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 Múspellsheim. However, the leaf-eating monster continued its advance to the point where it seeks to devour the leaf representing Múspellsheim. This was the backstory upon which the players adventured into the world of Múspellsheim in order to protect it.
Múspellsheim 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 Múspellsheim which were advantageous to them and refused to go to other worlds.
In the past, the Múspellsheim's World Champion was said to possess a World Item that he has used to turn himself into a boss-type World Enemy for fellow players to challenge against. This unnamed World Item inflicted the said World Champion with a dangerous curse that would put him in such an unparalleled state. The World Champion of Múspellsheim have manage to join the ranks of the Lords of the Seven Deadly Sins. This World Champion fought with a lot of enemy players who continued chasing after him for the kill. Eventually, he met his final end by the players' hands, failing to escape their hunt as he was their World Enemy that they must slay.
After the World Champion of Múspellsheim was defeated, his player character was decided to be deleted by the developers. Without a World Champion to represent Múspellsheim, it was time for a new tournament to be held in a special festival at that world. It was to decide on who gets to become the next successor for the now ownerless World Champion job class. In the end, the victor of this tournament happens to be a female player appearance-wise. She achieved the status of not only being 2nd World Champion of Múspellsheim, but also the first female World Champion to exist among the nine players who have it.
Within Múspellsheim, 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 price for exclusive use with that class. Only one player with the World Champion job class was allow to exist in Múspellsheim. There is also just one guild base in this world that contains 3000 levels of NPC Data Storage which is 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.
- In the Web Novel, there are three guilds that rivaled Ainz Ooal Gown. One of such was an unknown guild that owned the field which spawned Fire Giants in Múspellsheim as their guild base.
- During an argument with Ulbert Alain Odle, Touch Me wanted Ainz Ooal Gown to farm for Fire Giants or the boss of them to fulfill Yamaiko's needs for the rare drop from it to enchant her item. "The Flame of the Beginning" item is most presumably in this world based on the fact it is dropped by these giants.