Zodiark.

Zodiark is the Ascian's primal, no? Or is that just their diety? (At least it was heavily hinted toward the end of ARR story).. Lore buffs can correct me if I am wrong here.. I watch the cutscenes but am not as knowledgable about the Lore as some of you guys..
Considering the Ascians aided Ilberd in the summoning / obtaining Nidhogg's eyes.
It also being a primal that is more menacing than Bahamut, would suggest it would have to be a pretty damn strong Elder Primal.
And a big part of the Ascian's plotline / goals have been to summon their primal..

Just my speculation. I could be way off the mark.