Disclaimer: I am not saying that the Thirteenth World (The Void) is Ivalice from the past Final Fantasy games, but my theory posits that the Thirteenth World may be one big homage to Ivalice if I'm right.
So, I was thinking about what was learned from the Warriors of Darkness when they revealed that the thirteen worlds were being consumed by either Light or Dark, and then I started thinking about how that could possibly tie in to Unukalhai with his mention of a world destroyed by Auracite and the people who abused Aether.
So, if we go back to Final Fantasy Tactics for a moment, we know of the story of the Zodiac Braves (who are mentioned in Hydaelen lore) who used Auracite, or Zodiac Stones, to fight back the Lucavi to save Ivalice. However, as you learned in the second half of the game, the Zodiac Stones were actually catalysts that help form the Lucavi in the material realm. They have to possess a host, and the host is generally the carrier of the Zodiac Stone.
When you compare this to what Unukalhai tells you, you start to notice a lot of similarities between the world Unukalhai comes from and Ivalice of Final Fantasy Tactics. He mentions that he comes from a war torn world that was eventually consumed by darkness and destroyed. I'm guessing that the world he comes from was the thirteenth world that became The Void.
He mentions that the people who destroyed the "war torn world" attempted to use Auracite (Zodiac Stones, maybe?) to control the power of Primals. This may similar to how the Zodiac Braves of Ivalice bolstered their own power with the Zodiac Stones, but were later consumed by the Lucavi. With their power twisted, they turned in to Aether hungry beasts.
Now, as another homage to Ivalice, The Ascians are technically supposed to be "good" according to their legend. They all share their names, so far, with the Scions of Light mentioned in Final Fantasy XII. Who, ironically enough, all share the same Zodiac sign of the Lucavi from Final Fantasy Tactics.
So, here's my theory: I'm thinking that The Ascians might actually be what's left of the "Zodiac Braves" of the Thirteenth World. I'm thinking that The Ascians may have been heroes who, like the Warriors of Darkness, battled the Primals of their world. However, instead of just destroying the Primals over and over again until their world was consumed by Light, they attempted to take their power for themselves without realizing what it was going to do to them.
Then, put that together with the fact that Unukalhai is the name of the Alpha Serpentis constellation commonly associated with Elidibus, and you've got another similar situation between Elidibus and Ivalice. Remember, Elidibus (who can summon Zodiark), seems to act solely on his own without any real influence from the other Ascians. So, like with Final Fantasy Tactics and Final Fantasy XII, he is still a separate variable.
Now, I want to stress again, I am not saying that the Thirteenth World is Ivalice, but I am almost convinced that The Ascians, the stories of what happened between Hydaelen and Zodiark, and the Thirteenth World may possibly be one giant homage to Ivalice.
TL;DR: The Thirteenth World is a homage to Ivalice, and The Ascians were the "Zodiac Braves" who attempted to use the Auracite to combat and take the power of the Primals for themselves. They may be the reason why the Thirteenth World became The Void, and it may also explain why they can summon the Voidsent.