I watched as the Lamb opened the first of the seven seals. Then I heard one of the four living creatures say in a voice like thunder, "Come!" I looked, and there before me was a white horse! Its rider held a bow, and he was given a crown, and he rode out as a conqueror bent on conquest. When the Lamb opened the second seal, I heard the second living creature say, "Come!" Then another horse came out, a fiery red one. Its rider was given power to take peace from the earth and to make men slay each other. To him was given a large sword. When the Lamb opened the third seal, I heard the third living creature say, "Come!" I looked, and there before me was a black horse! Its rider was holding a pair of scales in his hand. Then I heard what sounded like a voice among the four living creatures, saying, "A quart of wheat for a day's wages, and three quarts of barley for a day's wages, and do not damage the oil and the wine!" When the Lamb opened the fourth seal, I heard the voice of the fourth living creature say, "Come!" I looked, and there before me was a pale horse! Its rider was named Death, and Hades was following close behind him. They were given power over a fourth of the earth to kill by sword, famine and plague, and by the wild beasts of the earth.
— Revelations 6:1-8
Hundreds of thousands of years ago, the world was full of great cities powered by wondrous magic, much of which entailed the use of smoke and mirrors. One of the most prevalent magical devices in all the cities of the world were teleportation mirrors that allowed people to teleport to faraway places that were reflected on the mirrors' surfaces.
Eventually a certain wizard grew too daring and invented a mirror that could filter away all evil from those who gaze upon it. The wizard experimented on slaves at first and soon found that while his test subjects were devoid of evil intent, they also became as meek and docile as lambs and lacked all leadership qualities. Realizing that this mirror could be used to create the perfect workforce, he used it on all the servants and clerks who worked at the academy. It was a wondrous experiment, and he was much praised by his peers at the academy for creating a new race of docile servants that would do only good.
What he didn't realize is that when a person is purged of all evil, the evil is not destroyed but trapped inside the mirror. The loose shreds of people's souls, with no body, no overarching governing influence to guide them, reached out to one another and formed an entity, an amalgam of the dark, evil hearts removed from those created by the mirror. This entity, a mass of chaos and destruction, grew too powerful to be contained by the mirror and escaped to feed on the minds of all the people it can reach. With their minds gone, the victims of the entity become twisted, murderous husks that can no longer be described as human.
When the entity first escaped, it sneaked upon and preyed on several unsuspecting wizards at the academy, thereby acquiring the knowledge and power of its victims. It used its newly acquired abilities to reprogram the central hub that controls the teleportation mirrors throughout the city. These mirrors became channels that siphoned off the minds of those who would use these portals, effectively becoming extensions of the entity itself. Within minutes, the entity's intelligence grew sufficiently to tap into the central hubs of other cities throughout the world to feed its hunger.
All the nations of the Earth became drenched in a bloodbath unlike anything the world has seen. The victims of the entity were like bestial monsters that were filled with murderous rage; the weak docile half of the soul after it was rended was left defenseless against the stripped qualities. Magnified by time spent as a part of the entity, the ferocious half was easily able to dominate the weaker half. In less than two hours, the world population dwindled to less than a hundredth of its peak.
With the world nearly destroyed, the wizard, and those few who were able to help, sealed the entity back into the mirror, losing their lives in the process. Nevertheless, the damage was done, and most of the knowledge gained over the centuries was lost. The world delved into a dangerous place of superstition and rivalry among the survivors and their descendants.
