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.
Perhaps even worse than what he does is the language used to describe him. Pterry's writing can be uplifting, heartwarming, and beautiful, but when he wanted to he could leave you feeling like you needed a long bath for your soul. The Cunning Man is one of the finest and most acute examples of this. He doesn't make your skin crawl, he makes it turn and run.
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.
The book manages to turn the Morality Pet trope into one of these. Every inmate in the Tanty is given a songbird as a pet, and men who have committed unspeakable crimes often care for their birds gently and lovingly. Truly heartwarming... until the warden of the Tanty points out a piece of Fridge Horror: if seemingly evil people are capable of kindness under the right circumstances, what are seemingly kind people capable of under the wrong ones?