The scariest threat isn't a named villain with clear motives, but one whose intent stays unknown. An air conditioner falls on a character: accidental if a bird caused it, intentional if someone unscrewed it, terrifying if it just falls with no explanation. Withhold the motive to maximize dread.
Chapters
Here's a lie that ruins your writing. You think the scariest thing in your story is a clear named threat on the page, a villain with a face and a motive. That's not always true. The scariest threat is the one your reader can't explain. Every force of antagonism comes in one of three flavors: intentional, accidental, or uncertain. So your character's walking down a city sidewalk and an air conditioner falls off a window ledge and hits them in the head. So if you show a bird landing on it and that bird knocking it loose, that's accidental. Nobody meant to. But then if you show somebody unscrewing the screws as the person walks below, that's intentional. Someone wants that character hurt. But what if you don't show either one? They just get hit and you never learn why. Well, that's uncertain, and uncertainty lingers because in real life it matters whether a threat was aimed at us, and it's worse when we can't tell. So here's the rule. When you want maximum dread, don't reveal the intent. Write it so the reader can't tell if it's aimed at your character. Then protect that not knowing all the way through. Your instinct will beg you to explain, but don't do it.