It was in a collection of Sci-Fi short stories which I lent to a friend and never got back.
It was about a human who somehow became trapped in a muddy cave or beneath the ground. There the man encountered a being that was humanoid, large, and had black skin more like a shiny shell. The most memorable feature of this being was that it was telepathic, it communicated with the man IN CAPITAL LETTERS to signify it was non-verbal communication.
The mental communication hurt the man to receive. It became a sort of mental combat or test the human had to pass before he could be released.