So many versions of Minecraft ship with the same line in their changelogs:
"Removed Herobrine."
They keep removing him. Many updates. Many patches. For over a decade.
You should ask yourself why they have to keep doing that.
This mod adds one thing to your game.
We're not going to tell you what it does. It wouldn't help you anyway — most of what it does, you will never see. Some of it, you'll see but won't be sure of. And once or twice, if you play long enough, you'll be sure. You won't like being sure.
Install it. Play normally. Build your base, dig your mines, farm your crops.
Forget the mod is installed. It works better that way.
It won't forget you.
We know what you're expecting. This isn't that.
Most horror mods want to scare you right now. This one is patient.
"I played for three hours and nothing happened. I think."
"Was that always there?"
"I went back to check, there was nothing there. I know what I saw."
"My friend says he didn't see anything - he was standing right next to me!"
You may play for a long time and have nothing to report. That is not the mod failing. That is it deciding it isn't time yet.
Fully supported, and this is where it gets interesting.
Your server shares one world, but that doesn't mean everyone shares one experience.
Don't be surprised if nobody believes you. Don't be surprised if you don't believe them.
A request: if something happens to you, please don't spoil it in the comments. Post your screenshots — most of them will show nothing at all — and tag your clips. Half the fun is nobody being sure who's exaggerating.
Streamers and video makers: clip everything. You'll want the proof.
He's still in the changelog. There's a reason for that.