To be honest, the candidate which comes to mind is the Primal responsible for the Calamity of Astral Darkness, namely Bahamut.
Which, based on the SMN 50-60 questline, we didn't even need to fight, but just to be in the vicinity of. Which apparently applies to literally everyone on Eorzea, and implicitly the Source (given the variety of possible backstories for the player character).
Still, I do agree that what we did get is at least handwaved enough that the seams can be ignored.
