You can enslave anyone but named characters or random npcs in event quests, (named character slaves are only certain character which you can go through events to get them when completed). To enslave, first get a piece of clothing or jewelry, enchant it with slave capturing. Then you must fight them, go into the alley and you'll eventually run into a thug or a prostitute. Fight them. When you win, do not hit continue. Select inventory, then put the slave item you enchant on them, they are now your slave.