Not asking for any spoilers, but I haven't touched coil at all, and not being a LEgacy character and the PC presumably being not from Eorzea ((I believe the game only states that you are not from your starting city, but still))it is possible that My character's greatest interaction with Bahamut is the Roar at the end of ARR's main section. And the destruction, I suppose.
So I am curious, I needn't be told HOW it is explained, but rather how well, is there a justification for how we can invoke the Dreadwyrm without having beaten it ((Maybe)) as opposed to having needed to beat up the other Primals to be able to summon them?