Please don't list this on a work's page as a trope. Examples can go on the work's YMMV tab.
Nightmare Fuel: Codex Alera
The slave discipline collars in Codex Alera. Once put on a person, they can only be taken off by the person who put them on in the first place; otherwise, the wearer will die. They cause the wearer extreme pleasure for following orders, particularly the orders of the one who put the collar on; they also cause extreme agony (that will kill, if it persists long enough) for disobeying said orders. If children are raised wearing one of these collars, they can be shaped into mindless machines so intent on doing the will of their master that they will happily hack off their own limbs in order to better follow his/her commands.
Thank you, Jim Butcher, for feeding my mind-control phobia.
Odiana has her own moment in Furies of Calderon during a battle with three Marat:
On the other side of the clearing, Odiana sat on her horse, humming quietly to herself. The ground in front of her had, it had seemed, quite abruptly transformed into bog. Neither Marat nor herdbane could be seen, but the silt and mud before her stirred vaguely, as though something thrashed unseen beneath its surface.
The water witch noticed him [Fidelias] looking at her and commented, her tone warm, "I love the way the ground smells after a rain."
The Marat nearly eating that girl alive in Furies Of Calderon.
The Vord from the same series are no slouches at this either. Imagine giant insects that are collectively smarter than 99% of humans, live only to consume everything in their path, can sent parasites to take over your body that, unlike the Yeerks of Animorphscompletey destroy your original personality beyond all hope of recall, and basically have the Sorting Algorithm of Evil as a superpower. Kill some? Great, but the Vord Queen will just breed new warriors without the earlier models' weaknesses. Fun times. And when the Vord start learning to work the aforementioned discipline collars- well, if we had a trope called something like "Crowning Moment of Nightmare Fuel'', that would be it.
Did we mention the Vord are smart? Come up with a defensive strategy, and they will devise a counter to it that you'll likely never anticipate. The assault on Alera Imperia itself in Princeps' Fury even has them Out GambittingGaius Sextus by packing thousands of crows with body-snatching takers, innocuously flying them over the city, and then, at the height of the assault, having them all drop dead into the city at once.
Amara's description of feral furies attacking a city:
"I saw an earth fury that looked like a gargant bull knock down a building being used to shelter orphaned children. I saw a pregnant woman burned to black bones by a fire fury. I saw an old woman dragged down into a well by a water fury, her husband holding her wrists the whole way. He went with her." She paused, musing over the placid, inflectionless calm of her own voice, and added, "The second minute was worse."
The Canim ritualists get a couple good (bad?) moments of this too. One of them in First Lord's Fury makes another Cane literally vomit up his own guts.Enjoythatmental image.
In the last book, High Lord Attis's description of what his wife's blade did to him.
"The lower half of my body has been sliced open from groin to ribs. My guts are an unholy mess and will doubtless begin to stink in short order. My heart is laboring too hard because apparently being bisected does terrible things to one's blood pressure. The injuries are too severe and extensive to be healed.
"I can't eat anything. Without all the proper tubes in my belly, the food would simply rot in any case. I can drink a little, which means that I will die of starvation a few weeks from now instead of from thirst a few days from now. Unless, of course, an infection takes me, which seems likely."
The croach. Any living animal sealed into it will shortly start to be digested, appearing to be intact while its internal structure rots apart. Not only do they remain alive for a significant length of this process, Araris Valerian's imprisonment indicates that they also stay awake.
Not necessarily - Isana realizes Araris's metalcrafting may render him immune to whatever mind-numbing toxin is in the croach. This does not detract from the original Nightmare Fuel, however.