X Antibody

When the Digital World became so overpopulated, the super computer that governs the Digital World, Yggdrasil, wasn't able to handle it anymore. His solution was to wipe out the vast majority of the Digimon with the X Program, and choose 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 had a rare gift known as the X-Antibody. This antibody, when activated, changed their appearance and strength.

Those who have the X-Antibody are generally distinguished by the X in their name and sometimes an X placed on their body somewhere or a gem (example: Dorumon's on his forehead, WarGreymon X's on the front portion of his front horn). There are few with the X-Antibody that aren't distinguished by an X after their name because there is no other version of them in existence.

The X Digimon fled to a new Digital World which was split into three Terminals (the three terminals are named after the Norns from Norse Mythology) consisting of Urd, a past plain which is a volcanic wasteland, inhabited by dinosaur and dragon Digimon, Verdani (mispronounced as Versandi) is the present which is a world of lush greenery and is home to beast, bird, plant and other nature Digimon, and Skuld is the future, a high-tech city where machine and insect Digimon dwell. Yggdrasil sent the Royal Knights to eliminate the X Digimon.

Also, in America, Digimon with the X-Antibody are prominently featured in the card series Digimon: Operation X Certain Digimon have the X Antibody, some Digimon are vulnerable to these Digimon, and another group of Digimon can easily destroy Digimon with the X-Antibody. Modify cards can also add, destroy, alter, or get power from the X Antibody.