Please don't list this on a work's page as a trope. Examples can go on the work's YMMV tab.
Nightmare Fuel: No. 6
The birth of the wasp, with the person rapidly aging and ending up as a shrivelled corpse.
So far it's happened to three people, one who died off screen. Shion's co-worker is bad enough, he collapses back and looks at his rapidly ageing hand in horror. We then get a blurred POV shot from him looking at Shion, and he shakily asks 'what's happening to me?' The second victim is far more horrific. She first notices the black spot on her face in a shop window, meaning that she's able to witness exactly what's happening to her as she rapidly ages and dies.
Also happens in a freaky scene to a woman at the end of chapter 10.
During the Holy Day celebrations the bees go one step further and...well, kill about a fourth of the No.6 inhabitants.
The pile of corpses in episode 10 and Sion vomiting upon seeing them. Doesn't help that people were actually still alive in that pile, moaning and groaning enough to nearly break Sion.
Small, horrific detail: the knitting needle Sion absently grabs onto while climbing that pile. Even scarier in that it's never overtly alluded to, as it's overridden by the horror of the climb in general...
It's pretty small, but Nezumi's reaction to the possibility of everyone dying in No.6 because of the wasps is to dance with Shion in joy. Nezumi's reaction becomes scarier after Shion says that he wants to make a serum to destroy the wasps, he gets angry and threatens Shion because he doesn't want Shion to help them.
Chapter 13 features Safu waking up in the correctional facility after being kidnapped and finding herself strapped to an operating table. A man tells her that they are going to operate on her and she cries Tears of Fear in response. *shudder*
The fate of the slumdwellers who are dumped into the room with the dead bodies - if any of them survived the fall like Nezumi and Sion, they are trapped down there with no food or water, surrounded by corpses. Logically, that would lead to rather desperate circumstances.