The thing is, how do you know if you're being a little too ambiguous? How do you know if you've given so few hints and clues that you've cheated your audience instead of giving them something to think about? When is it okay to info dump? Is it ever okay?
I think there are four things you're playing with here: character stakes, tensions, plots and thematic implications. Some quick, rough working definitions below:
Character stakes: what the main characters want and why they want it
Conflicts: what opposes the character stakes and the implications of that
Plot: how and why the stakes and conflicts came about, the options explored for resolving the tensions and how they finally resolve
Themes: what impact or meaning the attempts to resolve conflicts may have on us as readers
What follows is my personal opinion, but I'll try and justify it where I can.
Character stakes: You need these in detail. We have to know or be able to guess what all the main characters want
Conflicts: You need these in detail. We have to know or be able to guess who's worried, concerned, upset, desperate, furious, astonished, outraged, crushed, elated and why.
Plot: The reader needs to understand at least in sketch form what's happened and why. Else the tensions and outcomes may seem arbitrary.
Themes: I think that they should be clear in your head by the time you've finished your first draft, but you don't need to spell them out for the reader at all.
Since horror is about creating suspense and discomfort in the reader, it sometimes resorts to stinting a bit on either plot justification, or describing final outcome. This creates a cognitive dissonance that can suck the readers in, and also make them uncomfortable. That can be a win!
Horror can get away with this far more often than many other genres - romance or detective fiction for instance almost always spell out justifications and consequences in full.
My rule of thumb is that you can remove plot justification or outcome only if it
increases the suspense or discomfort for the reader, and
doesn't damage your conflicts or themes. Else you need a good way to spell it out. (More on that in a minute).
Classic horror has fabulous themes. They're rich with symbol and import in the reader's mind. The
Hellraiser series for instance, constantly plays human desires against the price of forbidden knowledge, and frequently plays sacrifice against fear. The desire might be ambition or curiosity or loyalty or love - depending on the particular story. Each
Hellraiser story features a diabolical puzzle-box, but often the history of the puzzle-box is almost irrelevant to the story. The box acts primarily as a symbol of forbidden knowledge and its mystery only helps increase the suspense.
On the other hand, in each story there's someone who falls afoul of the puzzle-box, and
how and
why they do are critical to both the suspense and the themes. The how and why link
stakes you see - and both conflicts and themes rely on stakes.
How do you explain plot or backstory without infodumping? My suggestion is to tie the information directly to either a
difficult action or a
dilemma. In other words, weave the tension and character stakes into the way that the information is imparted. Here's an example.
Suppose you need to explain to the reader that although they're thought of as gods, the Great Elderly Ones are actually aliens from a distant star-system.
You could write it in italics author-to-reader, or do it in 'As you know Bob' format, or have some villain gloat about it, or have some clever scientist explain it at the end.. But I prefer to have the main character beat the information from the bloody lips of an evil scientist say, who refuses to give the information up. Try to
force or
trick the information out of a reluctant villain and I believe that readers will hang on every word.
Or... you could spring the information on the main character at a time when he faces a dilemma: to get this life-saving info to the heroes who have prepared ineffective magical godstopping rituals, OR try to save his girlfriend from the human collaborationists masquerading as cultists... The information then becomes a source of tension itself. Its own value creates a dilemma. Audiences will weigh up the dilemma and in doing so, absorb the information to decide what the main character should do. Cunning, huh?
Hope this helps!