The almost unbearable tension from the start of the teaser to the first sign of gross otherworldly mayhem. Nothing's happening yet, but you just know this poor bastard is doomed... any... minute... now.
The first five minutes of almost every episode. And a good portion of the rest. But especially Flight 627.
A diabetic passenger on an international flight injects himself with what he thinks is his insulin, but is actually a fast-moving disease that first makes him ill, stagger to the lavatory, vomit on a stewardess, and then makes the flesh dissolve off his bones as it swiftly spreads to everyone else on the plane.
Another episode involves a woman who emits a form of radiation when she's stressed that literally superheats the water in organic tissue. As a test, this woman is bound to a hospital bed and a rat is released into the room. She freaks out when it starts crawling on her, and the radiation makes the rat explode. This example is only made slightly less sickening when you see the heroes trying to re-create the radiation to test it and save the woman, they end up blowing up a papaya in a rather sticky spray that was made ironically more amusing when Walter Bishop refers to said papaya as "The friendliest of fruits"....
Was that the same episode with the woman whose head exploded in the opening (after killing everyone else in the diner, thanks to that radiation spreading ability)? Yeah...
A gas released on a public bus crystallizes, trapping the passengers inside like flies in amber. I, being one who rides the bus regularly, still get the shivers when I remember the crazy of the day for that episode whispering "I know what's going to happen on the bus..."
A boy is surfing the web when a popup Blip Vert entrances him. A hand reaches out of the monitor, deforming it, and grasps the boy firmly by the head, boiling his brain inside his skull.
A college professor is lecturing about virology. He starts to choke and sputter, then collapses. His TA attempts to give mouth-to-mouth, but pulls off when something bulges out his neck and starts to climb out of his mouth: A spiney looking slug.
Ironically, it's a supersized cold. Which immediately makes the thing Nightmare Retardent for people with a biology or medicine background; for reference real viruses look like this. They also don't get much larger than a few microns across. You can drink that water in peace now, maybe...
It could be modified (multicelluar?) or something? *pushes away water glass*
The random newsagent who gets a two dollar bill and then his orifices seal up. His nose, his mouth, his eyes, and he suffocates. It gets worse when Walter asks if anyone checked his anus and penis to see if those sealed too. It gets even worse when they try and get the guy responsible; one of the agents is exposed to the same chemical, and the main character tries to save him via a tracheotomy. The skin simply eats the trach.
The Bishop family weaponise it in "The Bullet That Saved The World". Windmark calls it barbaric.
"Jacksonville" provides some absolutely terrifying Body Horror with a building from Over There colliding and merging with a building Over Here, fusing the two buildings together, along with all its inhabitants. Made that much worse by Astrid's refusal to dissect the body of a man fused with his alternate. When someone who works every day on fringe cases with Walter is upset by something, you know just how extreme it is.
The Russian cosmonaut that became host to some sort of Radiation Vampire from space.
Particularly Walternate's grin in 'Bloodline'. Brrr...
The look of utter loathing Walternate gives Olivia every time he sees her. Especially when he pulls her out of the tank in "The Abducted". And it makes a disturbing amount of sense considering she was the one who convinced Peter to stay.
Getting erased from existence is fairly disturbing in my opinion.
You think that's bad; suppose you manage to undo it, and then find out that no one you love remembers you. Oh, and you had a son you didn't even know about until after he'd been erased as well.
Olivia's situation in season 3 is somewhat similar. Imagine being all alone with a single ally in the entire world. Literally. Not to mention the light deprivation torture, experimentation, imprisonment and potential organ removal (while conscious!) with little or no chance of rescue.
Seeing a corpse that looks exactly like you is a pretty frightening idea.
The episode "Bad Dreams." Especially the beginning. Ironic Nursery Rhyme. A woman alone with her baby in a creepy, empty train station. The train is coming. Someone is coming up to her. Then fricking Olivia Dunham shoves her onto the tracks with this utterly blank look on her face.
The other nightmares, which include Olivia guiding a woman's hand to slit her own throat and guiding another woman's hand to stab her husband, are equally terrifying.
