Ok, random idea, I'm going to ramble a lot, but what if Undertale ghost monsters could possess living things?

I'm nicely blaming @eriscary for this idea because of Tear being a Sans plush possessed by Napstablook. It made me think about what other objects ghosts could possess, and why not something living?

Seriously, what with all of the AUs and stuff where Chara's ghost takes over Frisk, if human ghosts are allowed to do it, why not ghost monsters? And what would be the rules? Would the host and ghost share control? Would the ghost be able to communicate with the host? Would the ghost be able to leave whenever they wanted or would they be stuck because of some sort of thing going on? It's gotta be different than just possessing an object, right?

I think it would be easiest for ghosts to possess living things without souls (say, plants and stuff like that, brings up some questions about Flowey) and it would be hardest for them to possess other monsters. They'd be able to possess humans easier than monsters because a monster soul and a human soul can combine together (it would basically be an opposites attract, magnet type compatibility which is why monsters are hard to possess.) Actually, maybe it would be nigh impossible for a ghost monster to possess a fellow monster unless they were a willing host.

Plus, what would happen when a ghost monster became permanently bound to their living host? I mean, I think that the host would need to agree in some capacity, but if a ghost got emotional enough that that happened even without the host's consent, ohh boy. (And what if that binding happens to someone who has the power either to RESET or remember resets? What sort of paradoxical nonsense could we get into with that?)

Also, what about the damage rules? You can't hurt a ghost with physical attacks unless they're bound to a body, but if that body is something living, the host is definitely going to get that damage. What are the pain rules for ghosts anyway? And what about magic attacks? Seriously, Undertale ghosts are not obsessed over enough!

by napstablook's own words, you "can't kill a ghost" because they're "kinda incorporeal." not only that, but napstablook (and the other ghosts, i assume) can "lower their hp," or at least make it look like it, which could be used to bait an enemy into overextending.

ghosts are, seemingly, very powerful monsters. they can teleport long distances (after meeting napstablook in the ruins, the next time you see them is all the way in waterfall), they can vanish and reappear at will, they can float/fly at pretty high speeds (or at least mad dummy can), and can possess objects.

the only way a ghost can be harmed is if it chooses to fuse with its object and become corporeal, which in itself is a difficult process, seemingly requiring a moment of intense emotions.

in a genocide run, 2 of the 4 ghosts will still be alive by the end: the ruins dummy and napstablook. (the ruins dummy because, according to what mad dummy says before their fight, they hadn't fused with their dummy yet.)