From Dragon Quest Wiki

Yggdrasil (World Tree) is the term used to denote a great mythical tree in the Dragon Quest series. It commonly appears in the names of items which originate from the tree, such as Yggdrasil Leaf or Yggdrasil dew. In Dragon Quest IX the tree is part of the central plot and appears at the top of the Observatory.

Dragon Quest IX

In Dragon Quest IX, the Yggdrasil is considered the gateway to the realm of the Almighty, as when it blossoms the Starlight Express comes to ferry Celestrians. However, as part of the events of the game, when the tree blossoms, it instead causes a disaster in the Observatory. Yggdrsil is also the daughter of the God, Zenus, who's name is Celestia. She became Yggdrasil to show Zenus that even though mortals were flawed, some still have good in their hearts. She also produces a eighth fygg created by the benevolessence recieved from the heroes deeds, specifically made to make the hero mortal and eliminate the rule that celestrians can't attack their superiors.