So basically 4es base lore is in the Nentir Vale setting. And in that setting there are many planes of existence that are arranged in a circle. The highest plane called the Plane Above or the Astral Sea a plane where the gods are and shit. And the Plane Below. So the plane below at the dawn of time was just the Elemental Chaos where the elemental gods hung out. That is until a "seed" from not another plane but another thing all together wormed it's way into this one and settled in the Elemental Chaos and made the Abyss and started spawning Demons and shit. So its a book about elementals and demons.
D&D 4th Edition The Plane Below: Secrets Of The Elemental Chaos.
Daniel Cadded July 27, 2018