This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Teen Wolf
For the 1985 movie:
"An explanation would probably be long overdue."
Scott's first transformation. Unlike most of them in the movie this one is pretty long and painful. He just looks at himself in the mirror and hair sprouts everywhere as he cries in pain. Not to mention as Scott slowly turns into the wolf he looks pretty damn scary. The makeup makes Michael J. Fox look much more creepier than we've seen him.
He is also forced to open the door for his dad, who actually looks a bit more scarier. Even scarier than Scott's wolf form. Being older, he has grey hair and huge glasses. Having the grey hair covering him is just flat out creepy as hell.
For the TV series:
In the opening of the fourth episode, we have a scene of The Big Bad Wolf running alongside a car, nearly out of sight. Creepy.
Invoked in-universe when Derek asks Stiles to cut off his arm.
The video store scene.
Oooh yeah, a nice bit of Fanservice with shirtless Jackson...wait...is that...Oh God Why is there a BLACK CLAWED HAND COMING OUT OF HIS MOUTH???
Gerard cutting the Omega in half and leaving the other half of his body dangling from a tree. His spine is quite visible.
The Alpha had no qualms about disabling Stiles's car by dismantling the battery and throwing it into the school as a taunt. And then texting Allison under the guise of Scott so that she (and, by association, Jackson and Lydia) would come to the school. And then 'warning' the cops that any calls from the high school would be pranks. Yeah, sweet dreams.
Let's just say all of the "Night School" episode.
When the full moon comes around in "Lunatic", there's the subtle changes in Scott's behavior. He gets more aggressive, angrier, and at times his body language is downright predatory. It's unsettling to see that from a person who is otherwise a pretty Nice Guy.
The doctor's check-up scene for Jackson. Try watching it without cringing or grabbing the back of your neck.
The Alpha revealing his fangs to Lydia. Sweet GOD.
The hunters torturing the principal.
Allison and Scott's Kanima encounter in Season 2 episode 2.
Jackson opening up a snake aquarium and allowing the snake to slither down his throat and later come out of his eye.
The scene at the night club when the Kanima attacks, and the guys just start falling down one by one.
The library scene. Holy. Shit.
Allison's mom in general.
The scene with Isaac's father. He initially comes across as okay if a little intense but when he asks about Isaac's grades, he see the fear flash across Isaac's face when he has to say he is failing a subject. Mr Lahey starts throwing plates around and telling Isaac to clean it up and then throws glass at his face and nearly blinds him, all the while never raising his voice above a soft sadistic tone that shows how much he enjoys terrorising his son. The look of absolute terror lets us know that this has been going on for a very long time. The worst part? There is nothing supernatural or mystical about it. Isaac's dad was just a vile sadistic monster who managed to be the most despicable and horrifying person on a show filled with supernatural creatures.
The scene where Isaac's father is ripped apart and eaten by the Kanima. He completely deserved it and much more but it doesn't make the way he gradually realizes what a horrible situation he's found himself in any less chilling.
The scene where Matt nearly drowns. Made even more disturbing when his dying thoughts are flashbacks of him drowning.
Season 3 introduces us to the alpha werewolf pack. Two of which can FUSE together. It also features a a deer slamming into Alison's and Lydia's car as well as a swarm of crows bursting into a classroom.
The glimpse of Erica's body in the vault. Especially the way that her head just pitches forward when Allison turns to look at it.
In Fireflies, the titular fireflies. Which happen to be giant and kidnap a girl, who is later found murdered.
The method of murder used on Heather and the other victims. Nothing called "The Threefold Death" can sound pleasant.
Cora Hale's wolf form is positively terrifying. While most werewolves on the show look more or less human (and fairly silly) when wolfed out, she looks completely monstrous and more than a little like a lot of the scarier vampires from Buffy.
Nearly everything from "Motel California".
The motel keeps a tally of every guest suicide. While nobody is looking, it goes up by three.
Each room has a bible containing newspaper clippings of each suicide committed there.
Boyd hallucinates his dead little sister everywhere, and nearly kills himself by submerging himself in a tub and placing a safe on top so nobody can stop him.
Ethan imagines being split in two and having his brother emerge from him, and tries to bisect himself with a handsaw.
Isaac relives a fight with his father before being trapped inside the freezer. Again.
Just the fact that everyone else's hallucinations are enough to push them over the Despair Event Horizon, but Isaac is so used to being abused that he just lays there and accepts it.
Scott douses himself in gasoline and tries to kill himself, saying that whenever he tries to help he only makes things worse.
Made even worse since being burned is established to break the mystical compulsion. Scott's method means that not only can his friends not safely snap him out of it (the Darach is adapting!), but that he'll snap himself out of it just in time to burn to death, fully aware.
Plus, Lydia keeps hearing reenactments of the suicides happening all over the motel, including a young couple's suicide pact and a woman drowning herself and her baby.
When we find out why Deucalion is blind. Gerad stabs out his eyes. With flash-bang arrows.
