i had this idia on my mind for long time. it would be cool to implement but there should be right way to do it. and i think your game has all conditions for it.
when player get knocked out he can be finished (killed) by other player. but agressor will have a choice not to kill but capture victim as a slave. of course it will be not possiable in huge fights when you are captured slave in one minute and next minut you get killed yourself. so this is the only option when final of the battle is clear and everybody knows that fight is lost all victims are knocked out and waiting on a graound for their destiny so this is a right moment for agressor to deside of they wanna enslave all this bunch of players. however you cant have like 10 slaves per charachter. slaves are like pets or tamed animals. there are slots for each of them and you have to have high charisma or whatever to have at least one slave.
But not every person can become a slave. very good you have gods. so if you dont want to become a slave you have to comit spectial prayer against slavery. so you pay money or maybe sucrifice head of the deamon (whatever your god like) and it pevent you from slavery.
what is slavery exactly? only oposing fractures can enslave. for example good can inslave evil and vice versa. so basicaly good cant enlave good. but evil can enslave evil cos they are evil. slavery can be for some period of time. because deamons have riot souls they hate slavery so as a slave they are have smallest amount of time. if deamon enslave deamon this time even smaller. human are nutral so they can be slaves for twice as much time as deamons. and beasts they are good so they can be slave for three time more time if they captured buy deamons. so basicaly good they sort of obey cos they are good and evil dont like slavery cos they are evil. once your time of slavery is finished you released to your home like when you died you teleport to your house.
so how doest it work? once you have been captured as a slave your master can see what is your crafting gathering and fighting skills. it need for him to deside how to use you, does he wants you to be resource slave crafting slave of he wants you to become a gladiator and fight on arena for him. there must be some punishment for slave if he doesnt want to do what master is saying. maybe it will be a wip strikes and if slave get 100 wip strikes maybe he loses some atributes. some skills or whaever, maybe tempreraly or maybe permanent. maybe long term tempreraly is the best cos its not too seriouse.
so these my thoughts. remember how fun it was in Ultima when you got a slave. There you could enlave forever, i know its hardcore but it was fun. We were free and we could ask some strong guild for help and unlock this bloody door. It was totlay up to us.
hope you enjoyed my idea and use it some time in a future.