I've been looking for the title of this movie for decades. When I was a kid in the early 80s, I saw on TV a 70s sci fi / horror film that is the scariest movie I ever saw. I only remember a few things from it:

a tall brown haired, cleanshaven caucasian man sitting down taking instructions via a monitor from something gray/silver that looks like a 12-sided die. He takes instructions to kill.

a dock, small, like in a lake or pond

Any idea what movie this is?