An adventure designer might write a horror adventure more successfully by avoiding powerlessness.
Go ahead and assume competence and agency, and take stock of the capabilities your party has. Once that’s done, think about what your competent party might fear, loathe, resent, or be disgusted/revolted by.
Throughout the adventure, increase either the frequency that horrible thing is encountered, or the opportunity to observe its behaviors or effects. Do not explain anything. It should be consistent, but not symmetric or regular. (Very brief/stripped example: 1 distant howl, 1 closer howl, a call howl and a response howl, 14 howls from all around. It’s consistently howling, but there are either more of them or they are closer and it isn’t a pattern).
During play, when describing what the players see and hear, reverse cause and effect - where possible, put off / equivocate description. “Angry red slashes rake your chest open and blood pours onto your boots. You’ve been clawed for 23 damage, and whatever did it is shifting its weight for a follow-up.” “You feel sharp teeth and hot breath on your throat before sticky wetness cascades down your shirt. You’ve been bit for 18 damage!” Doesn’t have to be combat, either. “As you come down the hall, you feel a tug at your clothes like they’re caught and a tickle on your flesh. Another step and your progress is arrested completely. Thick, sticky, invisible bands hold you fast. You’re webbed.”
Let players conjecture as to the motivations (if any) of your horrible antagonist. Yeah, mostly they’ll be eat-monsters. But they don’t have to be. Just ensure their actions (or the aftermath thereof) are sufficiently repulsive or disgusting.
Now, on competence. It’s actually necessary for players to feel like they have some power or agency. Helplessness is actually counter-productive for horror in a game. (In a story too, but never mind for now). The fear is going to come from uncertainty and the uncertainty is the idea that the power and competence they have Might Not Be Enough. If they are sure they can win, or sure they cannot win, then we lose the uncertainty and we lose the fear. Try “breaking off the attack” by the horrible antagonist earlier in the adventure. Maybe it turns away when an ally shows up, or someone sheds some light somewhere, or speaks elvish - doesn’t matter. Try limited engagements that are cut short. Endanger things that should feel safe or that are taken for granted.
None of this stuff is very difficult. It’s actually easy (except to remember to reverse cause and effect) to write this stuff up. And basically none of it has anything to do with the game system, so long as you write for the expected power level. Meaning, goblins won’t be terrifying past like 3rd level, and certainly not at 10th. Likewise, something that’s obviously overkill on a low level party (like a Death Slaad versus 2nd level players) also doesn’t work. It’s about as scary as “rocks fall and you die.” Some agency/power is necessary to get the uncertainty. But past that, the game system doesn’t matter a lick.