When Selûne and Shar were created, it formed the possibility of the rest of the planes. The question became what to do with all of this possibility. Selûne wanted to illuminate the worlds and bring life to all that could be there. Shar wanted to keep the world in darkness and prevent life from growing. And so, for the first time since time began, the sisters fought. Some claim the division of planes stems from this conflict as a field of possibilities was fractured. There are stories about gods having been created and died during this time. The battles themselves weren't of the traditional kind. Eventually, creatures were conjured to serve as troops, but for the most part, this was a war of symbols and metaphysics. Every action also pulled the planes further apart, making it harder to travel between them.
Creation of the MaterialThe end of the war came as the material plane formed. Selûne create Amaunator to shine a light across its space. Shar had brought energy from all the elemental planes to create landscapes and those now formed the planets. Ailion ended up being the perfect place for Selûne to bring more life. She also created Chauntea as a caretaker of it. With the new life growing on the material, other planes managed to form some beings of their own as the energies of creation lingered. But on the material plane, life was starting to become a threat to itself. With only nurture and illumination, it could only suffocate amongst itself. Shar set the planet spinning and made it so that half of the time, it would be in darkness and shadow. Selûne made her moon to watch over the planet.
Separations and the end of the warThere were still aspects of the material plane that kept getting out of control. Other aspects were getting pale and dead as they couldn't reach sustenance. Shar pushed the first category out, creating the Feywild, while Selûne took the other category and made the Shadowfell. With that, they could both see that this new world was better off than it had been, and they declared a truce. The sisters might have come to a tenuous agreement, but they have drifted very far from each other. There are, literally, worlds between them, and neither will forgive the perceived betrayal. Other gods have been created to keep the balance of the world in check. Some have even been collaborations if unintentional ones. The only god they have made together was the elevation of the Raven Queen.
Various forms of life sprung up on several planes almost immediately. For those on outer planes, such as celestials, this was simply a matter of residual energies. In the case of the Abyss, the energies created a self-sustaining whirlpool of creation and destructive chaos. On the material plane, not only did many different kinds of plants and animals evolve on their own, but several gods eventually added theirs.
The world hasn't seen any conflicts on this scale since. But it is also much more complex and delicate now, which is they the gods agreed to secede during the Second Breaking . Even Shar wouldn't want to see secrets and shadows destroyed.
War of Light and Darkness