My present working theory is that Zodiark's corporal body is the moon (known to Eorzeans as Menphina) based on a few facts.

1. Both the planet and the goddess are named "Hydaelyn."
2. Zodiark is, based on the Ascians' comments, effectively a dead god.
3. If Hydaelyn-the-goddess' corporeal form is a celestial body, it stands to reason that Zodiark's corporeal form would also be a celestial body.
4. A ruined statue resembling the one seen in the Chrysalis can be seen when Elidibus speaks to the Warrior of Darkness on the moon. This suggests it may have once been habitable.

Thus, it seems plausible to me that the moon is, in fact, Zodiark's corporal body. Will we go there? Well... dunno about that, if it's a dead "world" so to speak it's likely uninhabitable and the Ascians and Warrior(s) of Darkness could only be there because they are already effectively dead. It's just a theory though... I don't know where the story's going.