The World Tree YGGDRASIL had countless leaves, but then a gigantic monster appeared which devoured these leaves, causing them to fall one after the other until only nine were left. These nine leaves were the previous incarnations of the Nine Worlds, particularly Niflheim. However, the leaf-eating monster continued its advance to the point where it seeks to devour the leaf representing Niflheim. This was the backstory upon which the players adventured into the world of Niflheim in order to protect it.
Niflheim was one of the realms where many players of the heteromorphic races mainly resided in. During the early years of YGGDRASIL, players belonging to heteromorphic races were hunted by various PKers.
The world was described to be ridiculously huge, and there were many places within it which were difficult to explore. For instance, there were many areas which required the use of special equipment along with a proper strategy and the determination in order to delve into its hard-to-find dungeons that drop valuable Data Crystals.
When non-heteromorphic players went hunting for heteromorphic beings to acquire strong job classes, the latter players of heteromorphic characters often hid out in the three worlds like Niflheim which were advantageous to them and refused to go to other worlds.
Within Niflheim, things became more dangerous the further one went from the center of its world. In addition to the wandering monsters, the very terrain itself became a hazard for players to explore.
A developer-sanctioned tournament was held in this world where one could become a World Champion and acquired a special powerful equipment awarded as the winner's prize for exclusive use with that class. Only one player with the World Champion job class was allowed to exist in Niflheim. There is also just one guild base in this world that contains 3000 levels of NPC Data Storage which is the highest amount by far available.
- This world has its own particular traits, but they often shared one similarity which is the fact that it is two to three times the size of Tokyo.
- Some guild members of Ainz Ooal Gown had once thought of trying to conquer one of the worlds in YGGDRASIL.
- Cocytus's job class "Knight of Niflheim" may have a connection to the world of Niflheim. The class's special ability [Frost Aura] used an enhanced extreme cold to instantly deal ice damage on adversaries and slightly lowered their speed.
- There exists a Frozen City in Niflheim, which could only be accessed through during a blizzard.