King Drasil

Yggdrasil (イグドラシル) is the name of a fictional character in several iterations of the Digimon franchise, named after the world tree from Norse mythology. It primarily factors into the backstory written for the Digimon virtual pets and card game, which is a background storyline that is less prominent and often contradictory to the various anime. It appears as a character in Digital Monster X-Evolution and Digimon Data Squad.

In the continuities it appears in, although always presented as being very mysterious and powerful, Yggdrasil is the master computer that controls the Digital World, the otherworldly realm home to all Digimon. Due to this position of power, it is always presented as the "god" of the Digital World, overseeing its survival and, in some cases, destruction.

In two of the continuities in which he appears, Yggdrasil has command over the majority of the thirteen Royal Knights.

Digital Monster X-Evolution
In the animated movie, Digital Monster X-Evolution, which is based on the Digimon Chronicle storyline created by Bandai in conjunction with the Pendelum X virtual pet and card game, the original Digital World was cultivated and populated with Digimon by Yggdrasil. However, the continuous multiplication of Digimon left the computer unable to handle the data load, resulting in an apocalyptic event known as the "Digital Hazard" that saw the Digital World begin to collapse. Yggdrasil initiated "Project Ark," hand-picking a small number of Digimon to survive the catastrophe and transplanting them to a new Digital World it had created. However, Digimon who survived the death of the old world by way of the mysterious "X-Antibody" have appeared in this new Digital World, disrupting Yggdrasil's plans.

In order to move Project Ark into its second phase - the complete extermination of the Digital World and all Digimon life - Yggdrasil created an experimental being, Dorumon, from whose data it created a vast army of DexDoruGreymon, as well as his personal protector, DexDorugoramon, who later became Dexmon. Discovering Yggdrasil's true intentions, the Royal Knight Gallantmon turned on him, and after becoming an "X-Digimon" himself, he sent Dorumon to his comrades, Omnimon and Magnamon, in an attempt to bring an end to the fighting.

In the Royal Knights' stronghold, Dorumon was revealed to be Alphamon, the legendary leader of the Knights. This was enough to convince Omnimon to face Yggdrasil, and he went with Alphamon to seek counsel with his master, defeating DexDoruogoramon and Dexmon, who barred their way. It was then that Yggdrasil initiated the second phase of Project Ark, and the Digital World began to die as the DexDoruGreymon swept across it, reality crumbling in their wake. To stop the computer's scheme, Omnimon - transformed by Alphamon's X-Antibody - struck his master down with his "All Delete" attack, destroying Yggdrasil and allowing the Digital World to be reborn.

Later, Omnimon and Gallantmon speculated on Yggdrasil's intentions. For all that he had done, a reason remained unclear to both Knights - but in the end, Gallantmon believed that Yggdrasil had, just as all Digimon had been, fighting against his own inevitable destruction.

Digimon World Dawn/Dusk
Grimmon mentions Yggdrasil in the search the Digiarea Quest.

Trivia

 * Though Yggdrasil is relatively new to the Digimon mythos, a similar character exists in the Digimon Adventure and Digimon Tamers continuities named ENIAC. ENIAC, as the first computer, laid down the foundations of the Digital World, and because of his unique nature, he exists in multiple universes all at once.  ENIAC appears only in the series of games starring Ryo Akiyama for the Wonderswan, and is part of one of Ryo's attempts to defeat the evil Millenniummon.  As an added note, ENIAC's appearance in the game, as a clear orb sitting upon a pedestal, is practically identical to Yggdrasil's appearance in X-Evolution.