Yeah, i think its in maybe one of the Hraesvelgr scenes. The dragon(Nidhogg) knows that he can go in and end ishgard, and he chooses not to. He doesnt want to merely kill those who betrayed him, he wants to make their descendants suffer an eternity of war. When his no longer living spirit takes over Estinien, then and only then does he decide that its time for the city to fall and all of Ishgard to die. And as you saw in the game, it was a fairly close thing.