The scariest things are the ones you can't see -- the horrors that lurk just out of sight, only glimpsed out of the corner of your eye. 
 One such horror lies at the heart of "The Great God Pan," where dabbling in things beyond "the veil" leads to unspeakable horrors, madness, degradation and death. Sort of like getting involved in national politics. Arthur Machen's prose tends to be rather dense and impenetrable, with rambling pseudo-poetic dialogue, but the web of creepy, unspeakable horrors ends up being utterly chilling because of everything you don't see. 
 The weird Dr. Raymond has a rather bizarre goal, namely to "lift the veil" of illusionary reality and see what exists beyond it: "You may think this all strange nonsense; it may be strange, but it is true, and the ancients knew what lifting the veil means. They called it seeing the god Pan." He plans to accomplish this via an experimental surgery on the brain of a young woman, but the surgery merely turns the girl insane and catatonic. 
 Strange things happen in the years that follow. A young girl with a sinister presence who terrifies a boy and leads to the monstrous rape of a young girl. You can probably guess what she is and where she came from, but it takes a little while longer for the characters. 
 And some time later, a man named Villiers encounters an old friend, Herbert, who is now a filthy, half-crazed homeless man. Herbert relates how he married "a girl of the most wonderful and most strange beauty," and how that woman somehow "corrupted my soul," bringing him face-to-face with horrors he can't even explain. A few days later, Herbert is dead. Villiers consults with Clarke -- who saw the original experiment on the orphan girl -- and the two realize that something inhuman and horrifying walks among them. Something from the great god Pan. 
 Let's be brutally honest -- Arthur Machen was no Lord Dunsany, H.P. Lovecraft or Edgar Allan Poe. His writing style is rather dense and slow-moving, with dialogue that often tangles itself up in its own poesy (" It was as if I were inhaling at every breath some deadly fume, which seemed to penetrate to every nerve and bone and sinew of my body"). The characters tend to launch into page-long monologues, never broken by others characters or anybody reacting to what is said, leaving readers to struggle through giant blocks of text. 
 So it's a credit to "The Great God Pan" that it still succeeds in being bone-chillingly terrifying, from the misogynstic brain surgery to the final grotesque confrontation with the supernatural ("The blackened face, the hideous form upon the bed, changing and melting before your eyes..."). Machen conjures up the sense that endless, unspeakable horrors lurk just out of the sight of our eyes and minds, and even the seemingly ordinary -- Helen Vaughn -- may have a terrifying monster behind a seemingly ordinary, beautiful face. 
 What's more, he doesn't give the sensation that this is an isolated incident. Even if the protagonists manage to foil the monster, only a thin veil protects us from whatever is lurking underneath us. There is evil, corruption and madness there, and the depthless horrors of an endless pit -- it's like standing on a piece of frosted glass that BARELY keeps you from seeing a dank, slimy canyon under your feet. 
 Most of the characters are fairly ordinary people -- middle-aged, white upper-class Englishmen who are ill-suited to deal with the freakier side of the world. Some are utterly despicable (Raymond, who coldly raises an orphan so he can experiment on her brain) while others are fairly likable (Villiers, who stumbles into the freaky stuff entirely by accident). And Helen is a murky, almost ghostly presence, but her malevolent, corrosive personality seems to seep into even secondhand accounts of her. 
 Arthur Machen's writing could be dense and awkward, but he knew the best way to scare people silly -- "The Great God Pan" is full of terrors and horrors just out of sight, which are only made plain at the story's end. It takes some patience, but the creep factor is off the charts.