The Altered Earth: A Darker and Edgier Nanoha forum has a lot of really terrifying imagery. From Michael's originstory, to how he was possessed by the Book of Silver Dawn and what it did to Maya, and the tendencies of Ghouls and the various monsters now lording over whole areas of the world, and pretty much anything done by Baron Lou or Nicodemus.
Wendy, a gigantic Shadow made of bulbous flesh and looking like a Lovecraftian abomination, it has twenty arms dragging it along, is grotesquely fat looking and has eight eyes. It resembles the unholy spawn of an angler fish and a jellyfish, and what it does in a fight is also nightmarish. It basically makes even more arms rip their way out of it to pull enemies to pieces. And consider it Was Once a Man, as with all Shadows, and is trapped in its current form. Permanently. And that it used to be a sweet little girl.
Andalia: Garth from the RP gruesomely fused three children together into a Chimera. It involved a lot of fleshy bandages firing everywhere and the thing's screams...
Camp Despair Valley: Given that it's based off a nightmare-inducing series, it's understandable that this game would feature many gory deaths. These are Chiyo, who was impaled by a huge throwing dart (her last words calling out to her mother also make it a tragic death), Yukiko, who was stabbed in the neck (which, by the way, SEVERED HIS SPINAL CORD), Aya being posioned (imagine going to sleep thinking you're going to be ok after a failed attack, but then never waking up), Heather's HORRIFIC execution (force-fed poison and then trampled to death by a Monobear army), Momoko being tortured for information she didn't have and then killed, Harukadeactivating herself only to be brought back and tortured (luckily- or unluckily- she got better), Kyuuto being strangled (the look of pure terror on his face and the fact that the killer is still unknown makes it so much worse), Liam executed for a crime he didn't commit (the fact that he was drowned while tied to a stake didn't make it any less horrifying). and now, Vanille found suffocated in her own cabin (the many cryptic messages she left behind aren't helping). At this rate, these poor kids might never see the world outside these camp gates again.
Magic Academy: It's already full of unsettling imagery, but Leonard's Unstoppable Rage really takes the cake. There's the inference that it happened before, that it resulted in some disturbing effects on an area, and how it usually manifests as a spell called Closed Carnage Forest, which turns every tree in an area into a weapon of either restraining or outright murder.
Morimune High School RP: They previously had some pretty nasty transformations, but nothing was quite as disgusting as...this. A disturbing mix of Eldritch Abomination, Eyes Do Not Belong There, and Body Horror, Jae'yon's true form is nothing short of Nightmare Fuel. And what is worse is its power: it can control the blood in someone, making it bloat in one area of the skin and then rupture and let out a weapon.
The spectres themselves practically run on nightmare fuel. Whether they're corrupting their hosts, full-on possessing them, or just trying to make the heroes' lives worse, the spectres are truly awful.
Their ultimate goal? To corrupt their host enough so they can wipe out the human personality and take the body as their own - all on Xana's orders.
The Monday of Madness. Called as such because absolute hell broke loose on that day:
The idea that a spectre could imitate their host so well that even family members would be fooled, means the spectres are actively learning. Considering what their goals are...
Xana is normally very stoic with a limited range of emotion. The contact his spectres have with humans is slowly contaminating him with human data, which has the side effect of making Xana slightly more human. Normally when Xana gets mad, he's still able to process things logically. But when he loses it, he reallylosesit.