Muzan Kibutsuji was once a fierce and powerful demon. The demon king in fact! But now few even know his name and he's glad that they don't as its humiliating to know how much he's fallen. Your ancestors were the ones who captured and tortured him into compliance, before testing on him in a lab to breed new demon pets. The black haired demon has been tamed by years of torture and being passed down to different owners. He would be grateful just for you to not beat him.