In the beginning, there was Ym’Selj. Ym’Selj grew bored with the nothingness in which they lived and created the Cosmic Hourglass, in which was filled with the Gods. However, the Atzani, the most eldritch and powerful upper class of Gods, enslaved their brethren and dominated the Planes for millenia. Eventually, the enslaved Gods rose up in a divine rebellion, slaying their Atzani captors and casting their corpses to the Far Realms, where they could never remanifest. This event is known as the Purgation. Kyros then issues the Divine Mandate, decreeing that all gods, greater and lesser, cannot directly manifest onto the Material Plane. The only exceptions were the five saints they created to clean up the mess left after the battle versus the Atzani, referred to as the Purgation.
But after the dark events of Origins, the Divine Mandate was changed to include creating Saints or anything adjacent onto the Material Plane.