I have come with another crazy dragon theory, as always. This one was inspired by my friend's genuine confusion to my theories which prompted them to ask, "So...wait... Primordial One...= Nibelung?". So here we are now.
Quick disclaimer before I explain, this theory is not the most likely one. The most likely theory is probably something far more tame and boring like: Phanes is the 1st, Seutervoinen is the 2nd, Sarelainen is the 3rd, and Traveler is the 4th. But tame was never my style, so have this crazy dragon thesis instead.
The 1st and 2nd Thrones:
Let us first read most of what we know of the 1st and 2nd descenders, while establishing some points that tie the two:
It is interesting to me that Phanes is said to have killed just the 7 sovereigns... what about the dragon king?
This description matches the one in the Frostmoon scions' book of prayers (right after this point). But I also find it extremely interesting that this sounds a lot like this:
And this describes a DRAGON pet.
The scions prayers include one to Phanes, and one to Nibelung:
It is also very interesting that NIbelung is called "Lord of two natures". That will be important for later, so keep in mind.
If you are wondering why I think this Takoja is Nibelung, well, read these:
In short, it is because he forged the tripled moons.
How Can They Be One:
Now you must be wondering, if Nibelung fought Phanes... how can they be the same? The answer lies in how souls work in Teyvat.
What first gave me this idea were two pieces of media: Thunderbolt Fantasy, and Onmyoji. If you know either, you know where I am going with this. If you don't, I am about to spoil the major plot twist of both.
Both share a similar main plot twist: The main villain and hero are in fact the same person... just split in two.
In both stories, at a time before the story began, there was a powerful person. Then that person, in order to achieve some grand notion, decides to split themselves and cast away the half that is hindering their progress. Their folly? Never stopping to think what would become of that other half, since they thought it simply refuse...
In the case of Thunderbolt Fantasy, it is even more similar because the one that split themselves was the Demon King... and he did it to conquer the world in a great war. The half that he discarded was not only similar in personality to Kaeya, but they also share a Japanese VA. (This similarity is important for later, even though it proves nothing).
Another important point is that the other half always came back for revenge. The two halves were always opposed. This reminds me of something else: Kuuvahki.
Kuuvahki always comes in two polarities. Red and Blue. But if you did a certain quest in "Nothing Passage", you will have also seen the original form of Kuuvahki, as both polarities merged right in front of you and sent you hurtling through space-time. The one primordial power of the dragons, now complete, leaving a noticeable mark on the land.
This is interesting because what if the power of the dragons somehow represents the state of their master, the dragon king, like how the hypostases represent the state of the archons (last part is proven btw)...
Remember how the frostmoon scions described the dragon king as "Lord of two natures"?...
But why split himself?... Maybe he thought it would help him avoid the future Seutervoinen talked about (see the skirk artifact set), the destruction of his people... And in doing so, caused that future to come to pass... (wouldn't be the first to do something like that.)
So, in short for this section, I think that the "Dragon King" split his soul in two, "Phanes" and "Nibelung", and that is how we are in this predicament. And that this happened "before the story began" as well. Now onto the final part of this theory.
Kaeya is Half of Nibelung:
Sounds outlandish, I know. But that is the hallmark of my theories so bear with me a bit.
Before I get into all the connections there are, I want to talk about the nature of souls, and the marks they leave, in teyvat. Namely, heterochromia.
Heterochromia seems to always represent an abnormality of the state of the soul.
Ororon has heterochromia as his soul is only a split half of the original. Naberius has heterochromia as her and Rhine's souls now inhabit the same body.
Now think, which important individual do we know of that is intent on covering their eyes so we may never know if this mark of abnormality is there?... Yep, none other than our lovely Kaeya. His eye was also the target of his brother's wrath when he found out whatever truth Kaeya told him that night...
There is also the connection with the abyss. Nibelung is said to have brought the abyss into teyvat... and Kaeya is somehow the "Heart of the Abyss".
they changed it because it gave him away
The Khaenrians are also very picky about "pure bloodedness", something that (according to Enkanomiya's research documents) is why Neuvillette was reborn in human form, because the vishaps were of "impure blood". (and you know I already think that the khaenrians are vishap people)
There is also this peculiar symbol, which reminds me of my first ever theory on hoyolab, the one that tied Kaeya to Phanes (I guess I have come full circle with my theories):
That mural is one of "Phanes" btw. You can find it on dragonspine.
It has also been years since I realized that Nibelung is very likely in human form. (My reasons are many, greatest of which is that he is nowhere to be found so far... and he is a DRAGON KING. Would've been quite noticeable if you ask me.).
Then there is the damning piece that I could never understand. The surname "Alberich". If you know the story where the name originates, you know what I am talking about.
That story is called "The Nibelung Ring"... after the Nibelungs... who were Alberich's people. Yes, Alberich, the original ruler and owner of the "Nibelung" ring. The one forged from the "Rhine gold"... And we also have an illegitimate Alberich prince called Hogni/Hagen who wears an eyepatch and wields the Dainsleif sword in that story...
But that story is too much of a mess to properly interpret in genshin terms.
Not to mention that I have other theories that point to Irmin being Nibelung, Irmin the "one-eyed" king... but he is dead and we have no clue where his reincarnation is... or do we?
There is also the matter of his brother's wrath. Diluc scarred his eye when he found out his "truth", then went on to hate the fatui's guts. But a certain Khaenrian leader of the fatui (Pierro, who also looks like an Alberich) is trying to bring back the "Old World", the one of the dragons. Maybe that is connected as well. Afterall, being a descendant of a 500-year dead nation is hardly called a betrayal... what Kaeya told him was probably far more jarring.
So, in short, Kaeya could be one half of the Dragon King... and we could be none the wiser for it.
(Just like that character from Thunderbolt Fantasy) (watch that show. it is insane.)
There is so much I left out so that the post doesn't get too long. So much evidence I left out and just told you the conclusion of. So much symbolism that points to Kaeya being far different than what he seems. Take this for example:
That is a Kuuvahki plant btw, one enriched with the power of dragons... so tell me why it looks like a peacock feather?
There is also that Lauma who's powers carry the same tripled moon symbol as Neuvillette... a dragon sovereign. If you ever dig into her lore, you are sure to believe this theory even more. Not to mention digging into the funerary war... Actually, lots of my theories dig into things relevant to this one. Like Khaenriah's WHOLE HISTORY. There is also MONDstadt and the MOONS. The scions PRAYED to NIBELUNG. So much weirdness that actually serves this theory quite well.
But in the interest of brevity, I will end this here.
Till next time I come with crazy dragon theories. 
