The Overworld is the first dimension in the 99 dimensions of Aetheria. It is home to a diverse mix of biomes such as lush forests, flat plains, arid deserts, and hostile jungles. Many monsters are native to the Overworld such as the spiders and creepers, though many more had been created due to Herobrine's alchemic breeding programs (such as the cake slime).
The Overworld was once populated by many diverse kingdoms and empires, with Aetheria City being a center of commerce and culture. Human and Highborn Empires reigned supreme on the continent of Ardenvell. Travel to the Nether had also been discovered, along with its lucrative resources, leading to the development of human civilization in the Nether.
Herobrine, a malevolent being, rebelled against the governance of the Overworld. During the First Great War, he had raised legions of monsters which tore at the cities. The zombies, skeletons, and other Undead which plague the Overworld to this day are the creations of Herobrine. The First Great War ended in Herobrine's death, resulting in his banishment to a phylactery in Icerahn.
The Second Great War started when Herobrine had been freed from his phylactery by his legion of monsters. The Second War was so brutal, billions were murdered. Millions fled to the uncharted Dimensions as a means of survival. The Overworld has never seen glory since. Many humans, such as the ones that fled to the End dimension had lost their intelligence and evolved into the hostile Endermen, while the animals which had been brought to the other dimensions gained their intelligence and evolved into various races
In the dark ages that passed, the surviving Aetheria City was the only beacon of hope for mankind. Many races returned from the different dimensions, some as humans who degraded into monsters, while others as animals who had upgraded into wise forms. The world slowly rebuilt, but life was still hard.
Dangerous monsters, raving warbands, and the deaths of many settlements due to hordes of monsters were all but a brutal fact of life. With the return of Herobrine's army, monsters continue to end the lives of millions in greater magnitudes than before.