The Great Departure

Celestial

Having seen enough of the Universe, and growing tired, many Celests had grown tired of the universe, having great knowledge and power, they can direct the very flow of existence in their realms, and nothing surprises them any more. The Material Plane had become another space of fighting to them, and it worried them that the corruption of the Lower Planes would continue to spread until it consumes them.

Troth, one of the most powerful, and even minded Celests revealed a discovery from their Meditation and observation. Troth had found a method of sending any beings Soul into the next Dream of Ao.
This became believed to be pure a paradise, and actual rest for the Celest's, though it was known that none could return from the passage.
  This time also saw the signing of the Primeval Pact with Asmodeus and the other Celests. Among many things, this defined the Celestial Planes, Neutral Territory, and the Lower Planes.
  The Pact also gave Troth a permanent role in the Universe, as it was his pleasure to endlessly watch the choices of mortal lives, and so became the judge of mortal souls. To keep the Celestial realms clean of evil, all evil souls were filtered into the Lower Planes, unless that soul had a claim to it from another Divine. Troth would also find worthy souls and offer them a choice to join with Ao in the next universe.
  The Pact, while full of loopholes and provisions, was of great benefit to the Celestial Beings, for it left the Lower Planes in a endless conflict between Demons and Devils, as Asmodeus tried to claim what was awarded him from the Pact, and the Demons cared nothing for such agreements, but his supreme right to rule those planes, allowed the Deities and Celests to wash their hands of it, and allow them to cease worrying about the Devils, and allow the Devils to worry about containing the Demons.