them down. Well-known examples include Kane, Brock Lesnar, Mark Henry since his 2011 FaceHeel Turn, and even Beth Phoenix.
- The Shield. While they've started to fall apart in 2014, they debuted as a brutal SWAT Team-like trio who would steamroll anyone with flawless teamwork and then Triple Powerbomb them through a table.
- Randy Orton. It's easy to forget that, prior to the summer of 2007 when he really began to establish himself as a main-event heel, WWE had for months and months been a carnival of kooky gimmicks: evil cheerleaders, a leprechaun, a "monster" who ate worms, Booker T thinking he was a medieval king, Mr. McMahon getting his head shaved...Even the (kayfabe) destruction of McMahon's limo and Chris Benoit's (real-life) death earlier that summer couldn't quite put a damper on the silliness. Then Orton loses his title bout against John Cena at SummerSlam, pouts about it...and, after Jonathan Coachman tells him that he hasn't "proven himself" yet, kicks Cena's father in the head just to guarantee that he'll get a rematch.
- Seth Rollins has gotten progressively unhinged, then came the last RAW of 2014. He holds Edge hostage and actually threatens to kill or at least paralyze him via Neck Snap. When John Cena tries to save him, Seth orders him to stop and laughs that if Edge is unable to hold his daughter for the rest of his life due to paralysis, it is all on Cena's head. Cena gives in to all of Seth's demands, only for him to calmly say, "Damn, you gotta know me better than that, John. I'm gonna kill him anyway." Fortunately, Cena was able to stop him from stomping on Edge's neck, but still.
- Kevin Steen was already a pretty dark character in his "wrestling's worst nightmare" persona, but then he came to NXT under the name Kevin Owens. He has since established himself as the darkest, cruelest, most heartless heel in NXT's young history- injuring anyone who he doesn't like, damn near killing his former best friend Sami Zayn, and generally asserting himself as someone who does not give a damn about who or what he breaks on his way to the top. Case in point, he vowed to beat John Cena at Elimination Chamber 2015 and he kept his promise, not only throwing everything he got, but also got a CLEAN win over Cena, which very few in the WWE have managed to do.
- Lucha Underground has three Knights of Cerebus
- First there is Pentagon Jr. a brutal luchador who became this when he started attacking and breaking the arms of those on the roster, snapping them like twigs and dedicating each one to his Master. Showing zero interest in capturing titles as the only thing that mattered was sacrificing limbs to his Master. He even proved that gender was no issue to him when he not only attempted to break the arms of the ring announcer but Sexy Star as well.
- Second is Mil Muertes, a giant of a man who flat out demolishes those he faces. Unlike others on the roster, he does not waste any time or give his opponents even though he outclasses them opportunities to fight against him.
- Last but not least is Matanza, who is the brother of The owner of Lucha Underground, Dario Cueto. Kept locked in a cell in the basement in The Temple, its been stated that he was responsible for the deaths of Black Lotus's parents and also her village. He's kept locked up and we never get a scene with him but the only sound you hear is his snarling and breathing from within his cell.
- Matanza finally proved himself to be this when Dario, furious after a henchmen of his had failed, forced them against the bars of Matanza's cell and Matanaza destroyed him with blood splattering all over the room.
- Jimmy Havoc became this for Progress Wrestling when he turned heel, before that most of the heels were just assholes, Havoc was Ax-Crazy, some of the things he did was threaten to set Mark Andrews on fire for a title match and he tried to cut Will Ospreay's ear off to make a point to the companies management. When he was threatening to set Mark Andrews on fire for a title match someone from the crowd shouted out "you're a psychopath".
- CHIKARA is pretty well known in the wrestling world for being a mostly lighthearted, family-friendly and often comedic wrestling promotion, filled with goofy colourful gimmicks, slapstick comedy routines during matches, and storylines that would not seem out of place in a superhero comic. And then in 2014 along came The Flood, a massive stable of almost every single major villain and rudo-stable the company had ever seen. And leading them was Deucalion, a massive body-armour-clad black-eyed super-soldier in a gas mask who looked more than a little bit like Bane from the Batman comics, with one goal: To destroy CHIKARA and all that it stands for, from the inside out. What resulted was a massive year-long war between The Flood and the technico-wrestlers. Now CHIKARA had seen it fair share of serious villains, such as the BDK and the GEKIDO stables, but there was one major difference: Deucalion was not afraid to kill (in kayfabe, anyway) any wrestler that got in his way, regardless of what side of the war they were on. And he had been responsible for the 'deaths' of six wrestlers before he finally met his demise at the hands of Icarus. To date he is still the darkest and most dangerous wrestler that CHIKARA has ever seen.
- Jake Roberts: A thinking man's heel, "The Snake" was ahead of his time with his masterful promos and character work. Some of his most "memorable" Kick the Dog moments include DDTing Ricky Steamboat on concrete, locking Ultimate Warrior in a room full of snakes, and having Randy Savage bitten by a king cobra in front of a live audience. It is telling that even The Undertaker, who was once his accomplice, eventually had enough of his actions and turned against him.
- 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.
- 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 millenia fighting over one galaxy, the Tyranids are implied to have consumed MULTIPLE. 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.
- Camelot is extremely lighthearted up until Mordred arrives, and then starts building toward a genuine tragedy. This could easily have caused Mood Whiplash, but it's handled extremely well.
- Also in Into the Woods when the female giant shows up in act II, lets just say things go down hill from there.
- Similar to the above, the entrance of Bill Sykes near the beginning of Act II of Oliver! means the story gets much darker from that point on.
- Don John from Much Ado About Nothing is definitely a Vile Villain, Saccharine Show and his Evil Plan causes Cerebus Syndrome to kick in.
- Great Britain is initially something of a Black Comedy romp about the poor ethics of the British tabloid press, with Paige Britain using her sex appeal and lack of morals to get further ahead. However, two characters manage to cause Cerebus Syndrome to kick in during the second half:
- The model Stella Stone is at first something of a joke, appearing to be a Brainless Beauty that gets a boob job to sell more photos and get better work on Page 7 of the Free Press. However, come the second act her character arc turns into a depressingly real look into anorexia. Her condition eventually worsens to the point that she cannot recover, one of her family members (it's not revealed who) leaks this to the press, and eventually Paige heartlessly buys the exclusive rights to her death story for £100,000.
- Kieron Mills, a man suspected by some members of the public of either killing his daughters or selling them, is something of a plot device for the first act, with him being used as a joke about the tabloid press going berserk about paedophilia. His being arrested is also a case of Black Comedy due to the absurd headline that he is given. However, the scene where he and Paige talk in the mental institution is not Played for Laughs, where Paige's questioning makes him go berserk. It later turns out that Free Press were completely wrong about him, and that the bodies of his daughters were found far away from where he was suspected to have done things. Of course, by the time they find this out, it is far too late and some fellow inmates have stabbed him to death in a misguided case of Even Evil Has Standards. This kicks off the climax and causes the humour to more or less disappear entirely.
- Mortimer's twisted brother Jonathan, being a ruthlessly sadistic sociopath with a short temper and very little (and quite dry) humour, feels like belonging more to a crime-thriller kind of story than even the lighthearted Black Comedy that is Arsenic and Old Lace.
- 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.