Either way, they still have to try to enslave the humble noble. After all, they have nothing to lose.
The noble will either help them meet the advancement requirement or not. If he doesn't, they won't lose anything apart from the spiritual energy they expend in trying to enslave him. Then they will kill him.
But before they make their move on the noble to enslave him, they want to make sure that they will really not lose anything in the attempt. They have reasons to be cautious because his behavior is suspicious and he sometimes has suspicious thoughts in his mind.
They are prepared to kill him if he turns out to be useless, but first, they want to ensure that making any attempt on him will be worth it or make sure that his death won't bring them trouble.
To meet one or both of these requirements, they need a fateweaver to help them decide. So until their rank 4 Fate Weaver is ready, they don't plan to make a move on the noble.
