Nightmare Fuel / Harry Potter and the Prisoner of Azkaban
You don't want to know what's behind that door. Unfortunately, you're about to find out...

Ron: I think... something's coming aboard.
  • Boggarts. Creatures that can exist without any true form is pretty unnerving, but the fact that they can take the shape of the thing a person fears most, which can change on the person's mindset, and inhabit any given corner of the globe is pretty damn terrifying. The Giant Spider form of the Boggart in the movie is a good example. See for yourself.
    • When Parvati Patil is faced with a giant snake Boggart in the film, she responds by turning it into an infinitely more terrifying Jack-In-The-Box, that bobs back and forth and, upon going forth, looks as though it's trying to grab you. Keep in mind, this was made from a spell that's essentially weaponized Nightmare Retardant. Who on Earth thought that would be funny!?
  • "Dementors... are among the foulest things that walk this earth. They glory in decay and despair. They drain peace, hope, and happiness from the air around them." Rowling tried to dream up a demon that could scare anyone. Her solution was a monster that literally eats joy. And souls. According to her, she actually based the monsters on the feeling of depression. So basically, these things are depression made into (semi)physical creatures. Ron even says when in their presence "I felt weird, like I'd never be cheerful again."
    • The actual results of the Dementor's Kiss.
      Lupin: You can exist without your soul, you know, as long as your brain and heart are still working. But you'll have no sense of self anymore, no memory, no...anything. There's no chance at all of recovery. You'll just — exist. As an empty shell. And your soul is gone forever...lost.
    • The GBA game of the same game has a rendition of the theme that has been catapulted onto many "most disturbing video game music" lists. Hearing it may help you to understand why.
    • Let's take it a bit further, shall we? Deathly Hallows implies that there's some kind of Afterlife in the Potterverse, which you can only enter if your soul is whole and undamaged. But Lupin states that when Dementors Kiss you they suck away your soul and presumably eat it or destroy it somehow, and then your soul is "gone forever... lost". Which begs the question: how can you enter the Afterlife if your soul's gone? You probably can't, and once you finally die you'll simply stop existing altogether. Not only that: the Ministry of Magic can sentence someone to the Dementor's Kiss; which means that not only they basically kill you in this life, they also take away your right to enter the eternal Afterlife. Talk about harsh punishments.
    • The books' description of the Dementors is horrifying enough but the third movie took it to a darker places when the viewers are treated to a close-up of one of their faces as it attacks Harry during a Quidditch game.
    • And Pottermore makes it even worse with the revelation that the Dementors are not the scariest thing in Azkaban, and refusing to elaborate beyond stating that the Ministry was too scared of the fortress itself to destroy it and the building. I repeat: unkillable soul-stealing wraiths are not the scariest thing in Azkaban, and even that pales to the building itself.
    • Want even more? In the book, Lupin states that if you suffer a Dementor's presence long enough, you may eventually become one... Sure, he may have been speaking metaphorically, but heavens.
    • It's established in later books that dementors breed where there's lots of despair; maybe they do it by turning people into more of them? Oh, and lest we forget, not only do they breed, but they're immortal and, by all appearances, unkillable.
    • Dementors can clearly be bargained with, based on their detailed agreements with the Ministry, and yet we never hear one speak onscreen. Their means of communication, therefore, goes straight into Nothing Is Scarier.
    • Every other creature in Harry Potter has some reference point in mythology (hippogriffs, trolls, etc). The Dementors are one of JK Rowling's few original creations. Imagine you haven't read the books and you see that cloak and the creepy hand on the train for the first time. You'll quickly realize you're in whole new territory, going "What the hell is THAT thing?!!"
  • Wormtail betrayed Harry's parents to Voldemort, even though they were his best friends. When his remaining friend, Sirius, chased him down after James and Lily's deaths, Wormtail cast a spell to cause an explosion that killed a dozen innocent people. Pinning his mass murder on Sirius and ensuring Sirius's twelve-year psychological torture in prison, Wormtail escaped. Worse, he escaped by turning into a rat and got himself adopted as the Weasleys' pet. For twelve years, the Weasley family unwittingly shared their home with a murderer who would bring about the resurrection of Voldemort. The fact he was in Harry's dormitory for three years. If he had heard of Voldemort recovering...
  • Remus' transformation, in both the book and the movie. It's both the way it's clear that becoming a werewolf is painful, and that he's trying to not become a monster, as his sanity goes and his pained whimpers slowly change to growls as the wolf takes over and... yeah.
    • Also the fact that - unlike the protagonists of most werewolf movies, who get bitten as adults - he's been going through that same anguish since he was a little kid.
  • While you find out at the end of the movie that he's a good guy, seeing the Grim before you know it is pretty scary. Especially when he turns up in the beginning, when Harry's alone and he looks ready to attack.
