Harry Potter (film)/Nightmare Fuel

You don't want to know what's behind that door.

Harry Potter and the Goblet of Fire

The second task of the Triwizard Tournament in the Black Lake in the film adaptation... the merpeople's design and their shockingly aggressive attitude when the Berserk Button is pressed.. Viktor Krum's transfigured shark head, the Grindylows, which, despite the fact that they were only seen from a distance or below, were extremely territorial...

Harry Potter and the Prisoner of Azkaban

- The Giant Spider form of the Boggart in the movie. Seriously, the dementors have NOTHING on this. Do not view if you wish to sleep in peace.

Harry Potter and the Half-Blood Prince

Katie Bell touching the cursed necklace, floating up with her arms outstretched, then DROPPING TO THE GROUND SCREAMING! The worst part in the movie is when we get a closeup of her face while she's being held rigid in the air. Her eyes are bulging and the angle makes her mouth look like it's open much wider than humanly possible. It's freakier seen than described. What? You say you want to see it?

Harry Potter and the Deathly Hallows

Really, anything scary in the book Deathly Hallows are ten times worse in the movies. Voldemort's soul was sick and we get Harry and Voldemort merging mid-apparation. FREAKY. But Fiendfyre is God-awful as well. It was like someone called up Satan and personally escorted his worst demons through the gates of Hell!
- In the Purgatory-esque scene, we get a much more vivid picture of Voldemort's soul (well, the parts that have been removed from the Horcruxes, at any rate). It looks like an abortion... combined with Freddy Krueger.
  - Ahem.
- When Voldemort uses his cloak as "tentacles" to attack Harry, he looked a little too much like a certain well-dressed gentleman...
- Voldemort's death is... very graphic. He starts dissolving into paper like shreds, with a truly horrifying, despair-filled look on his face.
- In the film, Voldemort's ultimatum to the school is accompanied by a chorus of inhuman shrieks, which is revealed to actually be coming from students. Apparently, whatever spell he was using had a side effect of mind raping random people.
For any twin, Fred Weasley's death is this, on top of being a Tear Jerker. It's not the death itself that's the nightmare, it's the fact that George is left alive and alone (well, without the person so close to him, they're all but extensions of each other). And, rather understandably, he never gets over it. It's especially bad if you are also rather terrified of loneliness.
The scene where Harry uses the Cruciatus Curse on Amycus Carrow can be very disturbing.
This picture of Fenrir Greyback. Don't look in his eyes too much.
Voldefetus. It doesn't help that this horrific...thing is seen in an extreme closeup, covered in blood against a completely white background. It's impossible to miss.
The ghost in the tower pulling a Jump Scare when she suddenly screams at Harry during his search for one of the Horcruxes.
The scene with the locket Horcrux trying to turn Ron against Harry in a last ditch effort to defend itself. Ghastly spectres of Ron's friends and family tell him that he's worthless compared to Harry.
- That Eldritch Abomination swirling cloud of darkness that EXPLODED out of the locket was freaky as hell. Swirling, talking, with things that looked like heads and bones thrusting out of it before disappearing... That scene wasn't scary in the book, but in the movie...
- Speaking of the locket, you know when Harry finds the Sword of Gryffindor in the frozen pond and jumps in to get it? When he gets close enough to the sword, the locket yanks him back. It doesn't get repelled—it drags him away of its own volition. It knows what he's trying to do. Just that is scary enough, but Harry looked like he was getting strangled and quickly abandoned the sword to claw desperately at the ice. I'm seriously glad that Ron came back in time. Five more minutes and Harry might have drowned.
- Here's a nice subliminal scare in the movie. When the locket opens, the shot lingers on the open locket for a split second before the darkness erupts out. Pause it there and you'll see an eye inside the locket looking right at the camera.
Death, as portrayed in the movie's animated version of The Tale of the Three Brothers. The atmosphere in that scene is generally creepy, but the way that hunched-over, skeleton-like thing moves...
- Try not to think of how much Death looks like a Dementor... although, to be fair, Death is rather more True Neutral than anything else, if something of a Jerkass Genie.
  - Dementors look like Death, not the other way around.
- Nagini attacking Snape. The movie has this as a Nothing Is Scarier moment—we see it only partially through a dirty window, and only hear the sound of the snake striking at Snape again and again. God, the screams.