Dark Divide
No one is quite sure how the Digital World was really created. Some have said that it was a world created by the hopes of humans, while others claim to have created the world themselves. What is known is that it is merely a general term for many 'similar' worlds, whose only true relation is their inhabitants. Each holds countless Digimon, creatures made of data who reside in said Digital Worlds. However, every Digital World is linked to a 'real world', filled with humans. They are also interconnected; on occasion, beings from one digital world will enter another.
Between these Digital Worlds are two voids; one of light and one of darkness. Within the world of light, there reside many organizations. The rulers of each Digital World are allowed in this nexus, where the Royal Knights, Great Angels, Digimon Sovereigns, Olympus Twelve, and countless more reside. Some of them are in direct contact with the mysterious entity known as Yggdrasil, which is the god of all the Digital Worlds.
However, the Dark World is filled with little more than spare data. Anubismon, guardian of the gate to this world, judges those Digimon who die in battle. If they are worthy, their data is returned to the Digital World in the form of a Digitama, or Digi-Egg. However, countless evil Digimon had arisen over the years, and so the data of those corrupted beyond redemption were simply deposited into the Dark World. There, the countless evil Digimon would reside, dormant forever.
Or so it should have been. However, with the countless entities that wandered the Digital Worlds, it was destined that one would find its way into the Dark World while still alive. As misfortune would have it, an adventurous machine Digimon named Bulbmon was present at the deletion of a Devimon. The data of the Digimon surrounded him as its body lost its form, and Anubismon appeared before the poor machine. Unable to differentiate between living and dead, Anubismon judged the Devimon, and therefore all within him, as evil. So, the living Bulbmon was thrown into the Dark World alongside the evil Devimon.
Deep within the Dark World, the two Digimon floated, for there was nothing but spare data. By fusing with the still-living Bulbmon, Devimon had kept his consciousness, and he took in as much of the dark data as he could handle. Soon, his own body was reformed, but he still held Bulbmon close to him, as the dead Digimon would only live with the life force of those Digimon who had not truly died. As his power grew, he found his body could no longer handle the energy. Desiring to consume the entire Dark World, he found a way. He forced Bulbmon to fuse with him.
The data of the two Digimon combined in the Dark World, and the result was Apocalymon. Slowly the evil Digimon grew, absorbing the memories and dark emotions of the evil Digimon. He soon realized that the Dark World also drew in the dark emotions of living Digimon. His form grew larger and more powerful as time went on. However, no evil Digimon lived beyond the Champion level. A vast majority of them were unable to surpass the Rookie stage, and some were In-Training who had died before their corruption could manifest. However, he was not content to merely take the data of the dead Digimon once he sensed that which should not exist: He found a trace of doubt in the mind of Cherubimon. He felt a hunger within a simple Gatomon. Apocalymon was both surprised and intrigued by the darkness within some of the holy Digimon.
And so began one of the darkest eras in the history of the Digital Worlds: The Age of Apocalymon.