i have found my people
anyway, fontaine world quests spoilers, but since Elynas is now my favorite sweet child, here is a little summary\speculations in case ppl mb did not catch some details from the quests and about abyss lore:
Elynas is the monster from the Abyss, some kind of dragon or a serpent. He is gigantic and Elynas island basically formed around him.
He existed in "cosmic cold", i.e. Abyss, until "Mother" summoned him and his "siblings". We know from the logs of fontaine fleet which was following "great beast Elynas" that he appeared during the cataclysm 500 years ago. As we know, Rhinedottir was known to open portals to Abyss that let abyss monsters in, so logically, Elynas was one of these, and "Mother" is most likely Rhinedottir and siblings are other monsters summoned.
Like other Rhine's creation, Durin, Elynas was actually very good natured and didn't wish to harm anyone, but apparently his abyssal origin made him see the world differently and the actions he thought were harmless, were in fact destructive to this world. Spoilers for "Book of Esoteric knowledge", in the domain at the end of this quest we learn that Melusines see places corrupted by abyss not as ruined, but as a beautiful garden and abyssal rifthounds as friendly puppies. And in fact, after we fight rifthound here, it also turns into a friendly dog, hinting that ALL abyss monsters are well-natured, but there is some kind of warp going on that shifts their perspective\actions when they enter Teyvat into being destructive, while they see it completely differently.
after some time, Elynas realized that he was in fact, damaging the world around him, which saddened him greatly and he intentionally sought death to stop suffering of others. He was killed most likely by the fontaine fleet that followed him which i mentioned before, but its not stated directly. He died, but his consciousness still lingered and he is basically aware of his body and whats going on around.
After his death, he didn't rot, but calcified, and apparently many factions, including abyss order, Narzissenkreuz Institute and "spirit of primordial waters" (oceanid? hydro dragon?) explored his body and were collecting his blood and flesh, for which he was glad bc he felt lonely and liked to have company. Especially he liked Narzissenkreuz's Jakob, bc he felt the presence of same cold cosmic power (i.e. abyss) in him and so thought that consuming his flesh will be good for him. Jakob as we know, turned into "Inquitous Baptist" after this, but Narzissenkreuz drama is a different story. But we can at least intuit that one way of turning into abyss lectors\heralds\etc is consuming abyss monsters flesh\blood, btw hey Childe, can we have a word of how you got your foul legacy form? did u eat that whale, Childe???
At some point, there was a battle between former friends from Narzissenkreuz Institute inside of Elynas (mentioned before as Narzissenkreuz drama), which led to huge explosion. As result of this, wounds inside of Elynas were opened by explosions, and Melusines were created from them. Elynas was delighted, thought of them as his children and shaped them into what he thought was a beautiful form. They could hear him telepathically at first and he used this to guide and teach them, but with time, most of them lost this ability. The only one who can still hear him, Mamere, is able to do so because she paints with his blood (she doesn't know its his blood ofc), and so has prolonged contact with him. He still loves them and cares for them deeply.
Jakob tried to restart Elynas' heart with unknown goal, but Elynas didn't want to come back to life, stating that doing so would mean he will keep harming the world, which he doesn't want to. He appeared to traveler in the spirit form in a shape of Melusine bc he thinks they are cute:3 He dreams of being able to exist in the world without damaging it and to show traveler his true form eventually
i did not yet find all 13 pages for "Book of Esoteric Knowledge" quest, so here is probs more info to follow, but I think this is overall what we know of Elynas for now
oh, another thing is that every Melusine was born with a "token", which they don't know what it is, but highly treasure. Elynas doesn't mention them. This token can power a mechanical dog tho and dog calls it "power source". Elynas does mention many "mechanical animals" in the battle that resulted in explosion, so mb these power sources from exploded mechanisms interacting with abyss flesh were the triggers for creation of Melusines? Open question for now, hope to have these Eldritch NFTs explained later