The Great God Pan

""Great God, what simpletons! Show them Arthur Machen's Great God Pan and they'll think it a common Dunwich scandal!""

- from "The Dunwich Horror" by HP Lovecraft

Written by Arthur Machen in 1894 and originally published in a magazine, The Great God Pan is known for being one of the prototypes of the Cosmic Horror genre. It was a huge influence on HP Lovecraft, who used it as the basis for his own story The Dunwich Horror, as well as for the deity Shub-Niggurath. It is worth noting that the main themes of the story - the idea that there are Things Man Was Not Meant to Know and horrors outside of our reality that we do not understand - are very Lovecraftian in nature, making these tropes older than you think.

The Great God Pan is also considered by Stephen King to be "one of the best horror stories ever written. Maybe the best in the English language." He has stated that 2008 novella N. was a "riff" on it.

In Wales a scientist, Dr. Raymond, experiments on a woman named Mary to enable her to "see Pan". Sadly, her mind is broken and Clarke, who watched the experiment, gives up occultism. Cut to several years later in London, where another man named Villiers meets an old friend of his who was led to misery by his wife Helen Vaughan. Curious, Villiers begins investigating. Meanwhile, an alarming number of wealthy, prominent men are being driven to madness and suicide following their encounters with a mysterious woman known as Mrs. Beaumont. Though Clarke is initially hesitant to give him the information he needs, Villers soon learns that Mrs. Beaumont is indeed Helen Vaughan, the daughter of Mary and the pagan nature deity Pan.

You can read it here.

Provides examples of:
""Yes, I married, Villiers. I met a girl, a girl of the most wonderful and most strange beauty, at the house of some people whom I knew. . . My friends had come to know her at Florence; she told them she was an orphan, the child of an English father and an Italian mother, and she charmed them as she charmed me. The first time I saw her was at an evening party. I was standing by the door talking to a friend, when suddenly above the hum and babble of conversation I heard a voice which seemed to thrill to my heart. She was singing an Italian song. I was introduced to her that evening, and in three months I married Helen. Villiers, that woman, if I can call her woman, corrupted my soul.""
 * Bitch in Sheep's Clothing: Helen (when she doesn't have the Uncanny Valley effect going on) often appears to be a beautiful and charming woman.

""It was an ill work I did that night when you were present; I broke open the door of the house of life, without knowing or caring what might pass forth or enter in. I recollect your telling me at the time, sharply enough, and rightly enough too, in one sense, that I had ruined the reason of a human being by a foolish experiment, based on an absurd theory. What I said Mary would see, she saw, but I forgot that no human eyes could look on such a vision with impunity. And I forgot, as I have just said, that when the house of life is thus thrown open, there may enter in that for which we have no name, and human flesh may become the veil of a horror one dare not express. I played with energies which I did not understand, and you have seen the ending of it.""
 * Break the Cutie: It was what got everyone into that mess.
 * Campbell Country: Takes place in Britain. Was way before Ramsey Campbell's works.
 * Cosmic Horror Story: As mentioned above, Lovecraft cited this story as one of his main influences.
 * Death By Sex: The unspeakable horrors that drive Helen's victims to suicide are strongly implied to be sexual in nature, although Victorian propriety prevented Machen from elaborating.
 * Did You Just Romance Cthulhu: More like "Were you just raped by Cthulhu" and "Did you just romance Wilbur Whateley."
 * Don't Go in The Woods: "Ah, mother, mother, why did you let me go in the forest with Helen?"
 * "The man in the wood! father! father!"
 * Driven to Suicide: Both Helen's victims and.
 * Eldritch Abomination: This version of Pan is definitely very different from the satyr god of Greek Mythology. In addition, it is strongly implied Pan is also the Celtic god Nodens, who would later join Lovecraft's works as one of his eldritch deities...
 * Gone Mad From the Revelation
 * Half Human Hybrid:
 * Have a Gay Old Time: Take a shot every time "queer" is used instead of "weird" or "strange". Also: "gay curtains".
 * Humanoid Abomination: Helen was probably one of the first in modern literature.
 * Mad Scientist: Dr. Raymond has a lot of this going on.
 * Mystery Magnet
 * Nature Spirit: A very, very dark version.
 * Noodle Incident: Quite possibly the most skillful and frightening use of this trope ever.
 * Occult Detective: Villiers would qualify if he knew what he was investigating was supernatural.
 * Platonic Cave: Raymond believes in this, and it is what drove him to perform the experiment.
 * Shaggy Search Technique: Pretty much every single clue in the mystery is found this way. This was actually a staple in pretty much all of Machen's early works.
 * Supernatural Fiction
 * Take Our Word for It: There's a lot of unspeakably awful stuff that's hinted at or only described off-screen.
 * Things Man Was Not Meant to Know

""Everyone who saw her at the police court said she was at once the most beautiful woman and the most repulsive they had ever set eyes on. I have spoken to a man who saw her, and I assure you he positively shuddered as he tried to describe the woman, but he couldn't tell why.""
 * Uncanny Valley: Invoked: several characters describe Helen as beautiful but . . . wrong.

""We know what happened to those who chanced to meet the Great God Pan, and those who are wise know that all symbols are symbols of something, not of nothing. It was, indeed, an exquisite symbol beneath which men long ago veiled their knowledge of the most awful, most secret forces which lie at the heart of all things; forces before which the souls of men must wither and die and blacken, as their bodies blacken under the electric current. Such forces cannot be named, cannot be spoken, cannot be imagined except under a veil and a symbol, a symbol to the most of us appearing a quaint, poetic fancy, to some a foolish tale. .""
 * The Vamp: Helen.
 * You Cannot Grasp the True Form: Pan is described this way.