A host computer named Yggdrasil hosts the Digital World and all the Digimon in it. It is rumored that Yggdrasil created Digimon, who one day appeared on the network as a computer viruses. Yggdrasil experiments on the Digimon and the Digital World.
However, the Digital World grew so large, and there came to be so many Digimon, that the Digital Hazard occurred. Yggdrasil was forced to create and implement Project Ark, which would delete all Digimon but the select few Yggdrasil chose to experiment on.
But, after the Digimon had been wiped out, surviviors were discovered! Their Digi-Cores had been modified by a mysterious program called the X Antibody, which not only made them immune to the X Program (the program Project Ark used to delete Digimon), it made them stronger and sometimes evolved them into new forms. (For example, a Mega Seadramon X will evolve to Giga Seadramon instead of Metal Seadramon.) However, to survive, you had to keep taking in the X Antibody. Also, some Digimon not yet attacked by the X Program were desperate for the Antibody...
All Digimon went to the new Digital World, which was divided into three areas - Urd, Verdani, and Skuld, each representing a different era in Earth's timeline. Urd was like the distant past, filled with dinosaur Digimon, Verdandi was like the present, and Skuld was like the future, with cities and machine Digimon everywhere.
Yggdrasil then sent the Royal Knights to delete all Digimon with the X Antibody, as to exterminate it. There are 13 Royal Knights total, however, the only Digimon known to be Royal Knights at this time are Omegamon, Dukemon, Magnamon, Dynasmon, Rhodo Knightmon, Alphamon, and Alforce V-Dramon.
Three children find their way into the Digital World - Kouta, Yuuji, and Shinji. Kouta's partner Digimon is DORUmon, and Yuuji's is Ryuudamon. Shinji, however, seems to partner all the Royal Knights once they become X Digimon.. The Digivices are referred to as the XAI (X Antibody Indicator) system.