The 'true form' of the Darach. The close-ups of the mauled faced and shredded flesh are terrifying. A true Squick moment happens when Jennifer kisses Sheriff Stilinsky and then pulls away as the lip less Darach. Even worse when you remember that Derek sleep with Jennifer
The town hosts a concert in memorial for the victims of the serial killer, whom the audience knows is the Darach. During the orchestra, the band starts playing what the audience, and one of the characters, recognizes as the music which plays when the Darach is about to attack. Meanwhile, the band looks possessed. It ends with a death trap in the piano killing the elderly music teacher. Even more disturbing in that the Darach is holding a knife to Lydia's throat and counting down the activation of the death trap to the second.
The sneak peeks for the season 3b include Stiles freaking out while a person tries to escape from a white screen and his classmates appear possessed. Playing in the background is a creepy child-like voice and the last lines we hear are Stiles saying "Wake up!" while the child sings "Life is but a dream"
Another promo for 3b features someone staggering down a hallway, intercut with scenes of Lydia attempting to scream while unable to open her mouth, Allison petting a wolf's corpse with dead eyes, and a creepy apparition of the sheriff. The hallway opens into a blank white room with a sink and a mirror, revealing that the person is Stiles - and he's covered in blood.
3x13, has Allison having visions of Kate. She's a lot scarier then before.
In 3x14 Alison has a nightmare of being autopsied alive by Kate.
Stiles' hallucinations are pretty creepy too - he starts having nightmares and sleep paralysis, which is already disconcerting, but then he stops being able to read because you're not supposed to be able to read in dreams. Which means he never really knows if he's dreaming or not.
And then there's his dream near the end of 3x13, he wakes up in a dream next to Lydia. He sees something at the door. She starts begging him not to open it. Whatever it is has even Dream Lydia frightened. Just what is behind the door is left horrifyingly ambiguous.
We're introduced to William Barrow in "Galvanize", a notorious mass murderer who blew up a bus full of teenagers because he claimed they had glowing eyes. He's already creepy with that and his unhinged personality. But then he's admitted to surgery for what the doctors assume is a piece of shrapnel, and it turns out that it's really a tumour full of LIVE FLIES. Holy shit.
It's not that bad. It's worse. Barrow is able to hide his scent and movements even from werewolves when he is right next to them. He could hold his own against Scott, and identified Kira as a target with almost zero knowledge of her. As a side note, when Allison searches the bestiary for information, the only thing about flies is something on the "Lord of the Flies", Beelzebub, one of the names associated with Satan.
When the lights go out after Kira defeats Barrow, severalfigures materialize out of nowhere and attack Isaac. According to the promo for the season, there might not be people behind the masks.
Which is followed immediately by the until now unstoppable Oni being utterly destroyed in a single blow. Superpowered Evil Side doesn't quite cover it.
The abuse the Twins endured in their previous pack is kinda horrifing when one starts thinking about it. One, the abuse ranges from physical to emotional/psychological to even implied sexual. Its also implied they are born werewolves, so their families were more that likely part of their pack, which makes it even more horrifying, because their either stood and watched or even participated of the abuse.
And, on the other hand, another fridge horror dose comes if those implications are true: Ethan and Aiden killed their own family to free themselves from their abusers.
"Riddled" starts out with Stiles held captive in a basement, with a bear trap on his leg and an inhuman, heavily wrapped up figure taunting him. Turns out it was just a dream, and he was only in the woods at risk of dying of exposure to the cold. Later, as he's having a brain scan, he has another vision in which the same figure, the Nogitsune, forces him to answer a riddle, takes his bandages off to reveal he looks like Stiles underneath. A power surge happens at the hospital, and Stiles is seen leaving the hospital with an out of character look on his face. Ladies and gentlemen, Stiles has leftthe building.
Scott's face says it all during this scene. Not only is he understandably horrified, he's completely speechless. He's downright terrified of his best friend.
In "Echo House" the mental facility itself. It's dreary, depressing and very frightening — and that's without it being a supernatural hotspot, which it apparently is also.
In The Wolf and the Fox The flashback of Nogitsune killing two soldiers at a camp. Before he attacks them they are telling riddles to each other. When Nogitsune has one of them cornered he has one of his own.
What's worse than somehow surviving being burned alive via molotov cocktail? Learning that the doctor sold all the morphine, too, the hard way.
Generally the Oak Creek camp was a terrible place to live in. The lack of respect and proper supplies, the doctor selling much needed drugs to the black market, the government covering up the cruelty and slaughter...
So much of this episode is disturbing mainly because, even without the supernatural elements, it still provides a pretty realistic demonstration of Japanese internment camps during WWII.
And there's the flashback of Nogitsune's first rampage, killing guards and prisoners alike. At first we see the aftermath of his initial rampage at Eichen House, with dead bodies everywhere and the scene where a group of people fleeing the Nogitsune through the halls. Eventually they run down the stairs to find a locked gate, with Nogitsune still catching up to them.
Devoid had big bad wolf Peter Hale freaking out right along with the rest as Stiles vomited up literally an entire person's worth of bandages.
