Relationship between Bahamut and Midgardsomr?

[Cilia]

I think it was fairly well-explained by Louisoix / Phoenix in the T12 cutscene.

"Desperate for salvation, the dragons prayed to one of the first brood - to their ancestor Bahamut. Infused by the power of their fervent supplications, Bahamut arose from the abyss of death and took wing as the Meracydians' newborn god."

It's not directly stated, but by calling Bahamut "one of the first brood," it can be inferred that he was indeed one of the Wyrmlords / Celestial Dragons. However he died from unknown causes at an unknown point prior to the late Third Astral Era, and when the Meracydians prayed to him for salvation he either literally came back from the dead as a primal or one was made in his likeness. I figured it was similar to "Saint" Shiva and the primal Shiva's existence.