Hmm, well since there's a bit of talk about the whole halo thing going on, I'd like to share my thoughts on it and a couple of possibly related things. May be a little convoluted, but bare with me here.
So first off, the area in question where we see what could be the halo as a stand-alone object, is Lahabrea's...workshop, basically, I guess you could call it. And everywhere else in his area of the Akadaemia is full of weapons or monsters of some kind, basically aggressive or offense related Concepts. Stands to reason, to me at least, that the halo itself could be some form of weapon-type Concept.
Now, going all the way back to ARR, in the Praetorium Lahabrea appears and starts talking about the Heart of Sabik, "an enigma whose surface even the vaunted scholars of ancient Allag failed to scratch". So seems pretty clear cut to me that the Allagans obviously had nothing to do with it's creation. We're also not told where it actually comes from. And all mention of it seems to have disappeared into the aether after defeating Ultimate Weapon, but that's another story.
He also mentions how its magic had 'lain dormant for eons' and just before casting Ultima tells us to behold a sliver of his god's power. So Ultima should be related to Zodiark in some fashion, therefore we can rule out that it's something from another star like Omega was. There's also talk about using the Primal energies to 'quicken' the Heart, which also sounds a lot like that side quest Fay was talking about, with the rope and how you use the random Blob to fuel our subpar Creation magick and make a trendy cloak for ourselves.
My current number one theory, is that the Heart of Sabik is something that survived the Sundering, much like the portions of Amaurot in the Tempest (at the Sahagins). Also seems very likely it was actually something created by Lahabrea, as from many of the things we've heard or seen, he seems like the most martial/aggressive of the Ascians, and we can be fairly certain by this point that he had a rather major role in creating the initial Concepts behind Zodiark, if not more.
Which also makes me re-evaluate something else he says in those cutscenes, namely he uses "my god" a couple of times, which in retrospect, could be taken as him referring to Zodiark as his creation, 'this is the god I made' type of thing. Though naturally he could also just be using it in the sense of 'the god I worship/was Tempered by' as well.
So I can't help but think that the Heart of Sabik, and Ultima, was either something he created or was working on pre-Zodiark (mentioned below) and that he might have used it as something of a prototype for the finished product, or built upon it at any rate. Having the ring/halo as the odd-one-out in his weapons display just seems kind of strange to me if it isn't a weapon itself.
I'm kind of tired, so sorry if it's a mess and I can try and clarify my thoughts for anyone if they need it, but in conclusion, I'm left wondering this:
We know that Zodiark was a Created being. He had a very deliberate design and purpose, most obviously to halt the destruction of the planet and later to return life to it. Not entirely sure if he just didn't have enough power after being made, or if they had to rush his Creation just so they could stop the calamity, as from some of the dialogue it seems pretty clear they were pressed for time and barely halted the destruction in time anyway.
However, neither of those purposes requires the use of any type of violent force, or weaponised Concept. In that case, why did they feel the need to grant Zodiark so much destructive power? It can't be because of Hydaelyn, as she came after Zodiarks 'design' and Creation. A threat from outside would be a great explanation for that.
Though i'll admit some parts of what I said might be considered a bit of a stretch here and there. And if you start getting into amendments being possible (which we have no evidence to suggest they aren't) than it's a whole new can of worms, least of which is that you would also have to admit to the possibility that Hydaelyn herself is an amendment to Zodiark. Even with that though, it still feels to me like they gave him a bunch of destructive power straight up. But maybe i'm just seeing what I want to.
Somewhat randomly, there's actually an NPC in Tempest version of Amaurot who talks about Lahabrea briefly, mentioning him as "a master of every recognized creation magick, possessed of boundless imagination and willpower to realize impossibly complex concepts". Now this is just a gut feeling of mine, but if someone like Lahabrea seems to be were to have mastered every recognized creation magick, he seems like the kind of person who would try to take it one step further and create possibly...the 'ultimate magic'? Or the ultimate Concept, I guess it would be back then. Kind of makes me wonder if he had been working on some kind of version of a Zodiark-like being before they even knew they needed one.