Dire Bare
Legend
1st level and tone, atmosphere . . .It's been gone over a lot here in the past. But in short, the essence of horror is a sense of powerlessness. You have to cut so much away from 5E to accomplish that, you're basically playing a new game at that point. You're better off, IMO, just going with a system designed around delivering that experience, of which there are plenty of good choices.
I had my players running scared with twig blights when I ran the Sunless Citadel back in the early days of 3E. Granted, we weren't playing a horror campaign, but that adventure had plenty of horror scenes. Fun times.
I can't remember the sequel, but the one with the "hobyas" also managed to freak out my players. I would just randomly tell them, in the dark woods, you hear the cry, "hobya" . . . .
You also need players willing to give in to the horror.
Of course, the higher they advance in level, the harder it gets. But with the right DM and the right players, totally possible.