Well it could be both. The start of it was out of their hands. Maybe it was a third party or maybe the planet simply was dieing? I mean planets do have lifespans too and it could just be that the planet was "crying" out in his last days. This sound then and maybe some starting catastrophes created a huge amount of fear in the people around the world. And as we learned, the ancient ones can create really bad things when they are fearful. I can imagine that this then started a chain of events. One city heard that sound -> fear -> monster coming from their own minds and other catastrophes. (We know that Ascians can summon meteors too) Then other cities start to talk about this, fear takes over in their minds and make it worse. Amaurot hears about this but continues to only discuss if they should help or not and caught one of the created monster to study it. They only seemingly do something when its right at their own door and the world is already beyond saving.
Also its interesting to see how they do handle bad situations. The side dungeon shows an NPC that cant believe that something went wrong, while inside the building Ancient ones die left and right to their own creations. (Funny how such a strong race seemingly die so easily to these creations) At the end the NPC is still not overly concerned with the situation, even though a huge beast got out (and stayed out since we can fight it) and quite a few Ancient ones died..They might have been powerful but the quest already showed that they were far from perfect and that simple things could change their creations immensly. Also they too had human emotions, including the negative ones and they were forced to wear the robes in the fear that individuality could lead to negative outcomes. I have no problem seeing that not every single being on the planet would conform to this.
Also if Zodiark had to rewrite the laws of the planet then wouldnt he destroy the source too, to stop it from happening again? And with that, even if it had been a third party, it would not be important to our story anymore.
Of course it could also be like Zohar_Lahar pointed out and they just used too much of that power over time. No matter the regulations you will always have those that dont care about this. And we do know that they too had people without that huge amount of aether. (Which is why that one NPC has a solution for us) Maybe those tapped a bit into the life of the planet itself and over a long time this small amount can leave traces. This could then be used as a lesson to learn from: Its better not have that amount of power and be happy with the way we are now.
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About something else: I watched a stream again about the shadowbringer story and I saw a interesting scene (which I did kinda forgot). Ryne talks in Il Mheg about her role. She says that if we could not be summoned, she would be the one to kill the Lightwardens.
So I started to wonder: Why did nobody, even her, ever think about the possibility of sharing the light? First I thought that she might not be able to hold in much because she is just 1/14 of the soul, but after joining with Minfilia (a source soul, who had 8/14 too) she should be on the same level as we are. Which means that after the joining and getting her own name, she could have easily taking in some light, right? Why not (seeing how it hurt us) at least help with the last two? Why not have at least a scene where she tries to do that but we stop her because we believe that its our burden alone? The light never was fully great for us but only after the last two lightwardens, did we truly felt it and have it negatively impact us.
I know its probably because the plot had to go a certain way but I dislike it when it feels like such solutions are completely overlooked. Or did I miss something?
Also another question that then popped up. Why was the crystal exarch not turned into a lightwarden? We know that anyone without the blessing would turn as soon as they got the light into them. He came and took a bit of this light out of us until he was stopped so why is he still fine? Also would he have really managed to even reach a place to die before truly turning?