I put some thoughts in this (ongoing) theory.

We know that primals need aether, crystals and prayers (actually, feelings are enough. See Enkidu). The aether is drawn from Hydaelyn continuously too.

The bigger/more powerful the summon is, the bigger crystal you need, and the bigger strain on aether resources there is.

Now onto Hydaelyn. As an enormous crystal, and with the precedent of Bahamut Prime (which was able to interact with us), we could perfectly think that she is a primal. Or something else (I'll develop that point later).

In that case, summonings would draw both crystals and aether from Hydaelyn, weakening her available resources every time and thus endangering her as a primal being (to kill a primal, you either need to cut off its primary resource by sucking all aether within it dry, or destroy its physical flesh to let the remaining aether return to the world). In other words, summonings would deprive Hydaelin from both her air and food (aether and crystals).

Though, there is a little detail that feels horribly wrong with that : where would the aether used to maintain Hydealyn's existence come from ? Perhaps from the gods themselves, or from Zodiark, sealed somewhere within Hydaelyn to serve as an infinite aether source. This also opens way for the statue we see in the chrysalis and which is supposed to represent Zodiark, and look nothing like Hydaelyn.

Hydaelyn would then become an enormous gaol, sealing Zodiark by depriving him of his aether and thus preventing him to free himself on his own.


Or, and that brings the point I talked about earlier, Hydealyn is truly something else. We are told that the gods were there at some point. Zodiark too probably. And midgardsormr came from space and stayed here to watch over the world and fulfill his covenant with Hydaelyn. Though legends also say that the Twelves themselves asked him to stay in Silvertear.

So I've been thinking, what if something happened between the Twelves and Zodiark at some point, and then they decided to seal him using all their power, forming the gaol with their bodies (or whatever else gods may have. Pure aether bodies maybe ?) and trapping Zodiark eternally within them. The gaol in this case would be the planet itself, and Hydaelyn as we know her would be the collective consciousness of the Twelves in their eternal slumber. The Twelves would have then asked Midgardsormr to ensure that nothing can free Zodiark from the exterior, and thus joining both stories as one.


Hydaelyn in this case would be the usurper as the gods would have stripped Zodiark from everything he had, keeping it for what was left of them (the mother crystal). Summonings and great destructions on the land would weaken the gaol and help zodiark free himself, while summoning too much would also weaken him as he is forced to be part of the planet in a lesser extent (being sealed in the Mother Crystal, source of the aether on the planet).


I though of this thanks to dalamud. Sure it was an artificial satellite created by the allags, but it actually was a gaol made to contain a super powerful being and used to gather energy. Similar to how Zodiark would be gaoled in the mother crystal, contained by the other gods (keep the FFXII's lore, he ought to be more powerful than any of them ), and "used" to nurture the life on the planet by providing a continuous stream of aether.