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 DexDorugoramon 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 Data Squad
In Digimon Data Squad, Yggdrasil is a pivotal figure in the storyline, particularly in the show's final story arc.

Yggdrasil is first mentioned by Merukimon, who, ten years before the start of the series, told of Yggdrasil's existence to Dr. Spencer Damon (Daimon Suguru), the father of series' protagonist Marcus Damon (Daimon Masaru). Spencer and Merukimon departed, with the former stating that he would seek out Yggdrasil himself.

Years later, it seemed that Spencer had found Yggdrasil with BanchoLeomon's help. After defeating Craniamon, Yggdrasil brought up the Server Tree. Unfortunately, Kurata's unprovoked attack sent Spencer to prison to await trial. Spencer pleaded to be given one more chance on his life which Yggdrasil granted, only to be shocked at the news of Merukimon's deletion at the hands of Gizumon XT. This incident convinced Yggdrasil that humans are a threat to Digimon, ordering the Royal Knights to seize Spencer. But BanchoLeomon managed to escape with Spencer's soul in his body. But this didn't go unnoticed by Yggdrasil, who possessed Spencer's body to see through human eyes and warned BanchoLeomon not to tell anyone or he'll destroy Spencer's body. By the time Marcus and his comrades sought Yggdrasil in order to try and find a solution with him, they are encountered by Yggdrasil's cohorts, the Royal Knights, and eventually Yggdrasil himself - in the body of Spencer Damon.

Using Spencer's body, Yggdrasil ordered the destruction of the Human World in order to save the Digital World, sending his Royal Knights out across the globe to destroy it. Marcus resolved to discover the truth of what his father had become, and embarked on a quest to find Yggdrasil again. Once he found his "father", Marcus fought him until BanchoLeomon arrived and revealed the story about his link with Spencer before sacrificing his life for ShineGreymon to take out Spencer possessed by Yggdrasil. But it was in vain as Yggdrasil assumed a new form, a robotic Digimon, that flew off to the real world. Once there, he easily defeated the Burst Mode Digimon. Upon the Royal Knights arrival, Yggdrasil saw that even its own devotee's were beginning to doubt its sovereignty as lord, evoking its rage upon them. By the time Marcus and the gang arrived to fight him in the final decisive battle, Yggdrasil revealed that it was actually the computer which was designed to control all lifeforms in the Digital World at its will. Because of its observations of human-Digimon interaction, it desire to start over and reset the universe and began to turn any Digimon holding up the Digital World back into Digi-eggs. The Royal Knights helped Craniamon keep the Digital World from colliding with the Earth; Yggdrasil calculated a very small chance of success. But his Knights tell him all beings desire to live, which the simplistic computer couldn't comprehend.

Agumon underwent a pseudo Burst Digivolution, blasting a massive hole in Yggdrasil. Yggdrasil's true form, a crystalline figure with a female appearance, shot out of the robotic body and Marcus promptly punched it in the face, shattering it and finally defeating it. Recognizing its defeat and the real power of the combined efforts of Humans and Digimon, Yggdrasil's last act was to bring Spencer back to life before entering a state of hibernation saying "it would like to see how things turned out".

Unfortunately, due to Yggdrasil's defeat, the Digital World was left leaderless, so the DATS Digimon resolved to return to rebuild the Digital World. Marcus came along and he and Agumon became peacekeepers.

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.