This is another possibility; I mentioned it in the list of suspects way way way back, but eventually I discarded the idea because when you take into account that (A) the sylphs are still peaceful, friendly creatures that preach never summoning primals due to the way they darken hearts and make tribes violent, and (B) Odin's home is the Shroud. It just seems to carry slightly more statistical weight. You never know, though. Maybe they just did it that one time?
Wow! Regardless of whether or not it ties into the plot, that's a great find - and one that would tie a few loose ends together, were it correct. Awesome contribution (^u^)b
The biggest strength of your case is the timing. We know the Garlean invasion of Mor Dhona took on Midgardsormr 10 years ago, 5 years after the conquering of Ala Mhigo. And we know that Midgardsormr isn't what convinced them to turn back - a legitimate primal was.
The echoes of the main storyline introductions for each city seem to be about ten years past, shortly after the Battle of Silvertear Skies. This is when Y'shtola loses the horn.
If you're correct, then Midgardsormr's death caused Swallowtail Roam (Seal Rock) to rise, and with it the Sea Serpent and the Treasure (Horn), which would have been stolen from Y'shtola / Sthalmann only weeks later during the initial post-Silvertear investigation of Eorzea by the Circle of knowing.
This would have occurred BEFORE the Garleans encountered the primal and AFTER the horn was stolen.
You've got a serious shot at being onto something, there, sir.
This is something we've toyed with on and off. To sum it up for you so you don't have to dig through all the pages, primals appear to be like the Titans of ancient Greece; "old gods" (primordial deities, daemons, etc.) that come from, to quote the sylphs, "amongst the aether" and beyond the void. Seemingly, they take root in our physical reality and cross the veil by attaching themselves to an isolated society and building a cult with themselves at the center, exchanging gifts of power and protection in exchange for worship and aether (which is how they maintain physical form in our world).
This has a dual effect - the society that has been turned into a cult grows violent, arrogant, and powerful, whereas the deity becomes obsessive and paranoid about controlling as much aether as possible while preventing any competition. Garuda and the Ixal are one of the more extreme examples of out-of-control primalism.
I'm sure Ifrit can smell his own - so whatever is taking an interest in us might very well be of the old ways; a primal, a mothercrystal, something. So whom? Siren made a lot of sense of a little while... it could be Odin, as well. Each tribe seems to have an element except for the Dragons and the Woken Races (Hyur, Miqo'te, etc.) so this would be a great Astral v. Umbral relationship.
What I'm interested in is what the Garleans are up to. They VERY CLEARLY want aether despite claiming to desire the annihilation of the primals. My theory is that they crave the destruction of all primals but their own; a primordial essence fused with Garlean magitek built in the method of the Allagan - Alexander.
But where does that leave Siren, of whom we have concept art?! OH MAN, GOOD TIMES ARE COMING.