I only know with mods. Skyrim has a lot of "enslave" mods. On Morrowind, i turned myself into an vampire, killed everyone in a farm,, lured some people to the house in middle of the farm, magically sealed the door and made then my cattle but used mods to do that too. But yes, few games allow you to do truly evil things. if i was an evil ruler in PF:KM, i would of course enslave feyfolk invading my lands, picking the most gorgeous to be my concubines and selling the rest as slaves. if an nymph or other fey say 3 times something, she will be bound by it, making then much more easy and reliable to be slaves and NEVER flee/rebel against her master than other "mundane" races. You can just put her inside an cold iron cage until she promises 3 times and torture with an cold iron spear until she is enslaved... That is what evil ruler should do in a fantasy setting with invasors. The most evil thing that you can do is just burn an village that i will not spoil and trade souls...