In the LV58 Job Quest for SMN, Y'mhitra explains your interaction with Bahamut aether, and there's actually a flag that changes the explanation based on the experiences of your character.


Legacy
You were there at the Battle of Carteneau. You were there on the day of the Calamity when aether infused with the dragon's essence rained down upon the field.

Non-Legacy
On the day of the Calamity, the dragon vanished in an explosion so massive that his aether was carried to the far corners of Eorzea. You, among many others, have already been touched by his essence.