In Norse mythology, 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.
During the Human-Isu War, several Isu became aware of an impending solar flare which would result in both the humans and Isu facing extinction. Realizing that a solution was needed to counter the oncoming coronal mass ejection, some Isu choose to ignore the war and instead focused on finding solutions to save Earth.
After many solutions had failed, Jupiter, Juno, and Minerva—the latest triad to hold the titles of Father of Understanding, Mother of Wisdom, and Sacred Voice, respectively—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 nine members from the Æsir branch of Isu—Odin, Tyr, Loki, Freyja, Heimdall, Thor, Sif, Frey, and Idun—managed to upload their consciousnesses prior to the computer's laboratory caving in.
Legacy and influence
Following the Great Catastrophe, the true purpose, design, and location of Yggdrasil was largely forgotten among the remaining human population. In time, it was passed down into myth, where it was reimagined as an immense ash tree that held the cosmos together, and became so important to Norse folklore that Vikings like the shieldmaiden Eivor Varinsdottir of the Raven Clan were known to tattoo a design of it on their body.
Yggdrasil is an ancient Isu supercomputer which can best be described as the Isu's version of the Animus technology developed by Abstergo Industries. The supercomputer is engineered with a myriad of advanced abilities, some of which include:
- Essence Containment – This allows users to upload and preserve their consciousness into the supercomputer. This can happen one of two known ways. The first is by sitting in one of the energy core-powered chairs surrounding the table which holds the computer's processing tower in the center. After powering up the chair and donning a faceplate, a small arm extends from the chair's head and takes a genetic sample from the user's neck, then feeds it into one of the four pods containing individual human fetuses that hang from the computer. The second method involves being physically entered into Yggdrasil via its branches, as seen when both Svala and Layla Hassan used Yggdrasil to upload their psyches to spend eternity inside the computer's simulations
- Reincarnation Inducement – Yggdrasil is able to reincarnate the psyches stored in it as humans across the globe. As of 2020, Yggdrasil successfully managed to reincarnate all nine of the Isu who used the device, 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 Assassin historian 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 to thoroughly study the calculations. Yggdrasil could also create a simulation of Valhalla from Norse mythology, as experienced first by Svala and later by Sigurd Styrbjornsson and Eivor Varinsdottir, themselves reincarnations of Tyr and Odin, respectively.
- Biological Preservation – Yggdrasil's branches could hold a user's comatose body intact for extended periods of time, long after natural death and decay should have occurred. In 2020, Basim's unresponsive body was still whole after being connected to Yggdrasil for over a 1000 years, with whatever minor visible decay vanishing when the Staff of Hermes Trismegistus regenerated him back to life.
- Assassin's Creed: Rogue (appears as ash tree on a medallion)
- Assassin's Creed: Valhalla (first appearance)
- Wrath of the Druids (appears as a tree on painted stone tiles)
- Assassin's Creed Valhalla: Geirmund's Saga (as the World Tree)
- Assassin's Creed: Valhalla – Modern day
- Assassin's Creed: Valhalla – A Brother's Keeper
- Assassin's Creed Valhalla: Geirmund's Saga – Chapter 6
- Assassin's Creed III – Modern day
- Assassin's Creed: Valhalla – Rigsogur: I. The King of Soft Reeds
- Assassin's Creed: Valhalla – Animus Anomalies: AA_Complete
- Assassin's Creed: Valhalla
- Assassin's Creed: Valhalla – Database: Yggdrasil