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Hmmmm
I was recently exposed to the rabbit hole that is Minecraft horror mods and it honestly really disappointed me. So much so that I thought up my own horror mod in response with some key distinctions to make it feel actually scary:
Be as subtle as possible to the point that the player can't tell whether a mod or a weird glitch is causing the effects. Not only is it scarier to take advantage of the existing horror within vanilla but doing this would really mess with veteran players who know the game in-and-out.
Rely on ambient psychological horror. No jumpscares, no threats, no escalation, not even anything flatly disadvantageous. The mod would just weaponize the player's paranoia against themselves.
Be really nefarious as to take full advantage of the insane adjustability that Minecraft has with all its internal options, gamerules and mechanics as well as use the player's acquaintance of the game against them. It should be a horror idea that could only work within Minecraft.
So here's the idea I got from all of this (mind you, I have no idea if a mod like this already exists):
Every in-game day there's a 5% chance (3% if you slept with a bed) for an event to occur from this mod. This chance is fixed and never changes. When an event occurs, its chosen randomly from a very exhaustive list of different events and (depending on what type of event it is) will either occur once or persist until the next event. The likelihood of every event is identical. So this system for pulling them is entirely static and random. Additionally, the chance is rolled again whenever an event occurs. Meaning there's an incredibly low chance for you to experience two or more events in the same day.
So what do these events do? Well, they're designed to mess with the player in the most subtle ways possible. Often specifically targeting their memory, understanding of game mechanics and overall familiarity with Minecraft. The intended effect is to give the player a creeping sense of powerlessness in a sandbox game that they are otherwise completely in control over. Some possible events could be:
Hearing a sound effect in the distance from a source that isn't real.
Having an item currently in a chest/furnace change its amount or position.
Replacing a current painting sprite with another one of the same size.
Having a door/trapdoor be activated (i.e. opened if it was closed and closed if it was opened).
Skipping a full day from sleeping rather than just the night.
Having a specific gamerule temporarily change from its default value until the player triggers it (with minor adjustments to make it less apparent. For example still having the player drop their inventory upon death when keepInventory is toggled on but having the items despawn near instantly unless another player was nearby).
Having a tamed/trusting mob despawn (but only if the player had not interacted with them or been near them for a while).
Surviving otherwise fatal damage or dying from otherwise near-fatal damage.
Randomly changing the difficulty or local difficulty (without it being visible in the options or the debug screen).
Randomly changing slime chunks.
Randomly changing the moon phase.
The list goes on.
These are not notable events but that's the point. The intended effect is to confuse the player and make them doubt themselves without ever thinking that a mod is responsible. To make things even more nefarious, you could have this mod be disguised as a typical QoL mod and sneak it into modpacks to really mess with people.
This is the kind of horror that I think suits Minecraft best. Not the loud, overt, in-your-face kind of horror. But the kind that has you slowly begin to doubt yourself and the world around you. No escalation, no climax, no resolution. Just you left alone to consume yourself out of your own paranoia.