uncanny valley = things that aren't normal almost getting it right
third person limited view
limited expressions
rot, mold, damage, age, static, flickering, espsecially in places it shouldn't be
limited sights for your mc - blindness, darkness, fog
being alone - the more people there are, the less scary it is
intimate knowledge, but only on one side
your reader's imagination will scare them more than anything you could ever write. you don't have to offer a perfectly concrete explanation for everything at the end. in fact, doing so may detract from your story.
genuinely so tired of the male love interest in every m/f romance being the most hugelarge tallman to ever growth spurt. I need to see some women swooning over little five foot five rat dudes who need to be tucked in their gf's pocket lest they blow away like a napkin in the wind.