im sorry but i have to agree with sibmaster. well how about instead of the spirit being a bad thing, maybe it can be aa good thing. Like the soul was possesed or something like that, and by killing it, the soul is free. it will tele you to a mystical place and thank you for releasing him. it will be human so for example, if you kill a great demon and you get this random, a human soul will come out and thank you. the reward will be temporary stat bonuses or permanet xp.