This is based on opinion. Please don't list it on a work's trope example list.
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, and painfully to boot. 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.
To drive home just how bad it is, Odiana reacts worse to the threat of having to wear one than she does to being raped.
High Lady Antillus Dorotea is collared by Sarl, who orders her to heal and do no harm. Sarl is then killed in battle, meaning the collar's impossible to remove. Then the Canim start liberating slaves, who are not very kindly disposed towards nobles and would be particularly hostile towards a close relative of High Lord Kalarus, who's notorious for his monopoly on slavery. Tavi, learning about this after the fact, shudders to think what must have been done to Dorotea (or what she was ordered to do) by angry and vengeful former slaves, who would have no mercy just because she was collared and couldn't refuse or defend herself.
When the Vord Queen gets her hands on the making and use of discipline collars, she uses Brencis Kalarus to brainwash and enslave stronger furycrafters into much the same mindset as his father's mindless 'Immortals', only now serving her. Amara witnesses part of one conversion process, which is creepy and disturbing enough on its own, and later comes face to face with one of her collared fellow Cursors in the heat of battle - who promptly and calmly tries to kill her.
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 send parasites to take over your body that, unlike the Yeerks of Animorphscompletely 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 GambittingMagnificent Bastard Gaius 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.
The aforementioned breeding power allows them to make warriors that can furycraft, meaning that you have magical alien locusts on top of everything else. And they also have a biological imperative to kill everything that isn't them.
The takers. They numb you up with poison and crawl inside you, where they obliterate your personality but keep your memory and skills intact. They also give the victim Super Strength and suppress pain, resulting in scenes of taken children and old people literally ripping people apart and disregarding any sort of injury that doesn't immediately kill them.
The vord are basically the Zerg from _Starcraft_. Even with bleeding-edge technology like power-armor and advanced weaponry, as well as centuries of experience fighting against these things, they're still a major threat; imagine what trying to fight them with normal metal plate armor, swords, and spears would be like. Even the power of the elements is often not enough, as these buggers are smart.
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.