Crazy Walter... lovable old man who likes candy. Walter with his brain rejuvenated? He frigging cuts off William Bell's hand and leaves him in suspended in amber.Holy shit.
Considering the douche was just standing around, chatting causally while Astrid was bleeding in Walter's arms (shot by his men) and turned Olivia into a Apocalypse Maiden... you can see where Walter was coming from. You can also add William Bell to Nightmare Fuel list.
The reason why Walter had those pieces of brain removed in the amber timeline. William Bell's plot was all his idea and he knew exactly how to pull it off.Damn.
The Observers, especially in "Letters of Transit".
Windmark smiles. After four seasons of September being emotionless, it's just... not... right.
Windmark: Hello, little girl.
Windmark killing Etta. Because instead of unceremoniously shooting her in the head like a nice monster who isn't emotionally invested in such things, he chokes her first, takes the time to ask about the bullet necklace (while he's nonchalantly choking her), realizes that the reason Peter bought it for her was because he loved her, and then shoots her in the stomach so she can slowly bleed to death.
Windmark versus Peter is terrifying for two reasons. One, for an emotionless Transhuman, Windmark sure looked pissed off. Two, Windmark shows Peter Etta's final thoughts. Apparently the only emotions Windmark is capable of are sadism and anger.
The bad guy from "A Short Story About Love". Dehydrates husbands alive, takes their pheromones and the tries to use them to sleep with (read: rape) the grieving wives. When the women wives realize that this guy is not their dead husband, he suffocates them with cling wrap. He tries to play the sympathy card and, unlike a lot of the other villains on this show, it really does not work.
David Robert Jones. His partiality to inflicting Body Horror on unsuspecting innocents, his electric torture of Nina Sharp from the alternate universe, the fact that he outplays both Fringe teams, his creepy politeness, his brutal beating of Peter Bishop with a crowbar even though he probably could have just shot him or infected him with some kind of parasite... he's terrifying.
Meana Sharp. Because a bitchy alternate version of your surrogate mother drugging you in an attempt to destroy your universe is terrifying.
Doubles as a terrifying situation for Nina Sharp. She's got her alternate who can get into the most secure part of Massive Dynamic, a sadist like Jones and the man she loves (who also happens to be a Mad Scientist capable of pretty much anything) all conspiring to use her surrogate daughter to destroy two universes (which also includes Rachel's family).
How the Observers torture Walter. Even complete with capillaries in his eye bursting. It makes it that much worse that he's having a seizure during all of this and trying desperately to protect his granddaughter.
Windmark: You seem much more interesting as a human being... than as a vegetable. But all things being equal... I don't care which one you end up.
The Japanese woman in the Black Market in 5x01. Just... sitting there. Not saying word. Just staring. There could be any number of nightmare-free reasons for this. Given the time and place... she's probably been mind raped.
The removal and reinsertion of Observer tech into Peter Bishop. The surgery itself (particularly the way the device seemed to activate on its own and attaches itself to his spinal cord, the dripping blood and the fricking twitching from the Observer) and the potential consequences. He's either going to Take a Level in Badass or bad things will happen.
Windmark watching and the Psycho Strings playing in the background do not help matters.
Peter reading Olivia's thoughts and then pulling out chunks of hair.
Some of Walter's hallucinations in "Black Blotter" include images of Carla Warren's burnt corpse, somehow ending up in Observer ridden Manhattan with no idea how he got there and evil!Walter starring back at him.
Those who had bad experiences with psychedelics might find Walter's sudden shift from downtown Manhattan to the boathouse and all the requisite disorientation distressingly familiar.
The goddamn noise you hear when someone is getting Mind Raped by Captain Windmark. That one, high pitched note... you hear it first and then you see his victims shaking and/or bleeding from various orifices.
The only way to stop Windmark from brutally reading you and dooming the people you care about and/or the entire world is to either piss him off enough so he'll kill you himself or shoot yourself in the head.
Michael, Donald's son, giving himself up to the Captain Windmark to save the others.