this is a very anti-early anime thing for me to say, but i actually really appreciate that demons are the most inherently Supernatural entities (well. entity.) in all of black butler.
it seems fitting that, at around the same time it's revealed that the werewolves of the forest were all a lie cooked up by a very human government agency, the backstory for all shinigami is also revealed-- specifically, that they were also all human once, transformed after death as punishment for their own acts of suicide.
paired with the fact that angels are an anime-only concept, as well as the fact that undertaker's bizarre dolls are essentially just soulless humans, the origin of every supposedly "supernatural" entity in black butler is therefore deeply rooted in humanity.
(also this, for more about how undertaker is decaying souls:)
this has interesting implications when it comes to the question of what sebastian even fucking is as the singular Demon of the series, an inherently difficult question to even begin to answer considering the following complications:
1. sebastian is the only (real, canon) demon in the entire series. sorry, claude, but if we're to follow manga canon we can only really rely on sebastian as our sole source of True information about demons-- assuming we can trust what he says about himself at all. he may be ordered not to lie, but if o!ciel never asks questions, we may truly never know.
2. by nature of the plot of kuro itself, it's to the benefit of the story if we never really know the extent of what sebastian is capable of. iirc, yana has directly stated in interview(s) before that she will never get into detail about sebastian's backstory, which makes sense, considering how much of his Mysterious Allure and Power relies on him being a mostly unknowable entity. even his true form is more amalgamation-shadow-tentacle weirdness than a singular Entity:
3. it is incredibly difficult, if not straight up impossible, to tell who sebastian genuinely Is as Himself, versus who he is in the eyes of others. frankly, you could even argue that there is no genuine entity known as sebastian outside of the expectations that others push upon him.
this is most obvious throughout his relationship and interactions with ciel, as he explicitly states near the end of EWA:
--not entirely denying that he is capable of acting like a "beast," but clearly his standards and current goals are pushing him towards something much more restrained at the current moment.
what's really interesting about this though is the fact that sebastian doesn't just act like this with ciel, in such a way that reflects the specific expectations put upon him-- he does this with william in the circus arc too, only making the move to seduce beast after will essentially calls him a seductress luring his prey into the darkness:
(and then, like five pages later:)
frankly, you could maybe even make the argument that sebastian being summoned as a “devil” at all is a result of the expectations placed upon him by the cult itself, that they kind of manifested him in his current demonic form by calling upon a demon-like entity through their child abuse rituals and sacrifice.
honestly, i'm of the opinion that there are only a few things we can truly trust when it comes to properly sebastian:
that he really really really wants to eat o!ciel.
that he is generally unreliable and cannot be trusted (o!ciel's words, at the end of EWA).
that he genuinely likes being a butler, or at the very least the game he gets to play with o!ciel in the process of taking on such a role. this is what the circus arc most clearly establishes, as well as the LLA flashback when undertaker gets into his memories.
sidenote, with regards to that last point: is it also not insanely weird that sebastian has a cinematic record of his own? like it makes sense that he has memories, and undertaker's experiments make it clear that memories are separate from souls so it's not like that confirms sebastian to have a soul of his own or anything, but still. shit's weird.
a part of me also still wonders what Exactly the reapers are cutting through when they attack him with their scythes also-- while he can clearly morph his appearance into something more demonic at will (e.g. arthur at the end of the murder arc), he must still be somewhat substantially attached to his current human body if injuries to it by a death scythe can still fuck him up as badly as it does on the campania. sebastian is certainly not all-powerful, though he absolutely should not under any circumstances be underestimated when it comes to his scheming.
ANYWAYS. this is all to say that, for all that kuro takes from a wide range of cultures and mythologies when it comes to it’s more supernatural elements, ultimately it seems like most of these aspects are still quite deeply rooted in humans and humanity. this world revolves around people, at the end of the day, with human lives (and souls) as the basis upon which everything else acts and turns.
