Remember the Relic Weapons?
They all got their form and name after slaying a greater demon called Zodiacs.. Absorbing the aura through their blood gave the weapons sapience and their glow as well.
The final stages of the quest line was to recreate that effect through the use of those four KI's, (Zodium included) and the mahatma.

My running hypothesis is either the Ascians and Zodiac demons were the same beings in a different age, and that Zodiark is the main source that they all spawn from. Or that Zodiark and Hylelyn are the same being or at least two halves of the same singular entity. Where one can never exist without the other, yet both are at odds with each other. Possibly over with what that lore was about with Silver Tear Lake and Midgardsormir. If so than I would also assume Zodiark would also have a pact with a elder dragon like Midgard. The only thing I can think of would be something like Shinryu being the antithesis to Midguard. There's nothing to support this other than a cliché of fiction. With everything being planned out into 4.0 we just need to wait and see what happens.