In Norse mythology, the concept of Yggdrasil was interpreted as a literal titanic ash tree whose roots and branches connected all of the Nine Realms, which consisted of Asgard, Jötunheimr, Múspellsheimr, Midgard, Svartálfaheimr, Vanaheimr, Nidavellir, Álfheimr, and Niflheim.
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During the Human-Isu war, several Isu became aware of an upcoming catastrophe which would result in both the humans and Isu facing extinction. Realizing that a solution was needed to counter the upcoming solar flare, some Isu choose to ignore the war and instead focused on finding solutions.
After many solutions had failed, the latest triad of the Father, the Mother, and the Sacred Voice had come up with a seventh attempt which had a chance of succeeding. This seventh method was called Yggdrasil, a supercomputer which could store an Isu's essence and reincarnate it into the human gene pool across eons. However, disaster struck before they had a chance to use it, though eight members from the Æsir branch of Isu managed to upload their consciousnesses prior to the computer's laboratory caving in.
Yggdrasil is an ancient Isu Supercomputer which can best be described as the Isu's version of the Animus technology developed by Abstergo. The supercomputer is engineered with a myriad of advanced abilities. Some of these abilities include:
- Essence Containment – This allows users to upload and preserve their consciousness into the supercomputer. The users can do this by uploading their psyche using a special mechanism that requires them to sit on a high tech chair and put on a glass mask to begin the process. A user's psyche could also be entered into the Yggdrasil via its branches. This was seen when Svala used the branches of Yggdrasil to upload her psyche into Yggdrasil and spend eternity inside the Valhalla simulation within the supercomputer.
- Reincarnation Inducement – Yggdrasil is able to reincarnate the psyches stored in it as humans across the globe. As of 2020, Yggdrasil managed to reincarnate Odin, Loki, Thor, Freyja, Tyr, Heimdall as well as numerous other unidentified Isu, all of whom could be identified by a black spot on their necks. It is unknown whether Yggdrasil had any part to play in Aita's reincarnations. Given the fact that Svala had successfully entered her psyche into the Yggdrasil, it is never discussed whether she herself has the possibility to reincarnate like the other Isu.
- Simulation Functions – Yggdrasil also create powerful simulations similar to the technology used in Animi. As discerned by Shaun Hastings, it appeared to have been used to "generate and explore possible futures, alternate realities, and counterfactual stories." This shows that the Isu could use Yggdrasil to effectively utilize the calculations. Yggdrasil also had branches which can be used to hook users into the computer and use its simulation functions. Basim Ibn Ishaq, the reincarnation of Loki, took advantage of being hooked onto one of the branches by studying the calculations thoroughly. Yggdrasil could also create simulations from Norse mythology, such as;
- Valhalla – Svala used the branches of the Yggdrasil device to enter the Valhalla simulation. Not long after, Sigurd Styrbjornsson and Eivor Varinsdottir—themselves reincarnations of Tyr and Odin, respectively—also managed to do the same.
- Niflheim – When Odin's psyche tried to forcibly persuade Eivor's psyche to stay inside Yggdrasil, he did so in a simulation of Niflheim.
- Biological Preservation – Yggdrasil's branches were also able to keep Basim's body intact in a comatose like state for over a 1000 years, with whatever minor visible decay vanishing when the Staff of Hermes Trismegistus regenerated him back to life from his comatose state.
- Assassin's Creed: Valhalla (first appearance)