User blog comment:IamJakuhoRaikoben/Chapter 398: The Final Duels, Review./@comment-24536822-20140903061611/@comment-25111584-20140903071714

Dunno. Books, like God Slayer Magic? Make a pact with a Demon? No idea at all, many possibilities. We don't even know how it works because, even though extrapolating from how Dragon Slaying works should be safe enough, it has never been stated ...

And I find the constant mention of Zero interesting in the fact that there might be an Ice Demon Zero ... Demon Force might be the demon taking over the body for the battle rather than the user actually becoming more like a Demon ... if Demon Force exists. All speculation though. :P

/me is too much into this ... -.-'