Millennia ago the astral sea was empty.
Gods and demons grew bored of the scattered pieces flying around the astral sea and started to create.
After many years of creation the joined efforts were successful.
Resulting in different planes of existence for more than just one form of life to roam around and entertain its creators.
Tensions between gods and demons rose and were settled. To avoid further conflicts the two groups took refuge within their own realms.
After the creation of the planes, peace held for centuries.
Different species could evolve and thrive on the different planes of existence.
Science and magic helped the thinking to build empires.
Legends talk of expeditions into the astral sea by elves and dwarves.
After a millennia of peace the demons got greedy and started to build an Army. Unknown to the Gods the demons prepared for attack.
The following war hit all planes more or less. The gods were overpowered by the Demons.
After nearly losing the war the gods brought the ultimate sacrifice and sealed the demons away at the cost of their own powers.
Most gods became mere mortals and died one after another. But the demons were cut off from all other planes and by extension from their worshipers.
The Material plane was heavily scarred and the races calling this plane their home fought long wars against the remaining demon army. Led by the now mortal gods they could defeat the demons.
Some of the surviving gods became Kings and Queens of Kingdoms they built with their remaining time.
Most of the pre war technology was lost. Some adventurers delve deep into the ruins of the past and bring artifacts of the old times back to the light of day.
After the gods fell, some powerful beings took up the mantle of godhood. A handful of the old gods who didn't partake in the later part of the war survived and went into Exile.
The story of the war has been lost to the ages and few scholars study the true story.
With the passing of time the magic of the once godly beings loses its power allowing lesser demons to pierce the veils of the planes and to wreak havoc among the mortals.
Old forgotten creatures of evil nature awaken, called by former masters.