As you have seen, erika is obsessed with finding a "fine" husband, in vol 2 while they are in a kingdom, out of nowhere she proclaims that she found her destined partner (which is the prince of the kingdom they are in) and leaves the party without even talking to them about it, since Souji and the others see this as sketchy as hell they decided to investigate and rescue erika by infiltrating the weeding, by the end when she is rescued it is discovered that said prince from the kingdom is connected to the demon lord (because cliche imma right?), and thus because of her obsession of finding her desired partner, she gets manipulated into leaving the hero's party. Because of this, Souji has to rely again in the power of the cursed sword inching him closer to dissappearing like Kojirou said.