Demon Behind The Curtain

[thecoffinflies]

Help! I seem to remember a story from an 80s anthology that wasn't quite McCauley's Dark Forces, Winter's Prime Evil, or Etchison's Cutting Edge but something like them, in which:

two men climb a mountain. The story is a series of diary entries. In a cave at the top of the mountain, a demon lives behind a curtain and they camp on the nearer side of said curtain. The story goes very strange and surreal, and the man writing the diary awakes to find the other has gone missing.

I don't know now whether this was a dream I had. If it was, I may just try writing it as a story. But if it wasn't, can anyone help place it? I've tried and failed for years (though it might actually be in the books I listed at the top, in which case sorry for being dim!).
any clues?