Pure speculation incoming...

-Zodiark existed before the world as we know it, thus all of reality occurred after Zodiark and (we can assume) the current world somehow is mutually exclusive to whatever Zodiark lived in before.

-The Ascians are largely not of this world and cannot interact with it.

-Hydaelyn is shown as a bright crystal in darkness, and a dark crystal is shown in a vision (possibly Zodiark or Hydaelyn...)

-There are as many Ascians as gods (twelve).

There's a lot of FFXIV that is based on duality, so it may be that Hydaelyn and Zodiark are actually the same being, with the Ascians attempting to return Hydaelyn back to Zodiark in order to retake the place of the Twelve (who may have supplanted them during this era). It could be that, before this era, Zodiark was Hydaelyn and the Ascians were in the position of the Twelve, and it was actually the Twelve who were attempting to break the status quo.

Just speculation, of course, but everything in FFXIV seems cyclical.