a multi-theory ramble about soul energy in aurora
let's start with things we know about soul energy.
soul energy exists ambiently in an unstructured lattice, everywhere
and when something has both a body and a soul, it has a mind
soul energy, in-universe, is understood to be separate from the elements, as no one can control it
these are the big facts i'm gonna be working with here.
it's also a pretty well-known theory that kendal-the-person's soul came from semi-structured soul energy that pooled up in kendal-the-sword, much like how the sentinel outside the city had developed a rudimentary soul and mind after so much time and usage from vash.
Let's take fact two. A god without incarnation is just a soul, no body, and thus does not have a structured mind. Add in fact one. Unstructured, loose lattice. I'm going to theorize that a complex soul leads to a complex mind. Animals probably have souls, but I imagine they're rudimentary compared to humans, elves, et cetera - and so too are their minds. Instinctive, focused on survival both of the self and of the species.
It's also well-known that, because Light and Void transmute into each other, they're likely two sides of the same coin, or two parts of a whole. The most common assumption I've seen is that the Light Dragon was created during the final fight against the Void Dragon, with Primordial Fire transmuting parts of the Dragon that then became something else.
While I'm not going dispute this theory, I'm going to build on it - I suggest that, whether or not Lucy existed prior to the prologue, this is the first time there were enough souls complex enough - and minds complex enough - for her to be conscious of herself and make contact. You can even see in the back of this page, all the humans, elves, and ancients have black sclera and yellow pupils characteristic of the Void Dragon's control - and on the next page they have normal eyes.
Why do I point out complex minds when discussing Lucy? In this post by Red, the Dragons are used to represent Destruction/Consumption and Insight/Inspiration. The latter seem like very creative or otherwise refined aspects of a complex mind, to me, whereas the former is a very basic representation of Hunger, something experienced by most living things.
People have theorized that Light's higher property is Willpower, or - after Dainix's talk with Tahraim on the boat - Love, and while those are nice, I do think Destruction/Consumption and Insight/Inspiration are the higher properties of Void and Light respectively, with Willpower being the higher property of the whole that the two of them make up. Destruction and Consumption, or just Hunger, is the willpower of the body (I'd like to note, here, how cave-corruption warps the body most notably out of all elements but Life, and brings the body back up after what should've killed it). Insight and Inspiration are the willpower of the mind.
The whole? Soul. It exists everywhere, as do Light and Darkness. And willpower seems like a good way to keep the Elements in or out as the soul barrier does, no?
Which brings me to how Erin is going to handle the Dragon. I don't think the Soulshaper Monks will be of a ton of help. They work with the body and mind to reshape the soul, yes, but when the problem is an external, malicious force - a conscious, living Primordial who is itself part of Soul, and half of the body-mind paradigm - there's only so much you'll be able to do.
I think that as Erin pieces together the Light-Void connection, the 7th element, the nature of the soul, he's going to come closer and closer to figuring out what to do about it. And I have two sides to this solution.
I'm going to lean on a theory I was introduced to by @amisplacedalphabet, among a couple others. By picking a mortal vessel, the Dragon is going to unconsciously grow closer to that mortal perspective, just as Life had intended to from the beginning. A vessel, a microscope, an anchor humanizing them both. The Dragon just doesn't know it's going to happen because that's not what it's trying to do, and he thinks he's above accidents like that. But I don't think that'll get us all the way.
I think Erin's going to have to find Void's truename - possibly Light and Soul on the way, based on how none of the three aspects of Water counted as the real thing. I don't know if he'll invoke it, because there's no way of knowing what a truename invocation will do when the Primordial is still alive. The dead ones had each been disastrous already.
But I do think he'll discover it. Have it in the back of his head as another threat against the Dragon to bring it to heel. And maybe, if things get bad enough and he has a solid idea of what the risk is, he will decide that invoking the truename - and the disaster that comes with it - is worth avoiding destroying the world entirely.
And perhaps, once the Void Dragon himself finds out truly what he is and what he means now, here, in this world, the way he extended his prison in his efforts to break out... Perhaps he'll find other ways to cope with that.