Revelations from Patch 3.5

[CGMidlander]

So technically when we defeated Bahamut he wasn't at full power. The primal that Ilberd was summoning was suppose to be more powerful than Bahamut, who caused a Calamity. If it is Rhalgr than that makes sense--a partial summoning of the Twelve was used to try to power the seal to permanently contain Bahamut because a full summoning would have destroyed all life on Hydaelyn. It took another primal with Tupsimati and basically a death-powered summoning to even make a dent in Bahamut at full power. Rhalgr is only one of the Twelve but worship of the Twelve predates the Allagans and it is implied that they are on a completely different level than other deities in Eorzea. With his sacrifice and Tupsimati, Papalymo only bought the Scions time. I want to know what Elidibus is playing at since "kill everything" is not his usual style.

/10character

"Bahamut full power" (The one that destroyed FFXIV 1.0) had thousands of years to charge in a moon-sized container full of dragon batteries.

Ilberd's thing did not. Unless he found some way of gathering huge amounts of aether in a very short amount of time that alluded even the Allagans, it's unlikely that it would have compared to full Bahamut.

I think people are underestimating it.

We don't know how much aether the eyes still possessed. At multiple times throughout the story it's been implied that the eyes are effectively limitless in their power. Ilberd is also the first summoner we've seen to perform a self-sacrifice as part of the ritual. The original extreme primals highlighted that sacrifice directly increases the strength of the summoned creature so there's no telling how much more powerful a deiform summoned by self-sacrifice would be. Ilberd seemed convinced that it would be more powerful than even Bahamut though, and the Dreadwyrm was so powerful that we couldn't face him head on as it was.

The eyes are unlimited in that they absorb surrounding aether over time, but they can still be drained over the span of something short like a fight. If that were not the case, then we could not have defeated Thordan or Nidhogg, whose forms were sustained by aether.

Self sacrifice isn't likely a huge augment. (Especially that of a mere one person. The other dead people were not voluntary.) It helps, but if it really does modify summonings to >Bahamut levels, then you'd have huge threats popping up all the time, what with all the crazy cults & desperate inhabitants everywhere in Eorzea.