|...but I'm getting a head of myself:|
Let's go one further: Gollum is a character in a story but the scene with Gollum in the Hobbit is a story to itself, a complete thing, so excellent you don't need the rest of the book really. We have an entire class based on *one guy* and entire character races extrapolated from the behavior or sometimes as few as two individuals over the four books that make up central Tolkeinism. What we do not have is *A* Gollum, a race of beings prone to subterraneanism, fish eating, riddles, delusion, and hoarding magic items. You can do that if you want but it will just be the Story, a pale imitation of it, because Gollum isn't special on his own. What makes Gollum something people clicked with was the story. The everything else AND Gollum of Gollum. Space, scale, context, atmosphere, everything that builds the world and therefore the story.
Acererak means nothing in a meadow. Useless.
There is a class of creature which is entirely inseprable from its circumstances and geography, of course, by definition, and it is the big thing I do not want to ever see in a monster list or random encounter table again, because ghosts are not monsters.
Sure, maybe in the Direct Object sense of narrative parts of speech, you can identify ghosts as the point of the subject "What is this scary movie about" which you can also do with wolfman or fishman or lotsofmansman or fangman. In that sense, fine. But it's a forest-for-trees thing. Ghosts aren't a thing you fight or negotiate with, they are not something you can predict. They do not belong in the same company with a 3-boglin scout troop, sleeping ogre, or half-crystal computer flying laser horse.
They belong on a list beside lost tunnels, melancholy hills, arid mausoleums, ancient bazaars, mysterious whorehouses, and dense lush jungle. You don't roll to hit the jungle. You roll to see what interesting, unexpected, fun surprises the jungle has in store for you.
Ghosts are the adventure. They are the setting, the story. They can take the form of gauntlets, of mysteries, of romance if you're weird, they can even be simply incidental to the main course of action, in the same way the red light of September Valley is an oppressive and constant expression but never figures directly into the search for Thundertomb.
They are not the fucking Predator. They are not a collection of player punisher abilities set into a book coupled with some resistances but ultimately vulnerable to the game's default provisioned ideas. Ghosts are specific to their use, expressed in whatever manner is most suitable to their environment or to the sense that their environment is out of step, and should be dealt with entirely in the logic of the sub-story you've set yourself in, not in the logic of "I played a different game that gave me a +7 vorpal ham so I got this." That is some Monkey Island shit that I will not permit to fuck up a good ghost.
While we're at it fuck Haunts, Spectres, Poltergeists, all those guys. Poltergeist is basically a trap/spell effect, you want it gone talk to your god or do the actual legwork to appease it, or work around it like you would a locked door or a sheer wall. Wraith can stay, wraith is like a whole other thing, a very specific one that we can work with.
You don't stab fire. You counter fire. You don't cast magic missile on the darkness, actually, you dispel the darkness because that addresses the problem at hand and uses your own intelligence and the tools at your disposal to solve it. You do not save vs. the kingdom being four days away. This is what a ghost is. They aren't potted ferns to festoon an otherwise drab or unchallenging room. They are the reason you ever care about the room. In many very real ways they are the room. You should never roll initiative against a ghost. You should come up with a plan to deal with the ghost or get the fuck away from it, and what each of those means should vary widely depending on the specific situation.
There are good movies out there where you deal with a ghost by just beating the shit out of it but they are far outweighed by movies where you have to employ some, yknow, brain and skill. Which I think is what we're doing here. Last year for Santicore I asked not for 1d12 awesome new ghosts to threaten my players. I asked for a ghost story generator to create a hundred adventures with. If I want to just fight my players with dead things I'll make skeletons tougher.