On Devils and Dying
I touched on this very briefly back when I was discussing the workings of Devil May Cry's demonic power and devil arms, where I mentioned the fact that strong demons cannot be 'killed', but merely defeated in such a way that they cannot return to the human world without being awakened by some kind of ritual.
Since the 'beauty of death's finality' post, I've been considering if the same holds true for the demons of the Black Butler universe, and I have come to the conclusion that yes, it does, meaning that Sebastian actually cannot die.
Early on we had these quotes from Sebastian:
Which suggests that even if he were somehow weakened enough to 'die' whilst bound by the contract and was therefore unable to continue with his ordinary duties, the 'him' that would 'disappear' would be his butler aesthetic form, but he would still be present in some other way (something that expends less energy - a more simple form like an animal or some less coporeal entity - or a case of possession, perhaps?), the liklihood of which is also confirmed by the fact that the contract cannot be terminated by O!Ciel by means of any order, which one would assume to include the order to 'die':
We also have this to go on, from the flashback of the initial contract:
But surely demons must generally able to freely travel from their realm to 'hunt' otherwise they wouldn't pose such an issue to Reapers, so we can now also assume that prior to the story taking place Sebastian didn't have the strength to manage this on his own (probably because of being weak from starving himself), making him essentially 'dead' and gone from our world until otherwise summoned with a suitable sacrifice, providing much needed nourishment in the form of a human soul in order to give the demon strength and boost the recovery process (the necessity of which was also sort of confirmed in the Campania bonus chapter 131.5 - That Butler, Resting - though it was never officially published in english).
Essentially, then, even if Sebastian was killed by the Undertaker's death scythe and harmed in such a way that he absolutely could not continue to take form here and was made to forfeit the contract, he would still exist within his own realm. He may be severely weakened and in need of time to recuperate, or perhaps even put into some dormant state until summoned again, but he would still be 'alive'.
Easy then to see why the concept of such a permanent end would be a fascinating one, especially when made even more absolute with the consuming of the soul, which could otherwise have been considered the one part of a person to 'live on' as it awaits its final judgement.









