I agree with much of Moose-senpai's theory ('tis an actual theory). However I would posit that Elidibus, being the arbitrator and intended mediator of the Ascians, is not particularly bound to our world. Instead he is the one who is supposed to coordinate every Calamity on each plane - that is to say his job is to come up with strategies for Calamities, while the local plane's Ascian (Lahabrea in our case) is the one who comes up with the tactics. Lahabrea notes after Nabriales' death that "The Ardor was not his to invoke," meaning he's either being possessive on who gets to wreak havoc (seems unlikely) or he's outright noting that Nabriales was doomed to fail.
There's also the matter of the Ascian Prime and the Warriors of Darkness...
The Ascian Prime is a violation of the laws of the world. It's a fused being born from two Ascians (Lahabrea and Igeyorhm in the one case we've seen thus far, but theoretically any two could pull it off) created by "breaking down the barriers of existence." It also tells us to "render up our souls, that Lord Zodiark might be reborn," and upon defeat screams "What walls are these?! Damn you, Hydaelyn!" ... obviously aside from sundering the world Hydaelyn has put up some kind of dimensional or causal barrier that stops things like the Ascian Prime from existing. We have, after all, "grown so strong within [the Blessing of Light] to deny [the Ascians their] power." Hydaelyn herself governs the laws of the world... and the Blessing lets us reinforce those laws, presumably.
As for the Warriors of Darkness, they're still a great big enigma (wrapped in a mystery). However based on them having formerly slain an Ascian only to join them later, I would posit that they are either refugees from a destroyed world out for revenge on Hydaelyn or trying to save their world by destroying ours in its stead. Those seem like the two most likely possibilities based on the little evidence we have.
As for Thordan's comment on asking what we were? With us shown in shadow? Hmm... well, if nobody is going to survive a transition back to the "way everything once was and ever should have remained"... might it be that, again, every living mortal holds a small fragment of Zodiark's essence within them? I know it was apparently a thread from XI, but if nobody is going to survive it makes a significant amount of sense - everyone needs to "render up their souls" for Zodiark to be reborn, as the Ascian Prime says. The Echo, which allows us to violate Hydaelyn's laws much like the Ascians, would then be a symptom of Zodiark's power; unlike everyone else, we possess a shard as opposed to a sliver of Zodiark's power, which allows us to perform some feats similar to those of the Ascians. The Blessing is a way to keep that power in check - notably we are the only one with all six Crystals of Light, necessarily so to keep the Dark within from overwhelming us. Even those with the Echo, such as Minfilia and Ysayle, rarely manifest one or more. Most people simply lack the gift or knowledge to see the "truth" - but Thordan, having become an aetheric being, was able to see past our outer "shell" so to speak and glimpse the Dark within us. (Hilariously enough this is supported to some extent by Sephirot's theme song of all things, which tells us to "Heed the Dark within [our] being." Both Ramuh and Kan-E-Senna also make references to the "Dark" that lurks in the hearts of men...)
Maybe. Possibly. Who knows. /rambling