Patch 5.4 Trailer, Predictions, and Hints

[Kesey]

snip.

It's brought up a few times in ARR by the Ascians to show how close they are to realizing their goals, but to be clear an 8th rejoining just lets Zodiark out of the moon. And the alternate timeline clearly represents itself as being in the middle of an apocalypse even if Zodiark isn't seen. Also the passage of time seen in alternate future is explained too, because the summoning of Zodiark is just a delay to the end days and not an actual solution to the end days. The Ancients that follow Venat and summon Hydaelyn explain this.

So most likely the chain of events are as follows: 8th calamity/rejoining is achieved, Zodiark comes out of the moon, Zodiark causes a world of chaos because he can rewrite the rules of existence, because Zodiark is a primal and rewriting consumes aether he drains the star of aether (this is the apocalypse and takes place over a long period of time), the lack of aether leads to the final days (albeit not immediately).

This is why Hydaelyn was summoned in the past, most likely to solve the Zodiark aether follow problem, as apposed to actually confronting Zodiark. Now to further back up this, the WOL in 5.4 remembers the cave painting in Rak'tika where it shows Hydaelyn and Zodiark standing together. This implies that Hydaelyn and Zodiark co-existed and then something happened leading to their confrontation. Furthermore, since Azem didn't participate in summoning Hydaelyn or Zodiark, this is most likely the part of the story Azem did participate in. Hence why we have the Azem stone and think about it while Scions are having another conversation.

So in theory the final days of the past play out like this: Creation magic drains aether from the planet causing the final days, Ancients summon Zodiark to rewrite rules of existence to stop the final days, Zodiark is a primal and even though he saves the world is still draining aether causing the final days to just happen in the future, Ancients tempered to Zodiark refuse to listen to Ancients not tempered about the reality of Zodiark, those Ancients follow Venat because debate and discussion are the corner stone of their culture and now that doesn't happen because of tempering, they summon Hydaelyn specifically to deal with the aether flow problem, then the missing event transpires (here is where Azem is most likely involved with a potential 3rd solution), that event leads to Hydaelyn and Zodiark confrontation, Hydaelyn triumphs and creates the shards so Zodiark can't access his power.

I'm sorry this was a longer answer than you were anticipating, but in my mind all of these bits of lore are inter-related and are coming to fruition.