I haven't read through horror (or psychological) fiction extensively. I have sampled some of H.P. Lovecrafts work and some stories in different mediums, such as the video game Alan Wake (which has also been written into a book).
I've noticed so far that a protagonist is either personally called by a dark force or they either stumble upon it by accident. For example, in Lovecraft's cosmic horror, it seems that individuals are personally called upon to do the dark forces work. The same goes for Alan Wake, where a dark force needs to use the skill of a writer to free itself from it's watery prison.
Some examples of cases where the protagonist stumbles upon a dark force. I recently watched a version of Dr. Jekyl and Mr. Hyde, where Dr. Jekyl inadvertently creates Mr. Hyde in a test tube. In a way you can say he stumbled upon the dark force. I'm sure there are better examples of this though.
Lord of the Rings, while it isn't horror, seems to be a good example of accidental discovery; where Gandalf is noted saying that the ring did not intend to fall into the hands of a hobbit. I assume he was talking about both Bilbo and Frodo. Clearly a case where the dark force was accidentally found.
Are there other ways for a protagonist to discover a dark force in horror fiction? I have only noticed these two so far.