This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / The Darkglass Mountain Trilogy
The murder of that kid in the first book, Ba'al'uz himself and how the pyramid works.
The demon Skraelings...just...the demon Skraelings...
When Salome is revealed to be an Icarii. It starts with her being raped by all of the castle guards, then raped by any citizen who wants to rape her, as well as being raped by a boar, having chamber pots emptied all over her, then being forced to watch as her son is raped by guards and then having his genitals and penis cut off and thrown onto her body, followed by watching him be murdered as she's left there to die.
The rape of Summerstar and the birth of the Lealfast race.
Hell, the first chapter of the book, with Ishbel's family dying of the plague. And then their corpses speaking to her.