You must have completed the Chains of Promathia missions and all the cutscenes detailed in the My Three Rings quest.
As you approach the Auroral Palace in Ru'Lude Gardens (H-7) you will have an automatic cutscene.
You will speak to Esha'ntarl and be given the quest.
Next, travel to Misreaux Coast and examine the Dilapidated Gate (F-7).
You will need to have a Giant Scale. If you already have one, then you will not need to farm any drakes on the way up.
In Riverne—Site #B01, you must travel to the Unstable Displacement (E-7) and fight Bahamut in 18man uncapped BC.
Reward: Whisper of the Wyrmking (key item)