This is a strange one, the result of a Wikipedia trawl some time ago. Machen, though all but forgotten now, is in many ways the forefather of 20th century horror in the same way Horace Walpole was for the 19th, and The Great God Pan is his Castle of Otranto. It served as direct inspiration for Bram Stoker's novel Dracula, and its preoccupation with strange gods, occult practices, and the corrosive effects of dark knowledge on the human psyche all presage HP Lovecraft.
The story opens with a man named Clarke, who witnesses an operation by Dr Raymknd on a young woman named Mary. The purpose of the procedure is to allow her a glimpse at another world populated by gods. The surgery is apparently successful, for the subject is promptly stru k by overwhelming terror and dissolves into idiocy. From there, the novel bounces between characters and locations, telling a fragmented story of another young woman named Helen, whose otherworldly beauty is ruinous to those around her, and speaks of dark forces.
The plot of the novel is a bit disjointed, and the multicameral nature of its telling makes it difficult to piece the different events together. But I can't deny its influence, and I can only imagine its novelty when it was published back in 1890. The prose is propulsive--slightly purple at times, but I have a weakness for that kind of writing, particularly in that genre and from that era. All in all a good find. I'll be sure to read more from him in the future.