In the personal literary treasury of the Pnictogen Wing, Arthur Machen features almost as importantly as H. P. Lovecraft. He was a decided influence on Lovecraft, who once attempted (during his literary doldrums in New York) to write a kind of Americanized take on Machen's The Red Hand, which became Lovecraft's "The Horror at Red Hook". If you'd like a good précis of those writings of Machen's which touched upon cosmic horrors, you can't do much better than the relevant passage from Lovecraft's Supernatural Horror in Literature.

But Lovecraft wasn't impressed by Machen's tendency to sexualize his horrors, which seems to reflect a squeamish conservative-Catholic repulsion from any manifestation of sexuality that's not wholesome and procreative. In The Great God Pan most especially, but also in The Red Hand and The Three Impostors, Machen alludes to the sexual extremes associated with the cult of Pan—often alluding to them so obliquely that it's a bit difficult for the desensitized modern reader to understand just what's so horrific about (say) a Tiberian spintria (q.v. https://en.wikipedia.org/wiki/Spintria) or a gold figurine named "The Pain of the Goat" (not described at all, except in terms of the horrified reactions of those who behold it.) It's a bit tempting to conclude that Machen was affrighted by nothing much, but I think that would be a mistake. Machen is striving to convey a sort of horror that even in this era of cinematic spectacles and computer animation is difficult to get across: the horror that comes with a dissolution of all boundaries, of all the distinctions that make up what human beings regard as sane.

The Great God Pan may seem, at times, as if the prudish Catholic Machen is merely freaked out by Helen Vaughan having improper sex. But really there's something much worse going on there…something more like Event Horizon than a mere Christian fainting-spell over female sexuality. (Mind you…one must still take that aspect of the tale into account.)

~Chara of Pnictogen

Ah, today is the anniversary of the birth of Welsh weird-fiction writer and sometime dabbler in ceremonial occultism, Arthur Machen (3 Mar 1863 - 15 Dec 1947…good heavens, he outlasted Prof. Charles Williams by a couple years.)

“Look about you, Clarke. You see the mountain, and hill following after hill, as wave on wave, you see the woods and orchard, the fields of ripe corn, and the meadows reaching to the reed-beds by the river. You see me standing here beside you, and hear my voice; but I tell you that all these things—yes, from that star that has just shone out in the sky to the solid ground beneath our feet—I say that all these are but dreams and shadows; the shadows that hide the real world from our eyes. There is a real world, but it is beyond this glamour and this vision, beyond these ‘chases in Arras, dreams in a career,’ beyond them all as beyond a veil. I do not know whether any human being has ever lifted that veil; but I do know, Clarke, that you and I shall see it lifted this very night from before another’s eyes. 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.”

(Arthur Machen, The Great God Pan)