Earlier in the episode Void/Stiles cuts his abdomen open and UNLEASHES A SWARM OF FLIES FROM INSIDE HIS BODY...Yeah sweet dreams.
Void/Stiles leading the Oni through the hospital in a murderous rampage in "The Divine Move"
The death of Void/Stiles: they scream in agony for a moment before their body petrifies, cracks, and crumbles into dust.
The skeletal...things that appear in the season 4 opener "The Dark Moon". They're only shown in brief glimpses moving through dark tunnels or in the desert, so it's hard to tell whether they're warriors in skeletal armour or actual animated skeleton creatures. One of them lodged a claw in Stiles' Jeep that is at least a foot long, Malia claims they smell like death, and according to Jeff they're under Kate Argent's control.
The man without a mouth in "Muted". In addition to his rather creepy design, he murders an entire family, then taunts the last survivor from behind a door. He might not be evil, though, since the family he was killing were wendigos.
Lydia and the policeman finding the hidden room in the family's home, with the bodies of murdered people hung up in bags.
The Wendigo Teen's blank, horrifying white eyes and rows of teeth.
A lesser moment, wherein Stiles realizes his notes he took were not math notes. It instead resembles some sort of mad scrawl of letters, numbers and other such things. It immediately brings the Nogitsune to mind, and whether its demise awoke something...worse.
Good news, they weren't his notes. Bad news, they were Lydia's. And after she decodes them, she realizes that it's a hit list for everyone supernatural in town.
The faces emerging from the wall in Lydia's hallucination.
The death of The Mute at Peter's hands, reminding us that yes, he's still a threat, and what is more, he apparently...tore somethingout of the Mute's...body.
When Lydia's mom lifts up Coach's shirt in "Weaponized". Might want to go take a shower after that, especially if you have dermatophobia.
The death toll itself is pretty horrifying - Simon managed to off a whole pack except their leader Satomi, and its clear that he's not the last assassin to come after the gang. As seen in the previews for the next episode, it looks like there might be another Mute on the way...
The opening of "Perishable". Deputy Parrish is tied to his car, covered in gasoline, and set on fire. He lives, but from the screaming you can tell he sustained major burns in the process.
Brunski, one of the sadistic orderlies of Eichen House, has murdered at least ten people there and covered it up, and plans to do the same to Lydia and Stiles.
In the episode "Monstrous", the pack that Scott is protecting comes under heavy fire. While fighting off one of the assassins sent to kill them, Scott gets a little carried away and keeps slashing away at him, and as he does that Scott starts to transform even further into something reminiscent of Peter's beast like Alpha form from season 1.
"A Promise to the Dead" has a guy who acquired a Third Eye by drilling a hole into his forehead. And yes, we get to see it up close.
The origin of the Berserkers. They're normal, brainwashed people (or possibly werewolves) who are trapped in the skeletal armor, and Kate plans to do the same thing to Scott.
Liam's hallucinations of the Berserkers.
The teasers of Season 5 are horrifying in their entirety. Apparently the werewolves and other supernatural beings in Beacon Hills will be preyed upon by a group of Torture Technician Deadly Doctor types. Some of the laboratory equipment resembles primitive torture devices, and its all done in minimal lighting, giving an extremely claustrophobic look to it.
The official appearances of the "Dread Doctors" are just as bad if not worse - their figures seemingly flicker and phase in and out of reality, can induce night terrors, and seem to cause crows to appear around and inside their targets.
Scott's childhood friend Theo seems fine, right? Turns out not so much - he has strangers playing his parents, and isn't very pleased when he finds out that one of them didn't learn how to forge his father's signature correctly. The solution? Well, signatures probably come out wrong when you're wearing a cast. Queue Theo approaching his "father" with a hammer, and the father submitting his hand accordingly.
The brief glimpses we have so far of Kira's kitsune side taking a firmer control are quite terrifying. To see sweet, dorky Kira, who is arguably the nicest main character with this large golden aura of fire who was willing to attempt to murder Lucas even when he was no longer a threat is chilling. Thank God Scott interrupted when he did.
And it may not even be her fault. The Dread Doctors did something to her, which she didn't remember for a long time (terrifying in and of itself), and her Superpowered Evil Side may be a side effect, or the main goal, of her "treatment."
Eichen House, just in general. A mental institution that has frequently been the site of supernatural encounters and fights between the pack and their enemies. One of its staff attempted to kill Stiles and Lydia in season four, and the bedside manner of its other employees range from dismissive of their patients to outright abusive. It also does not seem to be in the most sanitary of conditions, given the perpetual low and flickering lighting and the appearance of the walls. Since its introduction in the second half of the third season, it's been a regular presence of the show, but even Theo remarks on how unsuitable it is to be a medical facility.
The teaser for Season 6 reveals the Wild Hunt who, when they take someone away with them, have the power to erase all memory of someone from the world. Stiles apparantly falls victim to this with a scene of him frantic as he meets his father, relaxing as he calls Stiles "son..." Until it turns out it was just addressing someone he thinks is a stranger as he asks Stiles' name.