No true gods?

[Zhao]

It is worth remembering the attempted summoning of Rhalgr by the Ala Mhigan Resistence, this thankfully was stopped as it would have created a Primal(version) of Rhalgar the Destroyer. So despite the truth of the twelve, whether they be god or humans of heroic deeds(whatever their origins), their current existence in the populace of Eorzea's minds are deities to pray to/worship. This existence is perhaps most comparable to primals in this sense since the Primal is separate to the actual existence(similar to shiva primal versus shiva the person who is deceased). What is important in this circumstance, I believe, would be the unified image of said primal. Everyone knows the twelve for their symbols, but to create a humanoid deity would require a much more uniform image, plus crystals & ritual of course.

As for a one true god, I suppose it would come down to what limits of godhood you think of. On the most generic level, you have the creator and the destroyer, the light crystal and the dark crystal respectively. I say this because I think there is an argument of Hydaelyn and Zodiark being primalistic themselves. I'll just point to this link and say enjoy!
https://www.youtube.com/watch?v=7ZhT...bQQr-psMidCXH8

The original spell that Louisoix cast was infused with explicit prayers to the Twelve that Dalamud be returned to the sky. It stirred the long-present traces of worship in those areas, and then Louisoix was able to focus it specifically on re-binding Bahamut ("Dalamud") and exiling him again. When Bahamut broke this, it broke the entire focus, so all that was in the air was a massive amount of free-floating aether.
That aether then reacted with the prayers of thousands and thousands of people below, "I don't want to die. Please, let Louisoix save us." This focused all of that aether on Lousoix himself and transformed him into a deiform. This is where I have a big ? even in my headcanon. Did he technically die somewhere in here? Did Bahamut light him up? Did the aether flooding him instantly tear him apart and put him together? That would explain the vision, I guess.

Assuming that my memory functions as proper, neither Louisoix or Bahamut were killed. While Louisoix's methods are still shrouded in some mystery, we know that some image of the twelve were summoned in attempting to hold the primal. While still unsure about the worship, we are told that the aether poured back into Louisoix, and he became the Pheonix. (I would have to check, but the Pheonix was chosen due to the bird being the symbol of rebirth and life in most if not all people.) After striking Bahamut down, he released the aether intending that it would return to the land. However, Bahamut refused death and took in a fraction of this aether to survive. Mind you, Bahamut took in all the aether he could, including that infused with Louisox/Pheonix, Gaius Van Baelsar, and more.
In response to Louisoix's transformation, I believe it to be more of an ascension of existence. Perhaps it is to be compared to the summoning of Shiva? I would hazard a guess to say so.