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 just unlucky passersby, 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 process of making someone's taint "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, who screams "You're killing me" during it and once it's over her only reaction to being asked how it went is to smile in a completely uncharacteristic fashion 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.
Worse, one of the forum admins, in speaking about an event a couple pages later mainly Snadhya getting stabbed clarified that unlike those whose seeds are slowly taking them over (i.e. Naal, Sabbror, Rann'dirk) whose pain response is diminished, "merged" tainted have a normal pain response. So Kalki felt every bit of that, which either makes it more satisfying, more horrific, or both.
Snadhya snaps. It also seems to be the case in-universe, judging by Riz'riia's stunned expression when found by Ariel and co.
When Snadhya finally grows weary of Kalki's antics and kills her with a summon, she does so with absolutely no emotion whatsoever. And judging by the blood stains on the door and sounds, it's not a pretty or quick death. Even worse it looks like one of the summons ripped out her throat before she could even finish her last words!
Learning from one webcomic forum troper that drow with a "merged" taint have normal pain responses, we now know that Kalki having her arm stolen by Ariel was pretty painful. Now imagine how Kalki must have felt as she was brutally torn apart by Snadhya's summons.
Years of warfare have taken their toll on the Sarghress. In chapter 47 a squad of Sarghress troops butcher what is apparently a Drow commoner child from an occupied district for their own and their wolves' consumption.
Yet although every demon before now has claimed a host at the first opportunity, these completely ignore thousands of viable victims, only spreading outward as fast as possible. Pretty fortunate, right? But just when you start to breath easy, it dawns on you; those demons aren't benign, they're fleeing in terror from whatever's emerging from that gate.
And then Karla, the most unstable, unpredictable, and bat-shit insane Drow in the entire setting eats it and absorbs it's power. Don't waste time fearing the unknown; there is something much worse already here.
Then it turns out even she can't control that beast. Karla is its puppet.
Kharla was always intimidating and creepy, but she was at least benign, being kept in check by her own naivity, passivity, and insanity. In chapter 47, she finally shows what she is truly capable of, and you realize that she was only withholding an apocalypse of her own making on a whim, with her millions of victims becoming a prisoner within their own bodies. Nothing will protect you. Nowhere is safe.
Thus far, we only see the effects on Chel. But how much farther is she reaching? Her Aura could be covering the entire underworld, maybe even the surface, maybe much, much further...
Imagine what it would be like if all your remaining friends and family suddenly went into a frenzy trying to kill you. If by some miracle you manage to escape, a quick glance around reveals that it isn't just your family; the entire city wants you dead. Even your adorable toddler, who will try her damnedest to bite your fingers off.
After the events of Chapter 47 prove disastrous for the Beldrobbaen, Waes'soloth orders all the servants be thrown out and the clan fortress sealed. Then it emerges that at least one devess has been infected with the parasitic Jal'darya flower, and probably at this point every other person in the clan has been exposed to the spores. Waes may very well have finally signed her clan's death warrant.
The very last page of Chapter 48. Remember that vial Shinae was carrying and passed off to a servant after giving Zala some tea? Oh, and don't think Suna's safe either. Suna bit Zala during the demon apocalypse, after Zala drank that tea... It's hard to see, but Zala's got a little something on her head.
The author gives a vivid discription of what to expect if you get on her bad side;
Kern: She cause blood to "boil", veins to ruptures, arteries to break. squishy tissues like eyes , gums, ears cannals, they all bleed first. Then come internal bleeding. Then tissues break. Then internal organs fails under the pressure. Stroke would kill, or the heart would cease completely. Likely Kousei suffered a stroke himself, but survived. His body may never fully recover. So while it's very directed one on one kind of power, it's very deadly AND painful.
Apart from the shear agony and fear that come with her attacks, she can utilize her power in complete silence, from any direction, and from almost a mile away, only requiring a line of sight. There is absolutely no way to defend yourself in public; you will die in blinding pain, without ever knowing where she was.
It's more subtle than most instances on this page, but Zala'ess' cognitive decline after being infected by the flower poison is eerily similar to seeing a loved one with Alzheimer's or other forms of dementia.
We even get to see her dementia from her own perspective in chapter 49. With Sabbor having lost control of his seed and gone missing, that monstrous hallucination could have been all too real.
The forums universally agree that seeing Khaless in action certainly qualifies. She shapeshifts into her true form, terrifying the Sul healer sent to treat Quain'tana and basically devours/assimilates her whole, regretting the fact that the Sul is "struggling" because she will forever remember the pain and fear the Sul healer experiences during the experience.