- This exchange cements the fact that Croup and Vandemar decidedly aren't human.Croup: If you cut us, do we not bleed?
Vandemar: ... No.
- The duo's habit of eating everything alive. Rats, cats, humans, basically anything with a pulse. At one point, Vandemar finds a sandwich and instead of eating it, uses it as bait to catch pigeons and eat them whole.
- The whole of Richard's Ordeal, but especially the train carriage full of rotting corpses, every single one a suicide.
- The idea of being permanently unnoticed by everyone around you. You can't withdraw money, your fiancee of 18 months doesn't recognise you, your desk at work's been removed, you can't hail taxis, your house gets sold while you're still living in it...
- Whereever Anaesthesia went on Knightsbridge.
- Mr Croup gleefully eating an object he had doted on with apparent love and affection mere seconds before is so horrifying it borders on Nausea Fuel.
- The Big Bad the Angel Islington is extremely unnerving as a being so utterly self-obsessed and self-righteous it's gone well past the point of insanity.
- The BBC radio adaptation had it voiced quite terrifyingly by Benedict Cumberbatch, who manages to switch between his Faux Affably Evil and his Voice of the Legion Villainous Breakdown perfectly.
- The portrayal by Peter Capaldi is just as frightening, especially whenever it shows anger; its face twists back into a near bestial snarl that makes it quite clear it's insane, especially when it screams "They deserved it!
- Where the fuck is Ingress? Croup and Vandemar murdered her mother right in front of her as well as the rest of her family (I doubt she knows Door's alive) and kidnapped her, and stashed her somewhere. And she hasn't gotten the hang of her opening skills because otherwise Islington wouldn't need Door alive That poor kid is probably going to be SO friggin traumatized by the time Door rescues her. If she can even be rescued... Considering what Croup and Vandemar tend to do with loose ends, after Islington says she "isn't going to work" well... *shivers*
- Probably Ingress wasn't killed or left somewhere that would prove fatal, as the Big Bad couldn't be certain that Door would cooperate, or even survive to be used. (Lots of dangers in London Below besides Croup and Vandemar.) If Door died or proved impossible to compel, the villain would have needed Ingress, so she would've been stashed somewhere safe until she grew up enough to do what her sister hadn't.
- The Beast Of London is a gigantic, ancient boar that is permanently enraged and far smarter than its mundane cousins. When you consider how dangerous a normal boar was considered to be, having to face this thing on its own territory is truly horrific - to the extent that it killed Hunter, who made a career out of killing such beasts of other cities.
- There are shepherds in Shepherd's Bush. Hunter tells Richard he should pray he never meets them. In the short story, How the Marquis Got His Coat Back we find out why. The shepherds have the power brainwash people, essentially turning them into drones. The members of the "flock" are used as slave labor, blissfully content as they're worked to death. The Marquis falls under their control for a while and is put to work disassembling the bodies of other flock members who can't work anymore—taking anything useful like hair and tallow fat—and then dumping their bodies in a pit.
Nightmare Fuel / Neverwhere