A model of souls I always found interesting describes an entity's brain as a kind of antenna tuned to a specific soul, amplifying the soul, and the rest of the body acting as a kind of vehicle for the soul, allowing the otherwise discorporate soul to interact with the world.
I'm not sure if that could be applied in Armok, other than, if the soul were somehow disconnected from the brain, the body would enter a sleep state, sustained by the autonomic functions of the brain. This model doesn't say whether souls are conscious when separated from a brain, or where they originate from, so.. uh, I guess it's not that much use.
But.....
The player could be controlling a soul (put into a body of their choice at game start) and the soul, if it's been in a certain body for a length of time, will retain the thought patterns of the creature it has been, but they slowly fade, and when the soul has lost all its capability to think (and thus act), that's it, game over (unless it somehow bumps into an unnocupied body, like a newborn child or something) The aim, if one wished to continue playing, would be to somehow wrest control of a body from another.
As for learning how to move as a soul, fight as just a soul, and whatnot, there could be spells which allow the soul to move away from the body, but still remain tethered to it for easy return.
OK, that's enough of my ramblings for now.