  • Before The Reveal, the idea of Sirius Black himself was pretty terrifying. Voldemort's most faithful servant unhinged by his death, blowing his once friend to bits along with a dozen innocents and laughing mad when the Aurors arrived, escaping from prison solely for revenge on Harry? All the fear of a mass murderer who's out to get you, with added magical powers that the wizards themselves couldn't figure out. Not only was he seemingly unaffected by the dementors, not only did he escape from Azkaban, but he broke into Hogwarts in a way Voldemort had not (at the time) managed to. Twice. Ron's first meeting with Black is especially chilling. He hears the curtains rip, wakes up and sees Black, who is said to resemble a vampire or corpse with an extremely gaunt face, waxy skin and dead eyes, standing over him with a huge knife. And when Ron screams Black is able to instantly slip away and avoid detection.
  • The trance Trelawney goes into, when she speaks her second real prophecy, is rather creepily described. In the film the scene begins in a Jump Scare.
  • When Ron was taken away by the Grim. Sure, Sirius Black was a good guy, so they weren't in any real danger, but try thinking about it from the kids point of view: they're out after hours without permission; so no one (except Hagrid, but he's not exactly in good shape at the moment) knows where they are, and they get attacked by what is essentially the wizarding world's form of the Grim Reaper. The Grim jumps on Harry and knocks him down. He tries to bite Harry, but bites Ron's arm instead; cause he pushed Harry out of the way. Then the Grim runs off with Ron despite their best attempts to stop it and disappears into the Whomping Willow, which leads to the Shrieking Shack.
    • From Harry's point of view, he seems to be the dog's target. Then Ron pushes him out of the way and it bit his arm instead. Even though he's hitting with all he's got, the Dog's dragging his best friend away into unknown territory, preparing to do who-knows-what with him (main thoughts were probably visualising how badly Ron was mauled, or the dog killing Ron). And he can't run to get the teachers cause it might have killed Ron by the time they found him. Kid was probably terrified for his friend, feeling guilty for being unable to stop it, as well as out of control of the situation (helplessness).
    • From Ron's point of view, it's worse. Basically, no matter how hard he struggles or hits the dog, it's dragging him towards unknown territory. He hooks his foot in a tree root stop it and gets his leg broken for his troubles. Then he finds out that the Grim (which he believes is the Grim Reaper), turns out to actually be Sirius Black's animagus form, and he has Ron's wand. Keep in mind that Ron currently believes him to be an axe crazy serial killer who's probably ready to kill him at any moment, since he needs a wand in order to perform magic. This means he no longer has any defence since his wand was taken, and that he probably knows that he was bait to lure his best friend to his death. Basically, Ron's in an unknown location alone with a serial killer who could kill him at any moment with zero defence and enough pain from his broken leg and bitten arm to potentially pass out, and the only reason he's still alive is to act as bait. Sure, he turned out to be a good guy, but they obviously don't know yet. The poor guy was probably terrified out of what little mind he had left which wasn't trying to cope with a broken leg. Pretty terrifying, huh?
    • And don't forget: this is the second time in two consecutive books that Ron's been dragged away in the clutches of an apparent monster, and the last time it happened, his abductors really did try to eat him.
    • The film adds extra creep factor to the scene in the Shrieking Shack: accentuated by the music, Ron points to someone standing in the corner, the true form of the Grim, hiding behind the door that Harry and Hermione just walked through. In the dust covering the floor are pawprints, which the camera follows until it finds a man's shoes, then pans up to reveal that the hellhound that's been following Harry throughout the story is actually the mad murderer who they're all convinced is about to kill them: Sirius Black. The pawprints leading to a human being are downright unsettling—they create an unnatural sight even by the standards of a wizarding school.
    Harry: The dog. Where is it?
    Ron: Harry, it's a trap! He's the dog! He's an Animagus!
  • So, back to those Dementors... The climactic scene where Sirius and Harry are at the lake under attack, there are swarms of them. The book makes it scary enough, but actually seeing it in the film, the part where one attacks Harry and you can see part of his soul getting sucked out, like some sort of double-consciousness, is horrifying, along with the screams of agony. Then two and three more attack, all sucking. And just what was that small bright light leaving Sirius's mouth? If the Dementors had swallowed it, his soul might have been lost forever.
  • The lengths Draco Malfoy went to get revenge for when he didn't pay attention in Hagrid's class and got injured as a result was to have his father convince a committee to kill Hagrid's pet. Imagine if some arsehole got bit by your pet after refusing to listen to directions on how to handle it and he pulled strings to have it put down.
  • Deserved it might have been, but seeing Lupin and Sirius Black ready to kill someone is pretty chilling, only mitigated a bit by Harry stopping them. The Ministry of Magic song "Marauder's Map" encapsulates the sentiment perfectly showing just what lengths they almost went to.
    • Made even worse? Lupin is a werewolf and Sirius has been in Azkaban for twelve years and is a very large black dog Animagus. They both, given the forms they transform into, must know many gruesome ways of killing. Now... imagine if Harry hadn't interfered?