Quote Originally Posted by Edax View Post
According to the wiki : "At some point, a mysterious "sound" emanating from the depths of their star caused the Ancients to lose control of their Creation magic, unleashing monsters formed from their subconscious fears. "

Now what caused the sound, we don't know. It may have been the result of some malicious design to destroy The Ancients. It appears even the Ascians never figured out the cause in the years after the Sundering and if Emet managed to Rejoin the Star, history may well have repeated itself. I assume the Ascians were complacent because they assume they'll have Zodiark to prevent the effects and thus never looked too deeply into the cause. This is pure speculation on my part.
Something interesting to note about sound in general is that in real life, there are certain frequencies of sound that can induce anxiety, fear and even hallucinations in humans.

During the Magic DPS Role quests, we discover that a mountain is on the verge of collapsing because the aether is being sucked dry via the crystal and the various nasties that were attracted to it.

We also know that Primals devour aether. Primals are created via Creation Magic. From the Lakshmi stuff in Stormblood as well as just general evidence from ARR up to this point, The amount of aether used for the summoning only dictates the size and strength of the Primal.
It could be argued that a Primals devouring of aether is due to an intentional flaw in the process to specifically cause problems for Hydaelyn. But we also know that the Primal in question is formed by the minds of the those who conduct the summoning. So theoretically, it should be possible to summon a Primal that DOESN'T require continual sacrifice of aether to keep it 'fueled'. Which is sorta supported by Bahamut. IIRC, at some point I read ingame that he's considered an 'Elder Primal' and doesn't need external aether. (I could be misremembering)

During one of the Side Quests in Amaurot to make the robes, we get sent out to capture a Cubus to use as fuel for Creating the robes. A device is then produced that converts the captured creature into aether to fuel the robe concept. Only reason we don't make the robes is because we don't know how to use the concept.
Additionally, when the clerk mentions how we don't seem to have any aether whatsoever, he goes to on to give this line {We have special tools that may be used to address this very eventuality.}
That statement implies that the inability to inherently provide Aether for a Creation is something that happens often enough that warranted specialty tools to be developed to get around the limitation.

Which throws doubt onto Emets statements about the aetherial capacity of every Ancient being limitless. If it was limitless, why then would they have a special tool for the "eventuality" of an Ancient incapable of providing their own fuel for a Creation?

To bring this all back around to the sound and its cause, just because Emet and other leading figures in the Ancient world had near limitless aetherial capacities and thus created self contained entites that needed no external aether source, does not mean that every Ancient who ever created a thing was able to do that.
We also know that the tiniest lapse of concentration irrevocably alters the outcome.

If enough Ancients created enough things that drained inconsequential amounts of ambient aether from the land over a long enough period of time (we don't know the scope of their history), then conceivably the Sound was caused by an event within the core of the planet not unlike the collapsing mountain scenario from the Role Quest.

If such an event caused a volcanic eruption of significant force, ejecta from this eruption could easily be thrown into the atmosphere. This would cool and eventually fall back down to the planet, striking regions across the entire planet as we see in the dungeon.

If the Sound caused by such an event was at the appropriate frequency to cause those nearest to feel fear and hallucinate at the same time, anything they attempted to Create would be altered.
Consider the following hypothetical scenario:
-Ancient is affected by the sound and hallucinates.
-Ancient attempts to use Creation Magic to Create something to defend themselves from this hallucination, possibly thinking "Oh no, its going to kill me!" at the time. All while meteors are slamming down around them.
-Concept is tainted by the hallucination, instead of creating a guardian they literally give form to the hallucination which promptly attacks the Creator.

Now imagine that happening to an entire city at the same time. Even those unaffected by the sound itself will find themselves under assault.
The creatures created in this way decimate the city and then roam freely. Potentially seeking sources of aether which they would find in the citizenry of other cities.

But that's just a theory I had during a long work shift...