Just courtiers that cannot leave your court, pretty much.They should also be able to become councilors and lead armies as well. They should function as a type of double edged sword ideally. Appointing a slave to the council should give the ruler temporary negative opinion from his/her feudal vassals while there's a possibility of the slave councilor plotting against you. At any rate, there should be events where the negative opinion of the slave being negated if the ruler treats him/her kindly or even release them by giving them land(in which case, there should be HIGH opinion and the slave should also convert to the culture and religion of the ruler). Former slaves of high ranking nobles in human society generally become 'naturalized' by the society they are in.Freed slaves of Romans for example, are granted citizenship while Janissaries and Mameluks go 'native' so to speak.
