This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / I Shall Wear Midnight
The Cunning Man, especially when he tries to leap out of a copy of his old book.
A man with no eyes, no eyes at all. Only two tunnels in his head...
...But it's more what he does than what he looks like. The Cunning Man makes people turn against witches. He makes people think the old thoughts, the stupid thoughts, the thoughts of a hag stealing babies in the night. The whole Chalk turns against Tiffany, and it's that what makes I Shall Wear Midnight the scariest book of the four.
Amber Petty's miscarriage. She's thirteen.
The Cunning Man Why do they always want to take over her body and soul?. If she had lost, what could've happened to her soul, would she be able to die?
The rough music. What it makes people do to those it plays for, and what it makes people period.