/cracks knuckles.

Long have I waited for a chance to ramble on about this....


Major spoilers in the midst of this!


You've been warned!


My current speculation is that the Allagans fitted Bahamut with the crystal that is Zodiark, seeing it as a near endless source of aether. Just with the Heart of Sabik they utilized Ascian tools in their technology without fully understanding what it was. Thus the giant purple crystal in his chest.

First they allowed Bahamut to be summoned, then shackled him using the Allagan dragon control devices such as they use with Twintania. You can see Bahamut is covered with Allagan hardware. Then fitted him with their impromptu energy source in an attempt to sustain him as one massive super-powered weapon of war. However, being a sentient crystal, a world to himself, Zodiark corrupted Bahamut and began using him to drain the world and Hydaelyn, from within his massive nigh indestructible vehicle.

Seeing their mistake too late the Allagans used their lunar base, Dalamud, rife with aether absorbing technology, as gaol to bind the terrible beast and its evil spirit occupant. They managed to seal it, but not before the course of destruction and the aether Zodiark/Bahamut had drank rendered the world into an estate of desolation harkening an Umbral Era.

It took the Aescians millennia, but they finally succeeded with Nael Van Darnus in unleashing their god once more, which is why they cheered on the falling Dalamud from the hills overlooking the fall.