The army of demons the Nidraa'chal sic on Sii'lice's army in the prologue right here. Not just because they did that to the innocent citizens who never did them any harm, but because if you look on the far left not only is a heavily pregnant woman possessed but so is the baby in her womb. Which is half-climbing out, presumably to get some blood for itself.
It's a more subtle kind of horror, but what happens to Diva'ratrika while imprisoned in her tower is actually pretty disturbing. Just the idea of being trapped with no way out, with only one person that even knows you're alive while you're slowly losing your mind is bad enough on its own, but it gets worse when you consider what it must have been like for Ragini to slowly feel her aura being taken over, to the point that she buried herself in alcohol to stave off the pain.
Kharla'ggen can turn people into statues, with their souls still trapped◊ inside. She also has a hobby of making real people into dolls. She removes the bones, teeth and put in glass eyes instead of the real ones. And those people are still◊living◊ and aware of their situation and what goes on around them.
Bonus points for the poor girl we actually see being dollified — she's fully conscious. One eye has already been ripped out and replaced with a button; the other is weeping as she whimpers helplessly. One arm is sewn up after having the bones removed. She calls out to Kiel for help...but Kiel has already let the reader know that, terrifying as she finds Kharla's "hobby", she won't do anything to get on Kharla's bad side (in fact, she completely ignores the girl). As Kharla springs forward to grab the new toy Kiel brings, she throws aside her victim mid-mutilation...and the poor girl lands in a pile of her own discarded bones. Imagine living through that, knowing that you would spend the rest of your existence mute, blind and incapacitated, and the last thing you would ever see was your own skeleton and your deranged tormentor. We never learn the victim's fate...but it probably wasn't anything good.
Most likely inspired from Forgotten Realms drow matriarchs, who tend to have such entertaining objects as a gemstone on their throne that violently rips the soul out of anyone who displeases them, then imprisons said souls for all eternity. And have archmages who take captured prisoners, transform them into horrible, horrible monsters, then set them lose on the prisoners own people.
Don't forget the "Corrupted", artificial nagas who were once enemies of the Vel'Vloz'ress or unlucky passerby who fell victim to the giant eyeless snakes bred by the clan which transform them by consuming them only up to the waist and biting into the flesh with their fangs as both the victim and the snake symbiotically fuse into one artificial creature. The venom of the snakes makes the victim's mind go numb and turns them into docile and easy to train minions. Because the venom makes them clumsy, forgettable, and confused they're mainly used as psychological warfare against their former kin. The worse part is that even if a corrupted could get free if someone removed the snake, it would be a painful process as the victim would feel everything the snake feels and very likely die from blood loss. And even then the whole process would most likely be in vain as their brains may not recover from the neurotoxin.
The half-drow hale-spider Driders don't have it well for them either. With the exception of the completely sane and true race Ne'kalsaider, most were driven out by the clans who once revered them and inevitably driven insane due to losing the constant battle between themselves weakened by exile and ill treatment and the aggressive instincts of their spider half. These feral driders (or Streekaiders) are considered a lost cause as there's no way to save their minds in that state (the Orthorbbae Library even mentions that removing the spider half and using modern golem prosthetics will not help, and may even make things worse as the drow is now in full symbiosis with the spider). For the driders still part of drow society, even under the best care they are just barely able to contain their spider instincts as they slowly over time lose the ability to communicate and think clearly. Remember that they were all once normal drow who were magically fused with spider bodies to save them from severe injuries.
Think flowers can't be scary? Ohboytheyare◊. Especially since Word of God confirms that they're all connected, and worse, Wafay believes that that a Nidraa'chal has been developing them.
And to make it worse, the damn thing is covered in spikes, which makes it horrifyingly clear why Shinae was having trouble with the birth and bleeding so badly. Ugh.
Along the same lines in chapter 44, Chrys'tels miscarriage as a result of her taint.
Sarv'swati is terrifying. The previous page made it pretty clear what was going to happen, but the sheer brutality of it is still shocking.
Regardless of how much good it actually does, the effects of Snadhya'rune's "safe" is rather nightmarish as shown by its first victim—er, recipient Jiaan, Yami'ni's former bodyguard. As several people on the forum commented, a character who had been a Perpetual Frowner looks nothing if not lobotomized.
The actual process of making someone "safe" is also pretty disturbing, as it brings out the demon within them until they accept it. And the effect on Shinae, where her only reaction to being asked how it went is to smile in a completely uncharacteristic fashion and is one of the most subtly creepy moments thus far. Even Chrys'tel is unsettled by it.
The faceless golems from the tainting room in chapter 46 gain a close up of ones pregnant body which highlights how creepy it is then we find out the page is a GIF when a single red eye appears in the blank space of the face. Even Sarnel is freaked out.
While it's also awesome and deserved, seeing Arielabsorb Kalki's arm into herself is pretty hard to watch, with the latter's expression making it clear just how painful it is.
What makes it even more Nightmare Fuel is the Fridge Horror that comes from comparing how both lost their arms. At least with Ariel, you know that merely getting her arm cut off with a sword is a lot quicker and more merciful to the victim than having it forcibly ripped off from your shoulder while fused with another person.