Arthur Machen's 'The Great God Pan' is a seminal work of supernatural horror, deeply influential on authors like H.P. Lovecraft. The novella explores themes of ancient, cosmic evil intersecting with human experience, focusing on Dr. Raymond's experimental surgery that opens a young woman's mind to perceive the horrifying reality of the titular deity. This vision drives her to madness, setting in motion a chain of bizarre and terrifying events involving pagan rituals, monstrous transformation, and a profound sense of the sublime and the terrible. Machen masterfully weaves a tale of encroaching madness and the dreadful allure of forbidden knowledge, making it a chilling exploration of what lies beyond the veil of ordinary perception.