Fall of the gods
According to legends, there was nothing. But the first creatures in the world were described as truegods. There are four known truegods. Three of them created this world by sacrificing themselves, and thus the plane of Heaven, the plane of Hell and the plane between were created. The last truegod, also called the creator, gave life to the world. First by creating the god of Heaven, Pelor and the god of Hell, Nerull. He also created many other gods, but no one was given the right to control the plane between. The last plane was given to the mortal races. The elves, humans, dwarfs and orcs were the first to walk on the plane between. Soon second races were given birth, and the world began to function as we know it.