Once upon a time...

In the war-torn land of Equestria, there were two hateful Daemon Princes who ruled together, and created Chaos for all the land. To do this, the eldest used his sorcerous powers to bathe the sky in fire at dawn. The younger brought out a swarm of plague-flies, blotting out the fire to begin the night. Thus, the two Daemon Princes maintained imbalance for their Daemon world and their subjects, home to many types of Chaos Space Marines. But, as time went on, the younger Prince became resentful. The Chaos Space Marines looted and murdered in the day the older Prince brought forth, but they shunned and hid from his horrid swarm of flies. One fateful day, the younger Daemon Prince refused to disperse the swarm to make way for the inferno, the elder Daemon Prince tried reasoning with him, but the younger Prince was warped by his jealousy and became a monster, Festering Plague. He vowed that he would cover all of the land in eternal plague. Reluctantly, the elder Prince harnessed the greatest power known to Equestria; the Elements of Disharmony. Using the magic of the Elements of Disharmony, he defeated the younger Prince and banished him permanently to the top of the plague-fly swarm. The elder Prince took on the responsibility for both Fire and Plague, and disharmony has been maintained in Equestria ever since.