Admin Mawile: By prologue, do you mean a summary of how things wound up that way? If so, I’ll describe it here:
So Karlheinz, as we all know, loves to do horrible things to his kids. Specifically, he seems to like doing the exact things that will upset them most. He also has a tendency to put people like Yui in danger for no reason other than his plans.
Thus, while in the middle of the whole “sacrificial bride” plan, it occurs to him that, hey, his kids have been pretty troublesome lately, what with killing everyone he sends to them. He gets the idea that sending in a replacement “parent” of a sort would both provide him with some degree of entertainment in the form of his sons’ irritation, and possibly actually fix some things in the household. If it goes well, that is.
Karlheinz already knows that it won’t go well.
So, in the same way he picked out the sacrificial brides, he picks out someone to fill this new role, sets things up for them to head to the mansion, and sits back to enjoy the chaos. Considering how things went for Yui, it’s probably obvious that he expects things to end horribly for said person.
Also, this takes place in some kind of canon-strange Au where the three families are already involved with each other to some degree. DL canon is weird, so I can’t place it exactly. *shrug*