The Shattering

Before the creation of the cosmos, the jynra existed in a state that is incomprehensible. Adherents of The Duality would have you believe that the jynra existed as a single entity prior to the Shattering, but that is simply impossible to know for sure. What is best known about the jynra during this state is that they were diametric entities. Various cultures in Aedelor have come up with allegories to try and explain the nature of the jynra's existence prior to the creation of the cosmos, and they can differ slightly from version to version. Most, however, are similar to the following.  

Sarasaamun and Havastu, jynra locked in war with one another, existed long before all other things. Sarasaamun, the essence of creation, and Havastu, the essence of destruction, in a delicate balance. To tip the scale, Havastu tricked Sarasaamun. Havastu severed each of her five heads knowing that Sarasaamun, in all her mercy, would not let them lie. When Sarasaamun tried to heal Havastu's heads, they instead were risen into dragons each of their own chromatic colours. The chromatic dragons, fueled by Havastu's essence, nearly destroyed Sarasaamun. And that, we believe, is when Sarasaamun shattered. — Melisant Boudier, History of the World, Vol 1

When Sarasaamun shattered, the cosmos was born. The event was instantaneous but released unimaginable energy, enough to send both Havastu and the remains of Sarasaamun to opposite sides of the cosmos. The shards of Sarasaamun were sent in all directions and created the planes of existence, the first gods, metallic dragons, and both giants and humans. At the very point of the shattering was a whirling miasma of raw elemental forces, which contained within them the planes of Aechyrus and Aedurūn. Where these two planes overlapped, Aedelor was made: a plane in which creation, destruction, and all of the elemental forces work in harmony and allow for life. The point at which the shattering occurred, passing directly through Aedelor, would become the axis upon which the entire cosmos rotates. Many other planes of existence were also created as a result of the shattering, spread across the cosmos on both sides of this axis. The planes closer to the side which Sarasaamun was sent became influence by her nature and would become the Upper Planes, while those that were closer to the side which Havastu was sent would go on to become the Lower Planes, perverted by her destructive essence.