Werewolf transformations. You feel your bones crack and reset, and the failure of your heart, liver and kidneys. You're totally awake for the entire thing, and when you're not screaming, that's only because you can't, because your vocal cords are reshaping. It's explicitly said that humans should simply die from the pain.
Mitchell's description of the transformation. Body Horror at it's finest:
Mitchell: He should be dead within 30 seconds. The werewolf heart is about two-thirds the size of a human's; but, in order to shrink, first, it has to stop. In other words, he has a heart attack. All the internal organs are smaller; so, while he's having his heart attack, he's having a liver and kidney failure too, and if he stops screaming, it's not because the pain has dulled: his throat, gullet, and vocal chords are tearing and reforming. He literally can't make a sound. By now, the pituitary gland should be working overtime, flooding his body with endorphins to ease some of the pain, but that, too, has shut down. Anyone else would have died of shock long ago, but it won't let him. And THAT'S the thing I find most remarkable: it drags him through fire and keeps him alive and even conscious to endure every second. Nothing like this could just evolve; this is the fingerprint of God, an impossible, lethal curse, spread by tooth and claw. Victim begets victim begets victim. It's so cruel, it's... perfect.
George's prequel. He's out on the moor with a slightly obnoxious but well-meaning American tourist trying to find a loch but they get lost, then the American's camera shows that it's a full moon. The two find a sheep that's been torn apart, then run, for the sounds of howling get closer. Eventually the camera is dropped and we hear the two being attacked. When the camera is picked up again and turned on, the tourist is disembowelled, and George is on the floor, bleeding, saying "It got me."
The preference of vampires to work in the police force so they can easily perform cover-ups and lure in their victims. If there's anyone the average British viewer would trust, it's a local policeman, so seeing them repeatedly play on their victims' trust then brutally slaughter people is very creepy.
Mitchell and Daisy slaughtering everyone inside the train car.
Type 4s. Zombies that can feel themselves rotting, and even if they're sliced up into pieces, they're still aware.
George and Eve's lives at the start of series 4. He spends every second, neglecting sleep, watching the door of his baby daughter's room, waiting for vampires to attack. He hangs mirrors around the room to check for if people who enter have reflections, he always carries a mirror and a stake, and he's arranged everything in the room into the shape of crucifixes, including twigs at the third floor window and children's blocks on the floor.
George when he partially transforms in the first episode of Series 4
The possible future shown in Series 4. Vampires have taken over the world, with only a small resistance of humans and werewolves fighting them. London at least has been wiped, along with New York after it. Britons tried to flee the island in boats, but vampires sneaked onto the industrial shippers and slaughtered all aboard. Humans and werewolves were rounded up in camps, which were struck by cholera, and the prisoners were branded to mark them as non-vampires, and in some cases made to fight each other for the vampires' amusement.
Hal has become one of the vampires' leaders, a feared and brutal dictator; Tom was forced to kill others in the camp dog fights and eventually committed suicide in one, and Annie eventually dissolved into the breeze after burying too many friends.
And the Saviour of humanity knows that the dictator trying to kill her used to feed her bottles and change her nappy when she was a baby.
It's implied that Hal only came into the household because future Eve set it up and in her timeline did not.
What would have happened if Kirby had succeeded with his plan: an invisible, inaudible serial killer free to murder whoever he wanted, and practically no way of stopping him.
The Old Ones prequel. A cleaner on their convoy ship is shut outside on deck by his crewmates after the captain is told to offer up a sacrifice or everyone aboard would be killed. He then wanders around the freezing deck till he sees a figure ahead in a suit who, when he approaches, shows flashing black eyes and rotting teeth before ripping him apart.
Tom transforming in a club full of young people, slowly advancing towards the crowd. Luckily Hal and Alex get them out.
Cutler's death, knocking down a door after not being invited in and burning alive as he tries to kill baby Eve. Fortunately, or not depending on the bad future, Annie stakes him before he can. All because he wanted to be a history maker. If only his father figure had loved him more. I'm looking at you Hal.
The fact that the Old Ones are able to walk into any home or habitat without a care in the World. One of our few defences humans have and they can ignore it like its nothing. Holy Symbols doesn't deter them and you could be coming home one day with your family and find one watching your Telly waiting for you to come back. So they can tear you apart if you're lucky, if you're unlucky they will take their time because they happen to be bored..
The madman's possession at the start of series five. The devil enters him, horribly twisting his neck and back to the sound of his screaming in pain. Then his eyes turn blood red and he begins to laugh horribly.
The despicable werewolf Larry destroys Tom's self-esteem, insults Tom and Alex and deliberately provokes Hal into fighting him despite Hal's warnings not to. So Hal punches him then strangles him with a cable, all the while whispering an introduction of his dark side. This is not even Hal killing out of blood-lust, here we see first-hand Hal's capable of murder just due to his personality.
Bobby's backstory. He was scratched by a werewolf when he was 11. One month later it was his birthday, and his whole family was at his party...
When tied to a chair by Crumb, Hal's dark side fully resurfaces for about ten seconds. He flashes the black eyes and taunts Crumb that he's doomed to be a monster, but that's not the scary bit. The terrifying part is how Hal was exactly the same as when he met Cutler and completely psychologically broke him.
The Devil banishing Alex. She's trapped in her own grave, next to her own rotted body.
Every time he regains control, evil!Hal almost certainly kills everyone good!Hal befriended over the past however many decades, and when good!Hal returns, he remembers it all.
The eighth episode of the second series was probably the most Nightmare Fuel filled episode of the entire series. Lets see, we have Mitchell decimating Kemp's organisation, the mutilated ghosts of the pressurised werewolves (the process of which was Nightmare Fuel in itself) coming after Kemp and Lloyd, Kemp stabbing the psychic to death with no remorse whatsoever, Annie being forcefully dragged into the afterlife, Mitchell almost tearing Lucy's throat out, Kemp killing Lucy, and just to top it off, Herrick is revealed to be alive.