Harry Potter and the Half-Blood Prince (novel)/Nightmare Fuel
- The entire 'House of Gaunt' scene.
- Inferi (which are more or less zombies), especially when they come out of the water; Harry slashes at them, but they have no blood to shed, and they try to drag Harry down into his grave.
- Especially considering the fact that Voldemort's inferi are the bodies of his victims... hundreds of innocent people with families, floating in a mass grave, forced to do their murderer's bidding...
- And when the Sectumsempra spell does work... OUCH.
- The movie made the Inferi creepier, just by making them succeed in pulling Harry under the water.
- Oh, and then in Hallows, we find out that one of those Inferi is Regulus.
- The potion in the cave. It's freaking Dumbledore sobbing and pleading for Harry to KILL him. And Harry can't do a single thing but force more and more of the potion down his throat. It's a Tear Jerker where your tears are mixed with fear.
- An extra dose of Fridge Horror comes when we realize what was (probably) actually happening - he was reliving the moment when, in a fight with his best friend, either he or said friend accidentally killed his young, mentally disturbed sister, who was only trying to stop the fight.
- Creepy Child Tom Riddle. He made a rabbit HANG ITSELF.
- Dementors breed. It's to be expected, and it's more Squick than scary, but to be informed so bluntly of that fact...
- Well, from how they described it, it's more like they "spawn" from fog, but that arguably makes it worse. Imagine, waking up to find your town covered in a thick, gray fog. Suddenly, you realize you feel cold, colder than you've ever been...
- In Prisoner of Azkaban Lupin mentions that if you are around a Dementor long enough you become "like them". I had assumed that was what the breeding was. They had made people so unhappy and miserable they became Dementors themselves. Which is really just another form of nightmare fuel.
- The fact that Muggles can feel their presence, but can't actually see them. These creatures are wandering the streets at night, preying upon victims that can't even see what their captor is. Although maybe it's better for them that they can't...?