According to the Fabula Nova Crystallis mythology, the thing that essentially kicked off the entire series was the conflict between Bhunivelze and Mwynn. For those of you who don’t know the mythology very well, I’ll briefly explain. Mwynn was Bhunivelze’s mother. He killed her in order to gain full control of the mortal world, and she ended up in the Unseen Realm. This is what caused Bhunivelze to create Pulse and Etro. He sought the door to the Unseen Realm so that he may find Mwynn as he believed she put a curse of death on the world.
However, when Bhunivelze realized he created Etro in his mother’s (Mwynn) image he discarded her and gave her no powers of her own. This is when he created Lindzei to replace Etro. This is what led Etro to tearing herself apart and disappearing into the Unseen Realm. Lindzei then used Etro’s spilled blood to create the first human, Yeul.
I’d like to point out that in XIII, Gods do not have what we think of as “emotions.” They can’t fully comprehend them, and the only deity who was able to somewhat sympathize with humans was Etro - as she became fond of humans and gave them each a piece of Chaos. The rest of the deities and Fal’Cie in the mythology do not express emotions to this degree. Bhunivelze’s primary function is the governance of life and order. He, and other Fal’cie throughout the series, are shown to use humans as tools to their ends. It’s a mechanical logic.
Under the cut I will go into some very important parallels that are represented during the final battle and the XIII series as a whole.