X-Antibody

The X-Antibody (X抗体 X-Koutai) is a vaccine that makes Digimon immune to the X-Program. Digimon that are introduced to the X-Antibody undergo a change in physical appearance (e.g. crystals emerging on various parts of a Digimon's body) and usually become more powerful than their previous, non X-Antibody counterparts. Some Digimon already have the X-Antibody present within them.

Project Ark
When the Digital World reached its maximum capacity, the host computer of the Digital World, Yggdrasil, was unable to manage this. Its solution was to eliminate the vast majority of the Digimon with the X-Program, and select a very small percentage to be moved to a new Digital World, and destroy the old one. Those who were not chosen but still survived and moved to the new world due to their Digicores being altered to resist the X-Program, forming the X-Antibody as a consequence. The X Digimon fled to a new Digital World which was split into three terminals consisting of Ulud (a misspelling of Urd), a past plain which is a volcanic wasteland, inhabited by Dinosaur type and draconic Digimon, Versandi (a misspelling of Verdandi) is the "present" region which is a world of lush greenery and is home to beast, bird, plant and other nature Digimon, and Skuld is the "future" region, a high-tech city where machine and insect Digimon inhabit. Yggdrasil sent the Royal Knights to eliminate the X Digimon.