This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / The Princess 99
The Princess 99 can easily be summed up as "What if Tim Burton and H.P. Lovecraft ran Hogwarts?". Most of the familiars are nightmare inducing, especially if you are afraid of rats. Don't believe me? Let's begin with Brig's familiar, which is described as a rat with a giant eye, a mutated nozzle-like mouth, and nothing else on it's head.
Uni and Dual, two demons who do not have eyes.
You can let Eulalie talk about her lovely paintings:
Eulalie: In the quadrupled helixes of its being there are cradled mysteries no man should know lest he go mad and that coiled beast is called The Dark, for to speak Its name would bring upon shadows of insanity. And the Doom Clocks keep man bound only to the surface but one will speak Its name and It shall climb and swallow all that come forth. And It will open one of Its multiplying mouths and out of It shall come the shriek that will end the world.