FFXIV EVERCOLD SPECULATION!!!!
I think, just via watching the new trailer and the keynote on how this new saga is about "Uniting the stars," AND how the trailer specifically calls out 'The Traveler' aka Azem, so we're gonna be seeing what Azem was doing during the Sundering.
I think, since the Winterer's seem to be preparing for a 'great freeze' as in due to Zodiark dying, the Source and Shards aren't bathed in massive amounts of ambient aether imbalance everywhere, thus they start to 'wither' and 'nature's proclamation' is that the lives of the Source and Shards can't be currently sustained by the amount of aether now in the system of life. It's an unnatural way of living, not due to souls, but the fact that the environment itself suffers. Even though we can survive in harsh conditions, other life and the world can't always after all.
I.e we killed the biggest aether battery in the star, and now we're in a aether deficit.
Zodiark was born of the star being sundered into 14 Shards, the current arrangement of the stars is unnatural. It's not how 'nature' is supposed to be. By bringing it together it stands to reason it would be fix the 'withering' like if you cut off enough tree branches the original dies.
Now, 'that's what the Ascians wanted to do,' well yes but the way they wanted to was the equivalent of smashing a meteor into Earth and hoping it reforms slightly bigger while also killing off everyone they considered inferior.
With Azem the Traveler's key, someone whose entire job was to unite people, I think their magic is the key to bringing the stars together safety without mass death. It would make sense their magic works like that and why so many people are coveting the Key. It would also give chance to show how Venat did the Sundering in the first place, as she being the former Azem, would have access to the same type of magic. So it makes sense we'd be able to 'learn' and end it properly.
Azem's duty was Shepherd to the Stars in the dark and it's a Shepherd's duty to bring their flock back together safely.
















