@Tokanova think of it like this: Nagi's harem aren't actually slaves, they're dependent subs. They all could have walked away if they wanted to. Some(Rita, spoiler) took the initiative to enslave themselves to him. Others(Cecyl, spoiler, spoiler) came to him out of necessity and enjoy how he cherishes them. Especially Cecyl, seeing as he's her soulmate. The rest(Aine, spoiler, spoiler) need someone to look after them or they'll destroy themselves or be destroyed. It's mostly up to the girls and their circumstances. By the rules of this world this is for the best
And then there's the sword. Oh my goodness is there the sword. The sword is just... well it's the sword after all.