'It is Earth-year 2391. Through a vast tear in the fabric of the universe, alien warriors emerge to fight an already embattled humanity. The sky is burning. The Gateless Gate has opened. The cosmic rip meanders like a burning string across the galaxy and slices through the asteroid field near the orbit of Jupiter. The great opening becomes known as, "The Mumon Rift"...' (Decipher Press release, April 30th 2004)
The Mumon Rift was created by the Shi and Quay, in their battle to destory one annother. In a galaxy far from our own, the Quay had rebelled from their slavery and abandonment at the hands of the Shi, and had begun to build themselves up to strike at the race they had percieved as Gods that had slighted them so, even going so far as to broadcast the coordinates of their bases, including their adopted homeworld Talgon, to the Shi. Since the Quay's colonies and bases were all within a single Solar System, the Shi built a quantum device to cause their sun to detonate, converting it completely into energy, and quite simply wipe the entire race of pests off of the Galactic map. Unfortunately for the Shi, they had greatly underestimated their former slaves.
The Quay captured the device, and used it to destroy the star at the center of the Shi homesystem in a suicide run. However, the Shi were not out of tricks. A group of Crowns, including the Razor-Wire Shi, had built a sphere of energy conversion satilites around Seyal the Shi homeworld, which sucked in some of the oncoming detonation, and right before the planet was to be seared from existance, made a hyperspace bubble around the planet. Seyal blinked from existance, and was safe, but the Shi had never finished the mathematics of moving a planet through hyperspace, and what happened next was entirely unexpected.
Seyal appeared in a new and foreign solar system, Sol, through the Mumon Rift, along with the Quay colony ships that had followed them in through the hyperspace bubble.
The Mumon Rift caused irreversible changes in the Sol system. Where three groups of humans, the Earthers, Gongen, and Mavericks had once been jockeying for power over each other, the balance of their interactions was conpletely blown away. They now each had to deal with fighting two different alien races who had no previous allegiances or grudges, and would preferably like them out of the way to finish their grudge match. The quay saw humanity as a roadblock to killing Shi, the Shi however had a bigger problem. The rift had opened up too far from Sol's sun, and Seyal was slowly freezing. Conquering an inhabitable world and colonising it would be necessary for survival, and Earth and Gongen were the two best Targets.
The Mumon Rift also was responsible for causing a permanant change in humanity, making some of them into a new superpowered version of human called Kizen.