Is 'Hydaelyn' a misnomer based on a misconception?

[Cilia]

There is a void of information on that matter. We kinda know why she was summoned. But I don't think we have any idea what her concrete purpose and directive was (is) that the creators bestowed on her? I doubt she was just manifested as an ideal. Defeating something much stronger would imply some premeditation.

If we look at Emet-Selch's account of events as told after the Qitana Ravel, and use Hythlodaeus' account to filter out his pro-Zodiark bias and gain some additional context, Hydaelyn's purpose becomes explicitly clear.

[Zodiark was a] savior mighty and magnificent, deserving of reverence and gratitude... one would have thought. Yet some thought otherwise. From the fears of these naysayers would rise Hydaelyn - She who was to serve as His shackles. To bind Him and hold Him in check. And so they fought, and they fought, and they fought. And in the end... Hydaelyn was victorious. With all Her strength She smote Him - dealing a blow so devastating that it split the very fabric of reality. And thus was Zodiark banished and His being divided.

The cycle of life had begun anew, and we reconsidered the means by which we might protect it. The Convocation decided thus: we would nurture our world until it was bursting with vitality. Then, when the time was right, we would offer some portion of its living energy to Zodiark... In return, He would restore to us those brethren whose souls had fed His strength, and together we would resume our role as stewards. There were, however, those who disagreed with this plan. They argued that enough had been sacrificed to Zodiark─that this new world should belong to the lives newly born. These dissidents surrendered their life energies in the creation of Hydaelyn, an incarnation of their opposing belief. And for the first time in history, our people stood divided... Know you, then, how this conflict ended?

Hydaelyn was created to serve as a check against Zodiark's nearly almighty power and to stop his supporters from perverting the cycle of life and death.

Her ability to split and bind Zodiark stems from her elemental affinity - as we now know Light is the element of stasis and enervation, and it would be needed on the scale Hydaelyn can use to do so. According to Emet-Selch's account the finishing blow on Zodiark was so grievous it alone tore reality into the shards and Source, and unfortunately that's one bit Hythlodaeus' account doesn't give additional context to, but the conclusion I draw from the information we have is that Zodiark's whole was too great for her to bind, so Hydaelyn broke him down into pieces to do so... but because Zodiark is "the will of the Star made manifest," that did the same to the world in the process.

Further information may change or give additional context to this, but it is the clearest picture available of the origins of the Hydaelyn / Zodiark conflict. It's not vague in any way.

While I agree that Hydaelyn probably is not as powerful as Zodiark was, I AM having my doubts that she's a Primal, or at least if she is, she's a very different sort of Primal. The following is speculation on my part, but I think it fits the facts we have so far: Where Zodiark is a will of the planet the Convocation created to suit their goals, I think she's the TRUE will of the planet, given form. The planet was suffering, on the verge of being torn apart by the Ancients' Creation magic - magic they were using with abandon, not realizing the toll it was taking. Even BEFORE the Sound emerged, the world was on the path to destruction, thanks to these abuses. The Sound was the only way she could respond - at least until the Dissenters stepped in to provide her with a form that gave her more agency (and possibly sapience, as well).

While I am not of the mind that Hydaelyn is the "true" will of the planet, I concur that the Ancients' heavy use of creation magicks may have been directly or indirectly responsible for the Terminus. There is a cause behind everything.