The Great God Pan

The Great God Pan by Arthur Machen - A woman in Wales has her mind destroyed by a scientist's attempt to enable her to see the god of nature Pan. Years later, a young woman named Helen Vaughan arrives on the London social scene, disturbing many young men and causing some to commit suicide; it transpires that she is the monstrous offspring of the god Pan and the woman in the experiment.
A classic pulp fiction story.