The Great God Pan is a novella by the Welsh writer Arthur Machen. Its first chapter appeared in the newspaper The Whirlwind in 1890, and the full work was published in 1894 alongside “The Inmost Light.” Dr. Raymond performs minor brain surgery on his ward Mary, about seventeen, to open her mind to the spiritual world — what the ancients called “seeing the great god Pan.” She wakes awed and terrified, and soon her mind is gone.
The story then pieces together fragmentary accounts from several witnesses, following the madness and death that gather around a woman named Helen Vaughan. At the end it emerges that Helen is the child Mary bore after the experiment. The remains of a pillar in a museum near the village where Helen grew up carry an inscription to Nodens, “god of the Great Deep.” Machen recalled trying to pass on the awe he had felt as a boy among the Roman remains at Caerleon and Caerwent.
On publication it was denounced as degenerate for its implied sexual content, but after re-evaluation in the 1920s it became a classic of horror. Lovecraft praised it in “Supernatural Horror in Literature.” The witness-assembled structure of “The Call of Cthulhu” comes from here, and the idea of a monstrous child of human and god carried into “The Dunwich Horror,” which names Machen’s novella outright.