The Nothingness is a concept of what is outside of Mildorn, much like our idea of the Universe. Many people believe it was home to the Aerorians. It is most famous for the slumberplace of Milldoran. In the story, Mildoran curled up and slept there, while the growth on its back became Mildorn. This theory may seemquite strange, but some say the creature actually exists.
As far as what the Nothingness is like, it is said it's a warp hole of pure elemental colors. Sometimes, it can be seen in the sky once ever few years. On these days, the King's son would become the next King, believing the lights will guide them on their new journey.
For more, see: Ther Tul'ven.