Yggdrasil is a gigantic, mythical ash tree that plays a central role in Norse cosmology by linking the nine worlds together and also serves as the location where the Norse pantheon of deities hold court. Yggdrasil is also home to creatures such as Nidhoggr, Hresvelgr and Ratatoskr. The Norn goddesses Skuld, Urd and Verdandi are also said to live by a lake under the tree itself.
While the protagonist is exploring Sector Bootes, he is approached by a trio of Disir who ask him to challenge Yggdrasil, who at the point had siphoned the sisters' time-based powers, and was rampaging in an effort to consume everything. The protagonist was then teleported to fight the tree in Sector Eridanus' top floor; however, abusing his powers, Yggdrasil easily fended him off and brought him near death.
At that moment, Yggdrasil was hit by a blast that canceled out his time-based powers, forcing him to fight normally. A second opponent appeared and proceeded to beat Yggdrasil into submission. It turned out the protagonist was destined to survive the fight and reach Sector Eridanus at some point in the future, whence he would find the Norns who had been the Disir, who had by then retrieved their powers and allowed the future protagonist to return to the battle he had nearly lost and canceled Yggdrasil's time powers to allow the future protagonist to defeat Yggdrasil and return the stolen power to the past Disir, creating a closed ontological paradox in which Yggdrasil always lost.
Defeating Yggdrasil on Sector Eridanus unlocks him for fusion.
Shin Megami Tensei IV
"The ash tree known as the World Tree in Norse lore. Its branches cover the entire earth, and its trunk pierces the sky and holds up several worlds, including Alfheim, the abode of the Light Elves. Its three roots stretch into Jotunheim, Niflheim, and Asgard. It is said that it will live through Ragnarok."
—Shin Megami Tensei IV compendium
Shin Megami Tensei IV Apocalypse
Yggdrasil can teach Nanashi the Grand Tack, Dekaja and Tetrakarn skills through its Demon Whisper. It benefits from learning Gun, support and ailment skills.
