These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Nightmare Fuel and Eldritch Abomination together again! Horrors are a GM's wet dream, really: I can torment your character for no reason, for as long as I want, and the only way to make it stop is to kill an insanely powerful creature from another dimension. Each Horror is different, each with a nastier assortment of powers than the last.
One of the fiction books, Lost Kaer, would make a great horror movie: a Horror has inflitrated the kaer of the title and is enslaving the minds of the inhabitants one by one.
The fate of Wyrmwood/Bloodwood and its inhabitants probably also qualifies as Nightmare Fuel. To elaborate: the Elven Queen refused to accept Thera's bargain or magic, choosing instead to build a kaer out of the forest itself. (First result: every other elven nation in the world cut ties with Wyrmwood and made their own deals with Thera.) Once the Scourge started, the kaer began to break down under Horror assault. Just before it was about to collapse, the elves learned that Horrors can't get any use out of those who are mad or in great pain... so they performed a ritual that made thorns burst out of their bodies. Most died. The survivors were both mad and in great pain, driving the Horrors off at a hideous cost.