The Reasoning behind The Sundering? (*6.0 Endwalker Spoilers*)

[Lyth]

Before we get too excited at the opportunity to blame every adverse event in human history on Venat (not to mention all the missed possibilities for marketboard manipulation), do we know what actually happened in Elpis before we travelled back to ever so subtly alter it? The current sequence of events involving Elpis wasn't even possible in the original Eighth Umbral Calamity timeline, until G'raha altered it to the current one. So there would need to be an original Elpis timeline which predates our time travel decision, or else this whole thing is a paradox.

'You may find your world to be very different. Or perhaps the erasure of our friends' memories has sown the seeds of a conjunction between us.'

And it's that phrase again.

It also creates some questions about Pandaemonium. Why do we have to involve ourselves with it when it's been already resolved in the past? Why does our past self know that Elidibus can expect us to arrive? Or alternatively, by encountering him, have we unwittingly put the timeline at risk of diverging by taking our past self's place in the story? Why does it matter if the timeline diverges?

[Rosenstrauch]

Before we get too excited at the opportunity to blame every adverse event in human history on Venat (not to mention all the missed possibilities for marketboard manipulation), do we know what actually happened in Elpis before we travelled back to ever so subtly alter it? The current sequence of events involving Elpis wasn't even possible in the original Eighth Umbral Calamity timeline, until G'raha altered it to the current one. So there would need to be an original Elpis timeline which predates our time travel decision, or else this whole thing is a paradox.

It's only a paradox if you assume that there are two Unsundered Etheiryses, one for each wholly separate timeline in which every single event up to late Stormblood was exactly identical, and that the events that played out in both Unsundered Etheiryses are also completely identical even though our death in the Eighth Era timeline would make that impossible. There is no paradox if there's only one Unsundered Etheirys, one timeline up to the Eighth Era, with a second timeline branching off at the point where G'raha changed the past.

We didn't change the past at all. Or if we did, it obviously didn't count enough to create a new timeline, as much as I hate saying that because it makes no sense (but then, neither does the story the term "Butterfly Effect" comes from). When she says "conjunction", she's referring to a closed loop. When she later refers to the conjunction being completed, it's because we've confirmed for her that we'd come back from Elpis—the threshold for accidentally changing the future has already passed, and she can finally speak freely with us.

EDIT: I went and made a crappy image to illustrate the timeline as I explain it:

As far as "what did she do based on what we said", it's important to keep in mind that Venat couldn't possibly have a full roadmap to work with. The only events we could've relayed to her are those that we were actually there for, or were told about. We wouldn't know which Reflections were consumed by the first seven calamities, or how many attempts the Ascians made aside from the successful ones—to say nothing of knowing which attempts would be successful and which wouldn't for the first two. We also wouldn't know the names of the vast majority of past Warriors of Light—the only ones we learn about at all off the top of my head were Ardbert and friends, Cylva, Unukalhai, Tenzin, Ramza, and Ajora. Oh, and the eponymous (yet nameless) Warrior of Light from Norvandt's folklore.

Because of that, I wholly believe Venat when she says she'll take nothing for granted, and will try to make the best choices she can. That doesn't mean she succeeded in doing so—and obviously a lot of folks disagree with the notion that the Sundering was even a good choice, let alone the best.

On a related note: We're the first person in the story to ever travel forward in time via time travel magic. Because of that, I feel inclined to mention it was entirely possible that we could've change the past to the point where our future was "no longer possible", taken our portal to the future, and still arrived in our own time with events completely unchanged. Venat does not mention this possibility, which I feel is an oversight—or perhaps evidence that time travel isn't something well understood in-universe.

Things are getting really weird when it comes to Pandaemonium, though. It's just kind of silly that we casually travel back in time again, for no reason other than to sate our curiosity, only for it to turn out that a character who shouldn't have precognition was somehow able to accurately predict the moment and destination of our arrival and direct a man who is obviously Elidibus to partner with us for the exact thing that we were looking for, which he just so happened to also be looking into. And we're going to do it again. Twice.

The whole "Azem foretold your arrival" thing seriously has me scratching my head. That's not how our Echo works, which leads me to believe Azem knew we'd arrive because they were the one who sent the soul crystal to us. And it's really hard for me to buy that this is a closed loop because... well, what would even be the point when they could've just handled things themselves?

[Lyth]

The problem is this. In the original Eighth Umbral Calamity timeline, Azem didn't travel back in time to Elpis because Azem is dead prior to that point. So there's some sequence of events that originally happened on Elpis that didn't involve time travel, and in which nobody comes out with any knowledge of future events. We don't know how that played out. We don't even know if Meteion ever had the courage to give her report to Hermes or if she was ever identified as the true cause of the Final Days. After all, it's Azem's questions about Dynamis and knowledge about the future that lead to the situation being examined more closely and ultimately force the showdown on Ktisis Hyperboreia.

When G'raha alters the Eighth Umbral Calamity timeline, it creates a new timeline that diverges from the original. That raises a flag that actually allows for the time travel event in Elpis to actually occur in the first place.

As long as key historical events remain unaltered by Azem's time travel journey, you now have a nice closed loop that replaces the original sequence of events without any net change to the past. This is an important distinction, because you still could conceivably alter something in the past that only had relevance to an event that hadn't yet happened, such as your showdown with Meteion in Ultima Thule, while still keeping the original timeline intact. If there is a net change to past events, the timeline frays off into a set of divergent timelines depending on what changes. As Venat notes, Kairos fairly important, because the memory wipe allowed a conjunction to form between the two Elpis timelines. If Emet, Hyth, or Hermes retained their memories, the present would likely be significantly altered as a result.

