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Nightmare Fuel: Goblins
Kore's massacre of the ogres and orcs inside the tavern, which ultimately results in him killing a crying child.
The various horrific mutilations inflicted by Dellyn on his victims.
Samples include Yala, a female kobold whose legs and tongue have been cut out; a kobold whose upper snout was hacked away; a hobgoblin with half of his face nothing but raw, bloody meat...
The drow/orc/whatever woman's injuries trump all of those by wide margin. All the skin and flesh of her back, peeled away... Have a link, if you dare.
ALL of her back. Neck and head included.
When Kore discovers the aftermath of the battle at the Warcamp, Taps-with-a-Stick and Asks Nonsense return just as he is about to leave, the resulting chase from Taps point of view is possibly one of the most horrifying things in ANY comic.
The effects of the random shield during the major fight. Highlights include: a soldier being slowly transformed into an ogre and then stabbed to death by his terrified sergeant; said sergeant's armour being made bigger, more impressive, and most of all heavier, to the point that it eventually crushes him to death inside (look out for his eyeball popping out); a man slowly and, from what he says, agonizingly turning into a mass of snakes. This is all punctuated by cries of "What's happening to me?!" Good luck sleeping.
That a Soulspike Devourer shows up at all is an in-universe example — it's a 4E monster thrust into an explicitly 3.5E world. Imagine trying to fight a monster based on rules just different enough from the known rules of your universe that it cannot be classified. It's undead, but healing magic heals it, its attack, damage, hit points, and defenses are on a different scale, who knows how it would deal with spells with saves it doesn't have, it might attack defenses you don't have, if it gives a save ends effect you don't have the proper mechanism to recover... Which is why it can effortlessly just grab any character it sees and rip out its soul. Due to the difference in editions, there's probably no saving throw that can be made, so all it needs to do is get it's hands on you, and you're lunch.
Even Thunt is spooked by this one. As well he should be: if what he told his forum is true, he designed it from a recurring childhood nightmare that starred the creature. A monster that causes slow, horrible agony and dismemberment from a single touch? That's a monster you have to run from, not fight.
If its stats are to be believed, it only takes three touches before the poor victim's entire body melts away. Which means that K'Sellisis already dead.
Psi-Max isn't as evil as Dellyn and Kore, but he's still just as horrifying. To reiterate: He's killed his universe's Kin and Forgath well over 800 times and made sure to break the counter to A) make sure they remember every second of their death, and B) to memorize the Maze in its entirety. That's the scary thing: Psi-Max isn't in the maze to get the rare treasure every other Universe is looking for. No, his entire POINT is to memorize every nook, cranny, and hidden part of the Maze Of Many for one simple reason: To hunt down and eliminate every single incarnation of him, Kin and Forgath. He's become an Omnicidal Maniac in a little condensed universe who can blow up heads with a flick of his wrist, and his entire point is to kill everyone, forever. And the clinical detachment with which he does everything — including torturing Minmax and Kin just to see if he can learn anything from the other's reaction — brrr.
Dies-Horribly does. And, after reviving, the portion of the orb that saved Dies' life takes over... and goes into Clingy Jealous Girl-mode here.
Dies' artificial arm gets very... creative when killing people. It seems to be like a switch; when Dies is in mortal danger, the switch is flipped, and he starts killing every perceived threat within reach, even once they start running in pants-wetting terror.
One room in the Maze of Many has upside-down trees that grow whenever damaged. Kin gets trapped under one as it presses and holds her down, causing PTSD flashbacks to what Dellyn used to do to her. Even without showing anything graphic or sexual, it's the raw, unfiltered stuff that nightmares are made of.
This alternate reality.Dellyn isn't killed during the tavern brawl with Minmax, and hunts down Minmax, Forgath, and Kin, and kills Forgath and Kin. Shortly thereafter, Minmax finds a Crystal Ball which shows him the torture Dellyn put Kin through while she was his slave. Minmax captures Dellyn, and proceeds to do to him every single thing that he did to Kin and Forgath. This is told from the perspective of Dellyn. This is probably the most graphic, explicit violence that Goblins has ever done, and it still swerves into Heartwarming with the last two sentences. This is made even worse by the fact that it's the normally light-hearted Minmax who is inflicting this on Dellyn.
Alternately, the last two sentences mean that Minmax was planning on using the Jade Teapot to take him into whatever afterlife Dellyn ends up in and spend all of eternity torturing Dellyn there as well, which is even more horrifying.
Psion!Minmax's liquification power, which he first demonstrates by brutally killing his counterparts from his reality.
The mere fact that all Dellyns across the multiverse are depraved monsters is chilling. The Dellyn of Sapphire!Kin's world may be even worse then our Dellyn was.