"The Great God Pan" is a horror novel written by Arthur Machen. It tells the story of a scientist who performs a mind-altering experiment on a young woman, which gives her the ability to perceive otherworldly entities. However, the experiment has tragic and terrifying consequences, leading to the birth of a being that brings horror and chaos. The novel explores themes of the supernatural combined with elements of science and it's renowned for its unnerving atmosphere and suspenseful narrative.