‘The most merciful thing in the world, I think, is the inability of the human mind to correlate all its contents. We live on a placid island of ignorance in the midst of black seas of infinity, and it was not meant that we should voyage far. The sciences, each straining in its own direction, have hitherto harmed us little; but some day the piecing together of dissociated knowledge will open up such terrifying vistas of reality, and of our frightful position therein, that we shall either go mad from the revelation or flee from the deadly light into the peace and safety of a new dark age.’

–H. P. Lovecraft (The Call of Cthulhu)

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The screams, that fourteenth night, continued until dawn. They were totally unlike any sounds in my experience. Impossible to believe they could be uttered and sustained by a human, yet they did not seem to be animal. I listened, there in the gloom, my hands balled into fists, and knew, suddenly, that one of two things must be true. Either someone or something was making these ghastly sounds, and Brother Christophorus was lying, or–I was going mad. Hearing-voices mad, climbing-walls and frothing mad. I’d have to find the answer: that I knew. And by myself.
–Charles Beaumont (THE HOWLING MAN)

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Dim, dubious, bat-like creatures seemed to be flitting to and fro between one of the stone vats and the group that toiled like sculptors, clothing the bony foot with a reddish plasm which they applied and moulded like so much clay. Gaspard thought, but was not certain later, that this plasm, which gleamed as if with mingled blood and fire, was being brought from the rosy-litten vat in vessels borne by the claws of the shadowy flying creatures. None of them, however, approached the other vat, whose wannish light was momently enfeebled, as if it were dying down. –Clark Ashton Smith (The Colossus of Ylourgne)

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A Greek god horror story to kill all HOPE. Really dark stuff.

‘The old man continued on his way to the sea, coming after a time upon two men who were digging a grave for a third who lay dead.

“It is a holy office to bury the dead,” he remarked.

“Aye,” said one of the men, “especially if you have slain him yourself and are hiding the evidence.”
–ROGER ZELAZNY (But Not The Herald)

From the listless repose of the place, and the peculiar character of its inhabitants, who are descendants from the original Dutch settlers, this
sequestered glen has long been known by the name of SLEEPY HOLLOW, and its rustic lads are called the Sleepy Hollow Boys throughout all the neighboring country. A drowsy, dreamy influence seems to hang over the land, and to pervade the very atmosphere.
–Washington Irving (The Legend of Sleepy Hollow)