I have a similar idea but its a lost soul who can possess any bilogical being and use it in the hunt for its own body but if the body it chose dies then it must start again...but from where it was killed.
All mana lost and therefore power so it must feed off others emotional outpourings to gain streanght again.

The ghost in the machine was a game similar to your concept and it worked well, just never made the big time at the time it was released.