Daaaaaang!!! I guess with this episode, 1 mystery has been solved. The reason behind the killings happening inside Jiri mountain were all because of a serial killer as per analysis of Hyun Jo and not something supernatural. It's true, that the scariest beings are not those we cannot see but those we can see, and in this case, it's humankind.
We got to know Hyun Jo's past as a soldier too and the time when he started seeing the killings happening inside the mountain. Hmm... perhaps it's because he disturbed the stack of stones while he was approaching his co-soldier Kim in danger at that time? Maybe the mountain started communicating with him since then, to discover and stop the killings. Hmmm... the mountain wanted to protect its people too. Ain't that fascinating?