You Keep What You Kill

When you kill someone, you take their soul. This isn't an active thing, it's just what happens when people die, their spirits become part of the spirit of the person who killed them. Every time you kill something with a soul (most living creatures have souls, except the Welsh), roll a d10. On a 1, the soul you just stole has a useful skill that you now know (GM's choice) at a medium level. Not that this only applies to whomever makes the killing blow, everyone else just gets the satisfaction of a job well done. Also, the PC who gains a skill this way doesn't know they have the skill, and nobody KNOWS that this is how souls works, though some religious types have begun to suspect it through long, arduous trials (mostly involving sacrifices).