The ghost is essentially a train entity (I forget the exact name) with a trigger around it that causes the scare effect. He goes to a tiny room in the middle of the map whenever he's not being used or called upon.
I made one for my scrapped map ctf_remnants and I found it pretty simple. I forget a lot of the technical stuff of it, but you could probably get a grasp on it by just decompiling koth_harvest_event and taking a look at how the ghost works. I'm honestly surprised there's no prefabs set up for him, but if you understand how the trains in Well work it's essentially the same sort of thing, it just scares you instead of killing you.
The ghost and the trains have one thing in common, and it's that they both need to show up in more maps! Good luck.