I'm seeing a lot of discussion regarding your comment about Elidibus and his motives. Elidibus, much like his namesake from Final Fantasy Tactics, is something of a "third party" villain tangential to the other villains. Unukalhai is actually a sneaky naming convention by the devs because it's the "primary" double-star of the actual Serpentarius constellation or the so-called "13th Zodiac" which Elidibus carried in FFT. And in FFT, while Elidibus' story relevance was cut due to time and he was reduced to being a bonus boss, he was still a Lucavi demon separate from the remaining 12. He did not participate in the resurrection of Altima but he was still a villain all the same...just one with differing goals from his peers.

How does this relate back to FF14? The universe of FF14 is dominated by two crystals--Hydaelyn of the light and Zodiark of the dark. Right now, Hydaelyn has the advantage and is the "source" of life while Zodiark is weakened and sealed away. The Ascians trigger calamities to force Hydaelyn to absorb her shards (the different worlds, such as the world that is consumed by dark and the source of the voidsent or the world that the WoD came from that was consumed by light) which serve to weaken her and bring Zodiark closer. But the Ascians don't want balance, they want Zodiark to have Hydaelyn's dominant position. While this wouldn't end existence, it would serve to more or less end everything as we know it and change it out for something different.

Elidibus has a different goal. He wants to bring true balance...which would be a rejoining between Hydaelyn and Zodiark. THAT would end ALL existence. Or in typical anime/manga storyline fashion... He wants everybody to be "one" AKA the Evangelion thing of everybody becoming Tang.