Aether

[Niwashi]

As the red moon Dalamud was descending, Loisoix sent adventurers to pray at shrines to the Twelve, summoning them to aid Eorzea in hopes of preventing what came later to be known as the Calamity. Some subsequent conversations discussed the relationships between aether, gods, and the land itself:

Papalymo:
You have my thanks also. Truth be told, I had feared you would be reluctant to help. Summoning deities requires an ungodly amount of aether, after all. And having manifested, sustaining even a single deity's corporeal form costs the land dearly--as you've seen for yourself. Yet here we are, seeking to summon no fewer than twelve of them!

Channeling even a tiny portion of a deity's strength will tax you greatly. Fortunately, no more than that should be needed to stay the red moon's advance. At the very least, it should be sufficient to nullify Dalamud's power and ensure Eorzea's preservation.

For an added blessing, the process should see some aether returned to the land--aether that Dalamud had absorbed from the primals.

and a bit later:

Thancred:
These eyes will not be fooled--something weighs heavily on your mind. If I were to hazard a guess, the prospect of entreating the succor of the selfsame beings you once slew seems less than attractive? Heh, I'll not deny it's ironic.

As you well know, aether is the source of all life. Bereft of its blessing, Eorzea would be naught but a dried-up husk of a realm. By whatever name they're called, gods drink of the land's aether for their sustenance. The Twelve are no different. Summoning them would be tantamount to bleeding the life out of the land. Yet without their aid, Eorzea is most assuredly doomed. Dalamud will fall, and the land will die. Even should aether still flow, life may never again blossom here. We cannot allow that to come to pass.

Even as we move to stay Dalamud's descent, ever must we be mindful of the toll our actions take upon Eorzea's longevity. We spare the present at a cost to the future. It will be no easy thing, this balancing act. Yet we must see it through to the bitter end, no matter the hardship.

[Nayto]

I'll drop the info that I stumbled upon while speaking to Elric, the Monk quest giver. Unlike the other jobmaster's/guildmaster's. He seems to go over what he studied with you during the questline (and.... very long text to add, don't talk to him if you qued for something, lol).

Erik: I shall tell you yet again of the study of aether.
Erik: As I laboriously explained, aether is not only the source of all magicks, but also the fount of all life. Yet despite its ubiquity, it remains imperceptible to the senses of man. When a living thing dies, the aether comprising its life is released.
Erik: It has been learned that when this discharge takes place, a portion of that aether remains, lingering in the physical world. No doubt you have come across aetherial crystallizations in the course of your travels.
Erik: All scholars now agree that these are the physical manifestations of great concentrations of aether.
Erik: I have a theory, however, that they are not the only such manifestations.
Erik: Allow me to give an example even you can understand─that of ghosts, spirits, disembodied souls. Apparitions of the deceased come back to haunt and generally unnerve us? False!
Erik: These are nothing more than the luminescent glow of aether in the atmosphere. It is possible, though highly unlikely, that you are asking yourself why a portion of the aether remains.
Erik: Right, well, the amount of aether that can shift between the physical and aetherial realms in any given instant is limited. I call this the aetherial threshold. Any aether present in excess of that threshold is left behind.
Erik: The more violent or dramatic the loss of life, the greater the amount of inner aether released to take the form of crystals or remain imperceptibly in the atmosphere. And you surely recollect the scene I identified as the most apt to produce such violent deaths?
Erik: On the field of battle! It stands to reason, therefore, that through the aetherial measurement of such sites, I will be able to reconstruct details of the wars waged upon them.
Erik: And that, [First Name here], is why this military historian has a deeply vested interest in the subject of aether.
Erik: Despite all we do know, however, many mysteries remain. The study of aether will no doubt continue to yield fascinating and awe-inspiring discoveries for years to come─of that I am certain.