The questions I want EW to answer the most: WHY and HOW was the world sundered?

[Veloran]

Can she, though?

We neither know what the threshold to overpower the Echo would need to be, nor whether it's based on "power" in a general sense to begin with. For example, Zodiark was far more powerful than Hydaelyn, but the Ancient's Echoes were far more powerful than WoL's Echo.

but there's no reason to believe a primal couldn't bestow power upon a mortal without tempering them.

Do we have any examples of this being the case?

Tempering is kind of a big deal; when exactly did that happen?

If it's a subtle form of tempering, I doubt anyone would be able to tell.

Last... there are numerous instances of Hydaelyn's blessed chosen defying her will, as has been pointed out before.

And as has similarly been pointed out before, it may be that her tempering isn't actively controlling anybody. It's been building for years now that tempering comes in many different forms and some are all but undetectable.

The very worst she's been portrayed is as a cosmic level Urianger: deceitful but well-intentioned. Compelling anyone to do anything doesn't fit her character as it's been established.

Well-intentioned maybe, but not just deceitful. I fail to understand how you can say that compelling people to follow her will isn't in her character when A, she draws every Echo-user into following her cause without telling them the truth the moment they awaken it, B, she has given explicit commands in the past for her followers to carry out, and C, she forcibly sundered the entire world against the will of the majority of the Ancients. In particular with that last one, it shows a willingness to disregard people's lives for the sake of her mission.

[Cilia]

We neither know what the threshold to overpower the Echo would need to be, nor whether it's based on "power" in a general sense to begin with. For example, Zodiark was far more powerful than Hydaelyn, but the Ancient's Echoes were far more powerful than WoL's Echo.

Point is no one can definitively say whether or not that's the case. An undefined value doesn't constitute evidence. "But it's possible...": see below.

Do we have any examples of this being the case?

Remember: this is always technically true. This is important. Remember this.

If it's a subtle form of tempering, I doubt anyone would be able to tell.

Tempering has never been subtle. If you insist on pressing "But it's possible...", see above.

And as has similarly been pointed out before, it may be that her tempering isn't actively controlling anybody. It's been building for years now that tempering comes in many different forms and some are all but undetectable.

If it's neither actively nor passively controlling anybody, it's not really tempering. Bears repeating: Hydaelyn has never compelled anybody to do anything. If her blessed chosen could be compelled, why would she not compel those who defy her will and invite catastrophe to, you know... not do so?

Well-intentioned maybe, but not just deceitful. I fail to understand how you can say that compelling people to follow her will isn't in her character when A, she draws every Echo-user into following her cause without telling them the truth the moment they awaken it, B, she has given explicit commands in the past for her followers to carry out, and C, she forcibly sundered the entire world against the will of the majority of the Ancients. In particular with that last one, it shows a willingness to disregard people's lives for the sake of her mission.

People hear Hydaelyn's call and assume she is a goddess; that's a misinterpretation on their part due to limited knowledge.

I don't recall any explicit, let alone compelled, commands from Hydaelyn. "Fight the Ascians," "drive out the Darkness," etc. are either eminently necessary for self-preservation or incredibly vague. She never says "Go to Ishgard and kill the Ascians pulling their strings because I'm a goddess and I say so," or such. We haven't even heard from her (directly at any rate) since 3.0.

Bears repeating: now armed with truth, we haven't gotten Hydaelyn's side of the story. "She didn't care about killing lots of people" is the least generous interpretation of her actions, but it's just that - an interpretation without context. Maybe it was an accident. Maybe it was a grim necessity for a greater purpose. Maybe she was just following her programming regardless of consequences. Or maybe she really did say "Screw these guys!" and break the world for shiggles. We don't know.

[Veloran]

"But it's possible...": see below.

This is not a good argument. It's only natural to look at what is possible within the established rules of the setting, compare and contrast relevant information, and draw conclusions based on the available evidence. In the case of the Echo being overwritten by tempering, we know it is possible, if not strictly how, and so to look at Hydaelyn and Zodiark, both of them sharing origins with the Ancients and being immensely powerful Primals capable of massively altering reality, it is completely reasonable to suggest that if one could temper the Ancients, the other could likely temper WoL. This "Well ANYTHING IS MAYBE POSSIBLE" as a response is just anathema to discussion.

Remember

So no, you don't have any examples?

Tempering has never been subtle.

But it absolutely has been. The Heaven's Ward were tempered, yet their behavior was consistent with what it was previously and it went unnoticed. Nobody really knew whether or not Ga Bu was tempered or if it was simply the trauma of losing his parents for years. Multiple tempered infiltrators have maintained their covers underneath the Alliance's noses. And while some were outright zealous, Emet-Selch and a few of the other Ascians took actions that would simply be contradictory to the notion that they were tempered, at least how we would usually conceive of it. This is particularly relevant to the situation with Hydaelyn because the only Primal which is anywhere analogous to her is Zodiark, meaning that if his tempering allows for such behavior, it is reasonable to say that hers may as well.

Bears repeating: Hydaelyn has never compelled anybody to do anything.

She compelled Ardbert to stay his blade when he attempted to attack Minfilia. If you would say that wasn't due to tempering, I would say her power to do that would be extremely inconsistent with her stated status of being too weak to lift a finger towards anything.

If her blessed chosen could be compelled, why would she not compel those who defy her will and invite catastrophe to, you know... not do so?

It may be that in many of those instances, what those characters were doing was Hydaelyn's will. Consider: Ysayle's summoning of Shiva would set off a chain of events which led to the end of the Dragonsong War and united dragon and man, as well as saving WoL's life. The Warriors of Darkness worked with the Ascians, but in this instance the goals of the Ascians and Hydaelyn, to not see the First consumed by Light and destroyed, were aligned. Indeed the Ascians needed Hydaelyn to intervene in order to save the shard. Furthermore Minfilia would spare Ardbert the fate of his friends, knowing that he would be important much later down the line. Even the summoning of Ultima and introduction of auracite to the world would prove to be important much later, as the study and development of white auracite was vital in killing Ascians.

This is a universe where some people with the Echo can quite literally see the future. And Ardbert being spared with both he and WoL being told that he would serve an important role really is not explainable without some form of foresight on her part. I would even suggest here that the reason it had to be Venat who served as Hydaelyn's heart is because she was gifted in this ability.

People hear Hydaelyn's call and assume she is a goddess; that's a misinterpretation on their part due to limited knowledge.

The knowledge is limited and misinterpretation made because of Hydaelyn. It was the intent of her and her summoners to hide the history of the world.

I don't recall any explicit, let alone compelled, commands from Hydaelyn. "Fight the Ascians," "drive out the Darkness," etc. are either eminently necessary for self-preservation or incredibly vague.

I would think that saying the Ascians "cannot be suffered to live" and telling WoL to destroy them is a fairly direct commandment, if not one which is immediately actionable.

Or maybe she really did say "Screw these guys!" and break the world for shiggles.

This is a disingenuous interpretation of what I was suggesting. My point here is that she clearly considered her mission important to the point of taking lives - Everyone's lives - For the cause. Even if this is ultimately for completely justifiable reasons, if circumstances repeat themselves and we find ourselves in that situation again, do you really think WoL would be willing to accept that? Azem wasn't the first time around.