About a half a year ago I brought up the idea that Fischl might not be all she seemed. That theory's gotten a little spicier recently but today I want to go back on something I said in that topic.
This was part of my cringy joke setup for the theory but it turned out I was actually onto something and I just dismissed it.
In Albedo's Story 4 he explains that he was abandoned by Rhinedottir in Mondstadt after they found the Heart of Naberius. I think every last person did a quick search for Naberius and came up with the same thing: Naberius is an Archon. The demon which is depicted at the top of this topic is an actual marquis of hell from the Ars Goetia and on the same level as Marchosius which funny enough lined up with my joke about Oz.
So could Oz be Naberius? Naberius is depicted sometimes as a raven. But how and why this applies to the lore come to mind.
In my Fischl theory I talked about how she got her Vision while hiding away in the library because her parents stopped supporting her chunibyou stuff. The second she got her Vision is also when Oz appeared making it seem like he's just part of her Vision. The thing is none of the other mascots are as intelligent as Oz and even Guoba who isn't anywhere near as articulate turned out to be an Archon.
We know that the Abyss are after something that they were using a Mondstadt library book The Pale Princess and the Six Pygmies to figure out. That's my theory about Fischl. But she's just a normal girl now and nobody is buying her chuni thing so if left the way it is the Abyss can't get what they want from her.
Enter Rhinedottir and her discovery of the Heart of Naberius. There have been so many theories about how the heart must be a Gnosis and then tons of speculation on who owns this Gnosis. Long story short I don't think that's what it is. I think MiHoYo is playing us and that's because the basis for these theories is on how close their Chinese names are. But why is there that little difference? (I talked about this with Signora how MiHoYo doesn't make changes without a purpose. Crimson Witch of Flames vs Embers wasn't to be cute; the difference was to show they aren't the same person.) We also know there are only 7 Gnoses and Naberius isn't one of these Archons.
So then what is the heart? I think it's something like a Herrscher Core. (not a literal Herrscher Core just something like a core) A Herrscher Core is a requirement for a Herrscher to sustain itself and it even houses its soul if the body ends up destroyed. From the core a Herrscher can possess a new body and come back into being. I think that both Oz and Fischl are diminished like Guoba and in Oz's case his heart would be able to give him back his full power and form. So this wouldn't be a Gnosis. Keep in mind that an Archon doesn't need a Gnosis to maintain itself. Ei for example was still able to make a nationwide thunderstorm and sustain it without breaking a sweat.
If Oz turns out to be Naberius then the situations have changed. Now Fischl got her Vision but Oz wasn't part of that. He just showed up. If Rhinedottir is using his heart as leverage to get him to help with the Fischl operation then the fact that Oz and her Vision appearing and making her confident again in being a chuni ends up looking like part of the plan. And of course the theft of the book happening so much later in her life means they still don't really know how to get what they want out of her but at least they have Oz keeping her on the right path.
In the Ars Goetia Naberius is a demon that teaches cunning and rhetoric. Listen to both how he talks and how Fischl likes to talk. (and how that's not how she really talks) There's your rhetoric. Naberius speaks in a grumbly voice but acts eloquently exactly like how Oz is voiced in all languages. And the cunning part is Oz's part in Rhinedottir's plan. At the same time Naberius is said to restore honor. Fischl is trying really hard to be the "prinzessin" so that all really fits together.
Ok but bigger picture. We already know many of these Archon names link back to older gods in real world lore. So what about Naberius? He's actually Greek mythology Cerebrus.
Yes the three headed dog. While sometimes Naberius is depicted as a raven other times he's a 3 headed dog aka Cerebrus. And that's really interesting because I've been digging into other names and finding how they tie together. So for example Istaroth = Astaroth (Ars Goetia) = Ashtaroth (Jewish translation of Greek) = Astarte (Phoenician) = Ishtar (Babylonia)
Side note: And Naberius is a cute anime girl!
The things you find when you're looking this stuff up lol
Naberius is the Ars Goetia version of the Greek Cerebrus which is the Zoroastrian Yama or Jamshid. Yama is a guardian of hell who has 2 dogs that have 4 eyes btw. Not sure how "having pet 4-eyed dogs" turned into "is a 3 headed dog." Greeks be Greekin'. 
And then we go back in the other direction and Norse Ymir is based on Yama. And Venti talks about Ymir.
Ymir is another creation god only this time Odin kills him and turns his body into the world. Phanes in his 40 year fights against the Seven establishes his world order of the Heavenly Principles. What if at the same time he also killed other Archons and left them in diminished states waiting to take their revenge. (as long as they remember) So this could be Oz's setup for that.
Yama means brilliant, radiant and majestic and remember Naberius is an eloquent demon. Phanes is also considered a god of light so it could be interpreted here that Naberius also considered himself a higher up and being beaten down by Phanes was a severe blow to his ego. Given the chance to have a rematch Oz might take it.
That said all of that would play out so badly right? That's not the Oz we've come to know.... back in v1.1 which was his and Fischl's last appearance in the story. (yes that one's for you MiHoYo 
) Anyway to that I say Honkai. MiHoYo isn't a stranger to a heart-wrenching plot twist.
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