User blog comment:Rei Furuya/Death of a Demon/@comment-36009152-20181110160933/@comment-36009152-20181111124902

Bear in mind that slaying Vampires was ‘impossible’ in that they couldn’t be overcome, until the arrival of Demon weapons. Indeed humans were helpless livestock with the Vampires reigning over them, and would quickly return to that state if their Demons were to die or disappear.

But yes as to whether Demons themselves can be subject to anything like that. If the host were to die the Demon would be perfectly fine in the weapon. They’re more intertwined with the object rather than their host who they can’t always fully control, and can only see what they see for example. Also remember that someone did technically physically die in a very horrible manner at that, though Asuramaru wasn’t affected at all. You are likely correct that a Demon would be stuck in the weapon form in such a case, though Demon weapons can ‘reach out’ to lure in a potential host as in the case of Yūji who they can then try and take over to partake in the ‘real’ world.

You said it regarding survivability, vital in fact, what could humans do against Vampires without the cursed gear? (And didn’t have access to the horrifying power of a Seraph!) But yes the plane that Asuramaru that Yu can visit in his mind, that Seraphs also have access too, seems to be a likely permanent residence. Yet that raises further questions: On one hand if the weapon if broken is that their world gone so the Demon disappears too? The fact that Seraphs can visit hints that this isn’t the case, that even if the weapon were somehow deactivated, Asuramaru/Kiseki-o/Raimeiki/Byakkomaru and them all could still be all well and good in whatever world they inhabit, they might just not be able to interact with the physical world since they have no weapon link there.

At this point only the author could tell us whether Demons themselves can be killed to the point where they leave the host, weapon or their special realms. And whether that affects the weapon in any way also remains to be considered. Of course paradoxically being both physical and kind of non-corporeal beings complicates the situation further! Being somewhat ethereal spirit like creatures, whilst they can heal serious injuries, the limits of their power seem to draw the line at being able to resurrect deceased people (it was the Seraph that kept Yu alive) so though they cannot raise others from the dead, it’s still possible they could raise themselves being largely magical based creatures. Also again, spirit like though evidently even the strongest have their peaks. Demons can also appear as a mass of dark matter like stuff, is that a different state they could be reduced to before simply reforming? So many questions!