What is Zodiark?

[Cilia]

If we're going to posit our whole hypothesis on the myth arc...

Zodiark is exactly what the Ascians claim: the one true god.

Going into more detail, before you fight him, Ramuh hypothesizes that light and darkness only became separate with the advent of men. If this is true, then Hydaeyln could very well be the "original" primal created by men so long ago that nobody really remembers it. They created Hydaelyn because the world pre-Hydaelyn was so awful that they couldn't stand it, thus making Hydaelyn the primal of "Light" and "Hope." This changed Zodiark, formerly an intangible concept, into the "god of darkness and chaos," who then created the Ascians as a sort of antibody to set things right. To take a line from King Vendrick of Dark Souls II:

Fire came to be, and with it disparity.
Heat and cold, life and death, light and dark.
Dark was seen as a curse. Shadow is not cast, but born of fire.
And the brighter the flame, the deeper the shadow.

Thus, Hydaelyn leeches Zodiark's power (aether) to keep him asleep and keep the disparity existent for the good of man, because that's what she was designed to do. Ascians are sentient antibodies created to return the world to its true, intended shape. Whenever the light shines too bright, the dark overwhelms it, and whenever the dark reaches too close to the light it is burned away. Thus turns the wheel of fate, the cycle of Astral and Umbral Eras.

... just my hypothesis, running off one of Ramuh's own and various lines from the Ascians.