The Edge Chronicles/Nightmare Fuel


 * The Deepwoods can kill you in a million ways, each more terrifying than the last. Special mention goes to the rotsucker, which traps you in a cocoon while you're sleeping and digests you alive.
 * In general, everything that happens to Twig in the first book. Everything. The scenes when he is very nearly eaten might be the most upsetting.
 * The death of Amberfuce, boiled alive, may also qualify. The deaths of the goblin leaders as well to some extent... Overall the series is heavy, heavy on this.
 * The Gloamgloazer, more terrifying than any other monster because it's cleverer than you are.
 * The glister, which forces its way into your head and sucks your consciousness out of you, with accompanying illustration by Chris Riddell.
 * The reality-altering Twilight Woods, a forest which gradually drives you insane and will never let you die. Anyone who enters it becomes immortal as long as they remain there, which isn't so peachy if you're so crazy that you can't remember who/where you are. Also, you get to stay alive while your body decays. Sweet dreams, children.
 * If the concepts and writing are scary, Chris Riddell's illustrations add a large exponent to the fear. There is no such thing as Gory Discretion.