(Spoiler) About amaurots downfall

[Nalien]

My completely baseless theory is that we cause the Terminus event.

A flip on the original Final Fantasy story; Garland is defeated. The Four Fiends send him back in time. He becomes Chaos. He sends the Four Fiends forward in time so they can send him backwards in time.

We are defeated. We Echo backwards in time. We trigger the Terminus event. We set into motion all of the events leading to the summoning of Zodiark and the rise of the Ascians. The Ascians defeat us. We Echo backwards in time...

Now, if only there was an alternate timeline where a world-ending chemical defeated us...

I cannot help but wonder what the "sins stacked to the heavens" that Emet Selch mentions during Amaurot were.

It implies the ancients had been doing something really wrong, but likely never even realized it until it was too late.

I simply took that as a follow on from their deepest fears manifesting. They weren't necessarily sinning before hand, just that after the Terminus event started, their concepts of "wrong" began to manifest, so their sins (their idea of what sin is) began to manifest and stack to the heavens. Everything they feared started to manifest, the more of them that gave into fear, the worse things got. Eventually all it takes is someone to fear that the planet is doomed and, well... That manifests too...

I'm not actually convinced it was a problem with the planet at all. We're told they had enough aether individual to manifest their creations without using external sources (Crystals/the planets aether), so it sounds more like this was a case of them being infected with something that robbed them of the ability to control their creation magic, rather than the planet Nope'ing them for some reason. Emet does say that the very fabric of their star began to unravel, but that could easily be their doing. Their plan was to weave them anew, so it stands to reason that they could also do the opposite, and given their fears were just manifesting out of their control... "I fear the stars laws are unraveling" is all that needs to be said to start that process... Essentially a cataclysmic positive feedback loop... You're scared of something, so it manifests, now that its manifested its real and scares more people, feeding that manifestation and creating more, carrying on exponentially...