Devourer of the Nine Worlds

Devourer of the Nine Worlds (九曜の世界喰い) is a World Enemy boss that bears its 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
Created by 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."

The World Tree YGGDRASIL held countless leaves, each representing whole worlds, which were consumed by this world-eating monstrosity. The remnants of these leaves (worlds) that were devoured would go on to form the two hundred World Items of Yggdrasil. This was the backstory upon which the players from each world must carry out their duty of protecting the Nine Worlds 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 finalizing his plans to take a paid vacation to the Elf Country with Aura Bella Fiora and Mare Bello Fiore, Ainz Ooal Gown was nervous of successfully passing the plan through the "Last Boss" Albedo, a boss he regarded in his opinion to be more fearsome than facing the Devourer of the Nice Worlds.

Mass for the Dead Arc
Upon encountering Enhela in the "Chaos Environment", the protagonist and Darkness grew wary of him, as the priest was out of place in the dimension that only they should be able to traverse. Even more peculiar was that Enhela lacked a protective gear to allow one to function on the "chaos environment".

Likewise Enhela noticed that the protagonist lacked wearing protective gear and this was enough to convince him that he and the adventurer shared a bond and allegiance to the same entities. Since the protagonist lacked an understanding of what he was talking about, Enhela then told his 'brother' about the true god he worshipped.

Abilities and Powers
The Devourer of the Nine Worlds is an unimaginably powerful creature that even a legion, comprising of six parties of six players each, would stand little chance against it. 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.

According to Enhela, the Nine Celestials are existences that exceed the Six Great Gods, who to him can't even compare even if they tried. From his testimony the Nine Celestials were entitles that are above the likes of not just the gods of his homeland, but the Eight Greed Kings and Thirteen Heroes.

Trivia

 * 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.