The Life Force of anything is what makes them who they are. The Life Force of each person is made up of several different parts: their natural element, their personality, their dreams, etc. The only way to interact with the life force is if the being has lost their face or are dead (unless they are in a state of living death, like a ghost, where they count as living). Not all the parts of the life force can be taken away from a living being, however, and not even the strongest of magic can do such an unthinkable thing.