Now there's the question of whether there are consequences for splitting a timeline. It seems convenient enough to be able to just rewrite events at will and ignore the frayed ends. For example, Elidibus explicitly warns you against trying to prevent the Final Days. Is there a reason why we couldn't just do what G'raha did in this instance, and more to the point, are there consequences that we'll have to pay down the line for splitting off the Eighth Umbral Calamity timeline?

I don't think that Azem predicted that they would be time travelling back to Elpis. It's also unlikely that Venat told Azem this. The most likely explanation is that in the original sequence of events, Azem told Themis to wait for their signal to explore Pandaemonium, and then the two of them headed off inside to explore it together. This time around however, you've messed up the sequence of events by showing up and meeting Themis in past!Azem's place, which probably places the present in jeopardy unless you can find a way to reconcile the two timelines.

[Rosenstrauch]

The problem is this. In the original Eighth Umbral Calamity timeline, Azem didn't travel back in time to Elpis because Azem is dead prior to that point.

What makes you think the Unsundered Etheirys we arrived in isn't the same one from the Eighth Era timeline?

[Lyth]

You can't time travel if you're dead. That's why you can't take it as a given that the Elpis time loop always existed. There is no time loop in the Eighth Umbral Calamity timeline because Azem is dead.

[Rosenstrauch]

You can't time travel if you're dead. That's why you can't take it as a given that the Elpis time loop always existed. There is no time loop in the Eighth Umbral Era timeline because Azem is dead.

This is irrelevant if there is only one Unsundered Etheirys. Only a WoL from one of the branching timelines has to travel back in time for both branches to be affected by it, because both branches are the same timeline prior to the moment they split.

[Lyth]

There are two issues with this.

First, the Eighth Umbral Calamity timeline still exists, even after G'raha alters it. In that timeline, Azem is still dead and cannot travel back in time to Elpis. You can't solve this problem from another timeline. You've just created a Grandfather paradox. That's why Biggs the Third writes 'We shall remain forever on different pages of history─and different books, besides'. The events of one no longer affect the other.

The second, what you're proposing hinges on the idea that everything up to Elpis was always destined to happen and nobody had any free will in the matter. Until G'raha makes the decision to go back and change the timeline, the time travel event is not possible. As a result, there has to be a set of events on Elpis that predate this decision. It can't be a completely closed loop. That's also why Venat says that 'a conjunction has begun to form', and not 'this conjunction was always destined to form'.

[Rosenstrauch]

There are two issues with this.

First, the Eighth Umbral Calamity timeline still exists, even after G'raha alters it. In that timeline, Azem is still dead and cannot travel back in time to Elpis. You can't solve this problem from another timeline. You've just created a Grandfather paradox. That's why Biggs the Third writes 'We shall remain forever on different pages of history─and different books, besides'. The events of one no longer affect the other.

Yes, it still exists. See: my image, where I account for this.
The WoL being dead is irrelevant. They don't have to travel to the Unsundered Etheirys, because we already did so. See: my image, where I account for this.
The Grandfather Paradox ("If you travel back in time to kill your Grandfather, you will not be born, and thus can't travel back in time to kill your Grandfather") is only relevant in scenarios where traveling back in time negates the possibility of doing so. There are a few solutions to this paradox, such as "you can succeed, but you'll cease to exist", "you can succeed, but your past will cease to exist", "you can succeed, but you'll split the timeline", and "you can never succeed, something will always stop you from doing so". And probably more.

G'raha traveling back in time to undo the Eighth Umbral Calamity ran headfirst into the Grandfather Paradox. The writers' solution to this was the continued existence of the time traveling G'raha Tia, the time traveling Syrcus Tower, and short story "An Unpromised Tomorrow" demonstrating that the future he left continued to exist. All of these combined are the third solution I presented.

The WoL in the Eighth Era timeline (who is dead) not traveling back in time to Elpis is not an example of the Grandfather Paradox, because they're not the one who traveled there in the first place.

Biggs the Third is not an omniscient observer. He has no idea what the timeline actually looks like, and anything he says about it is pure speculation.

The second, what you're proposing hinges on the idea that everything up to Elpis was always destined to happen and nobody had any free will in the matter.

Not quite. From the perspective of an omniscient observer, every action is strictly deterministic. From the perspective of those acting within the timeline, their choices are entirely their own.

Until G'raha makes the decision to go back and change the timeline, the time travel event is not possible.

That a person's future actions aren't possible until past actions have taken place is irrelevant. This holds true even if the future actions take place in the past, and past actions take place in the future.

As a result, there has to be a set of events on Elpis that predate this decision. It can't be a completely closed loop.

This is unfalsifiable. There is no observed set of events that were negated by a time traveler causing a closed loop.

That's also why Venat says that 'a conjunction has begun to form', and not 'this conjunction was always destined to form'.

What she actually says is this: "Or perhaps the erasure of our friends' memories has sown the seeds of a conjunction between us."
The word "perhaps" is important here. As with Biggs the Third, Venat is not an omniscient observer. At the time of our departure from Elpis, she is not aware of whether or not she's within a closed loop. It's only after we meet her post-Aitiascope that she confirms she was—and by that point, the loop is already closed.

[Cilia]

From our perspective the events are a closed loop. That doesn't mean it's the only possible way events can play out, just that it's the only possibility we can observe.

[Veloran]

A random thought here, but don't the two timelines need to be distinct due to the presence of Alexander? At some point in the future Alexander is going to unfreeze from his time bubble and travel back into the past to deposit Dayan and Mide there to found their tribe which eventually leads to the events of the modern day that precipitate Alexander's summoning to begin with.

But because there are two timelines there should now be two versions of Alexander. Ours and the one in the 8UC timeline. So if the timelines are supposed to be one that branched due to G'raha, how can two Alexanders travel back in time to the same place along one timeline to do the same thing, with two pairs of Dayans/Mides?