This is the most famous of Arthur’s short stories and there is a reason for that. At the same time I wouldn’t be surprised if you haven’t heard of this Welsh writer or read any of his stories. There isn’t much to separate him from you Edgar Allan Poe or Stevenson or any of the other famous horror short story writers, including the one who resembles him the best Lovecraft.
Then again, that should not scare you away or to think that you shouldn’t read him. Would you not read Poe just because Lovecraft exists, or would you just enjoy the fact that there is diversity for those who enjoys being scared?
The story is about a man who, in his youth, agrees to be a witness to a mad scientific experiment (is there any other kind of scientific experiment in these kinds of stories?). The scientist is trying to break down the walls so get in contact with the roman god Pan. Many years later the man meets a woman who seems to him eerily familiar.
I’m not going to spoil this for you, just take comfort that this has all of the good stuff you want from this kind of story written in an very compelling way.
After all it’s impossible to come to any resolution in a case like this. I hate melodrama and nothing strikes me as more commonplace and more tedious than the ordinary ghost story of commerce.