The Mind's Eye - a monster concept

I often get these weird -and possibly very stupid- ideas about things to do with trpg games, be it encounters or locations or entire mini-settings.
Just idle thoughts really, but sometimes one makes it's way to my mind that I feel like is worth noting down just to ensure I won't accidentally forget about it.
So here's one such thing. A creature, or more specifically an encounter with a creature.

I had this thought come to me as I was waiting in cold winter weather for a person to come show me around my possible new apartment, this thought about a party of adventurers entering a large room -or maybe a cavern- and they suddenly start feeling ever distant from themselves as if their life was happening from a third-person perspective. They would feel like they are but flies on the wall, watching themselves enter the room. The perspective starts to slowly lower towards the floor...
Then for a second their vision returns normal as they hear a meaty *thud* of something dropping to the floor behind them, and they are once again observing themselves from distance, watching themselves from behind as their selves turn to face their vision with an expression of disgust on their faces.
They watch as their third-person selves draw their weapons and start charging at their vision. Roll initiative!

The party is now in combat with... itself?
Within this confusion it would slowly become apparent that the party is actually fighting a monster, but also being the monster at the same time. 
You wish to attack, but what are you attacking? Yourself and your friends charging at you? 
Unsure, you go to draw a weapon just to defend yourself if nothing else, but quickly realise that not just are you not armed with a weapon, but neither are you wearing armour of any kind. All you can do is raise a ghoulish hand in front of you in protest.

After the person who won initiative is done with their actions, everyone's vision returns back to themselves for a brief blink of an eye. What the party sees in front of them now that they're observing the situation from their own vision is a vaguely humanoid shape, withered and shadowy even in direct light of their lanterns. The person next in initiative gets to have their turn as their regular self facing against this strange ghoulish creature.
And then we swap back to what is by now quite apparently this thing's vision.

I have no clue how this sort of creature would be percieved by the players but it would certainly be an interesting encounter, twisting even the mechanics of the game against the PCs.
What is the right answer here? Let the party kill this thing which's vision you are sharing? Fight back and possibly injure yourselves? Is there a trick to this thing?

In it's eyes are the people under it's control, or is it those controlling it?

For real, what is this thing?

Here's the fun part, I have no idea!
Maybe it's some sort of a bodysnatcher that moves in to the things it kills and lives off of the body's nutrients, sucking it's host dry and then needing a new one to keep sustaining itself.
Perhaps instead of being predatory, it is in fact a prey animal that uses the exchanged vision to drive off attackers.
Or it might be some sort of being from the other side of the veil posessing the body and simply wishes to mess with humans for whatever reason such being might have.
It might even simply have been another adventurer, all withered and worn from being held in this hellhole by some other monster which is causing this weird switch-up.
Or maybe there's some sort of weird drug in the air which is causing all sorts of dis-associative issues within those inhaling it.

Mechanics-wise I would probably play this as a creature with HD equal to the amount of enemies it is facing, and attack equal to the player whose turn it is to control it.
Roll individual initiative for this combat, and the players each control the creature on their turns and the creature controls the party on it's turn.
When you come around to the player that rolled same or closest to the creature on initiative, everyone briefly regains their vision and that player gets to act their turn normally as themselves. (This represents the creature blinking, during which it cannot exchange visions.)