For those who are kinda confuzzled about why he turned them into slaves, it is for two reasons:
1) He is distrusting of brand-new people so the slave seal makes it difficult for betrayal to happen.
2) He has shields available that are able to increase the worth of a level up if they are slaves to him rather than just party members(as in, if a level up is just a +1 normally, it is a +2 or even +3 when under the effects of his shield). I think the skill was called growth boost or something like that?