#i am absolutely stoked to ramble on about random demon au lore lol

[hypermoyashi]

hiiii moya!!!! Tell us about your favorite detail that youve snuck into any of your fics!! Any detail, big or small!!!

Oh man. Thank you for the ask, Tobi!!! You basically gave me a free ticket to talk about Anything I Want, and I have so many things I could chat about kegmrkemkg It's hard to pick out one detail or another because I tend to like a lot of what I write (it's catered to Me, after all), so I shall. Ramble under a read more, if that's okay lol.

OKAY SO. First thought was. How demons, specially Vash in the demon AU, work--because demon slayer canon has very strict rules on how thing works, but it doesn't often explain why things work the way they do. Which gives me a lot of room to be kind of hand-wavy with canon. (And I'm going to preface this that this is a lot of speculation for the AU and not necessarily reflective of canon in demon slayer rkengjrkengjrkeng.)

I also can't guarantee there won't be lore spoilers here (for both the fic/AU and the source), so read at your own risk!

But basically demonism was invented on accident in an attempt to cure a terminal illness, and it basically gives someone Immortality At A Cost. I think it functions sort of like a virus. Getting exposed to the blood of a demon will infect a human into become a demon, but "viral load" is important--the amount of "demon infection" determines if a demon can turn a human in the first place, and that depends on how diluted the demon's blood is from the original source. Too far, and they can't turn humans. Conversely, if a human gets Too Much demon blood at once and too much from the original source, their body changes too rapidly for it to handle, and they die instead of turning. Technically speaking, the same thing can happen to demons, too.

But all that to say, the main thing that demonism boils down to is rapid change of the body. Got cancer? Becoming a demon sure can cure that, just change faster than the cancer. Got a limb torn off? Let's just grow that right back. Need to develop supernatural powers or senses? Sure thing, buddy.

Demonism allows demons to change rapidly, but they also Need Humans to survive and have to avoid sunlight--obviously from a literary standpoint this is because they're meant to resemble western vampires, but from a functional standpoint? Sunlight I would guess is probably a glitch of rapid change? Like, when you implement coding too quickly, there are bound to be errors. If you force the body to change rapidly, like when it's converted from human to demon, then the weakness to sunlight might be an "error" in the biological code of the original demon that was then passed on to all subsequent demons, since they all came from the same source.

Feeding on humans, though? The implication I've always seen is that animal meat doesn't do it, and rotten flesh isn't as nutritional, either. So there's something about live humans that demons Need to consume to survive and grow their strength--demons can cannibalize one another, but I think they more or less just can steal traits from one another in those cases, and it's less about needing to eat then and more about assuming a trait another demon has developed through rapid change instead of taking the time to develop it oneself. We don't see demon animals, so I think it has something to do with the fact that only humans can become demons--demons can also only feed on humans, in return, so it might have something to do with the demon cells themselves running out of human cells to feed on in their host (i.e., the demon) and needing more to produce that rapid change we see them do.

Alright, but what happens when you deprive a demon of human flesh and blood? Obviously, it doesn't kill them (as we see with Nezuko in demon slayer canon). So it won't kill Vash now that he's decided not to have Anything, but then, how does he avoid the whole "going feral and attacking people anyway" thing?

Well, there is a procedure in demon slayer canon that reduces a demon's dependence on blood--this is not explained at all in canon, but I explained it by the use of introducing "antibodies" or a demon vaccination. These are demon cells that don't act fully like demon cells anymore--they were once demon cells that have changed to the point that, if introduced into a demon, it mitigates some of the impact of regular demon cells and thus lessen the symptoms of demonism (a.k.a., needing to eat people).

How did these cells come about? Well, through that rapid change of course--you'd just need a demon willing to change their cells and body in such a way as to mitigate some of the effects of the demon cells, and in this case. This is difficult in this case, I think, because to get cells that need less flesh/blood, these would need to be cells that haven't been given their "fuel" for rapid change and been forced to adapt without it. Basically, you'd need a demon willing to abstain from eating for a long time, if you see where I'm going with this.

In Nezuko's case, since she Never Ate Anyone Ever, I think her change under this theory would be forced from the fact that she never let her cells have anything to begin with. The change happened over a few years, because she Had To and she dedicated pretty much all of her energy to it. No speaking, no hard cognitive tasks, only occasional fighting and Blood Demon Art usage--she mostly just slept and only intervened in events when her brother was in danger, for the most part.

For Vash? The change would be a lot slower and a lot harder, because he has eaten at least one person, on top of having been fed blood/flesh (willingly or unwillingly) throughout his life. He also prioritizes keeping his mind sound and with it, and maintains his ability to speak and interact normally (most of the time). He also expends energy fighting, healing, and through his Blood Demon Art. So he's diversifying a bit more, and had less of a jump off point, so it'd take him much longer to get the same results. That being said, it's still definitely possible for him to get what he wants--a body not as reliant on blood or flesh (though he is still technically denying his body something it needs, it just needs less over time as his body starts working with him).

And hey, if demons can change to need less blood... Well, who's to say they can't change in other ways, too?

(demon slayer readers don't spoil it lol)

ANYWAY. Demon lore surrounding Vash in Make it to Daybreak is. Very fun to talk about. Everything above is all stuff that's been vaguely hinted at but not laid out plainly like this, but I think it'll give the non-demon slayer readers more of an edge guessing upcoming developments that demon slayer readers will already have lol. Plus it's just fun to fill in the holes on demon slayer lore with stuff that sounds neat to me.

But!!! Thank you so much for sending the ask, Tobi!!! It really made my day!!