Existential horror is well and good, but if a video game really wants to inspire creeping dread, forget about writing; all it needs is a very large open 3D area – preferably underwater, but up in the air can also work, provided the ground isn’t visible – with a very large monster moving around in it.
You’re giving me horrible Emerald Weapon flashbacks.
A excellent example, albeit a mild one; for best effect, you want a monster that’s much larger compared to the player character, and also a tight over-the-shoulder camera. That way you can easily lose track of the thing, then around and suddenly have it filling your entire field of view.