I couldn't find any online resources that say specifically where this is. So, I'm going to ask here: Where is the Mountain of the Father? The place where you obtain the summon Bahamut?
2 Answers
In the Dark Gate, the room that links you to the 4 voids, a portal opens up at the north tip after you have entered Nil for the first time. This will take you there, and there you fight Bahamut.
You'll find mountain of the father as you progress along with the story, it's almost impossible to miss as the game makes it a point to show to you that the portal to it has been opened.