The Split

As Salas and Tenebis struck each other down, at the end of the First Eternal War, something happened. Their very souls were split apart, shattering into hundreds of thousands of pieces. This gave life to the multiverse and created the various Gods and Planes that are known today. It is universally agreed upon that Salas created the Upper Planes of Mechanus, Arcadia, and Mount Celestia and the Gods who reside there. While Tenebis gave birth to the Lower Planes of Gehenna, The Shadowfell, and The Abyss and the Gods and Demon Lords who live there. And the Neutral Planes of Limbo, Carceri, and Hell where created by both of the Gods, neither one able to overpower the other. Even though the Gods were dead, the remanents of their souls still battled on(See The Second Eternal War).

Salas- Goddess of Light and Good
Tenebis- God of Darkness and Evil