Devourer of the Nine Worlds

Devourer of the Nine Worlds (九曜の世界喰い) or the Last Boss is a World Enemy boss that bears origin from YGGDRASIL in terms of lore and backstory.

Appearance
This World Enemy was described to be a dragon in the form of a gigantic monster that toppled the World Tree YGGDRASIL.

Background
To the Developers of YGGDRASIL, the incredibly powerful last boss of the main campaign storyline in the game was known as the "Devourer of the Nine Worlds." As it has the status of being a World-Class Enemy, the Devourer of the Nine World is classified to be a World Eater.

The World Tree YGGDRASIL containing countless leaves was entirely consumed by this leaf-eating monstrosity. The leaves it ate happens to represent the overall nine worlds and the two hundred World Items from the game. This was the backstory upon which the players from each world must carry out their duty of protecting their homeland against the Devourer of the Nine Worlds.

The Dark Warrior Arc
While there was a conversation going on between members of the Swords of Darkness, Ninya brought up the likelihood of their group facing a powerful race like dragons. However, her companions dismissed that scenario to be next to impossible as the chances of their encountering dragons were extremely low. Still, it caution Ainz to think about the World Enemies of YGGDRASIL. He initially thought about the Devourer of the Nine Worlds, but decided to set it apart from the rest of its World Enemy counterparts for unknown reasons.

The Bloody Valkyrie Arc
Ainz recalled a brief history about the existence of the World Eater, noting as to how it happens to play a very major role as the final enemy to be defeated in YGGDRASIL's main storyline campaign for players after they had embark on a long journey over the nine worlds and vanquishing other boss monsters before it.

The Half Elf God-kin Arc
Whilst preparing for the trip to the Elf Country, Ainz Ooal Gown mentally noted that it was not always possible to beat an opponent no matter the level of preparation. He used the the Last Boss as an example as it could eat worlds.

Abilities and Powers
The Devourer of the Nine Worlds is an unimaginably powerful creature that even a legion, comprising with six parties of six players each will still not be enough to stand a chance against. Like other World Enemies, the Devourer also possessed complete immunity to players who attempts to use the respective power of each World item on it. As it is look upon to be a World Eater, the Devourer possessed the potent power of eating up worlds like Helheim and the World Items held in there if left unguarded by mass numbers of players against.

Trivia

 * By YGGDRASIL standard, it is most likely the final end boss of the game.
 * It can be estimated that at least around two hundred leaves of the World Tree YGGDRASIL have been eaten by the World Eater.
 * The Devourer of the Nine Worlds, it's most likely based on Níðhǫggr, a dragon in Norse mythology that is said to gnaw at the roots of the world tree and in some translations herald Ragnarök, the end of the world.