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 and Mare, 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.

After Enhela was driven away by the attacks of the Pleiades, protecting Darkness, news of this was brought and the name of Kuyō to Momonga and the Floor Guardians. While the NPCs did not recognize the name, Momonga was all too familiar with it, causing him to be emotionally dismayed. He did not take Enhela to be just an idle threat, but a very serious one. While the Overlord did not know Enhela and the protagonist's relationship with the World Enemy, Momonga decided to use the latter as bait to draw Enhela and his master out.

During a massive spread of miasma spewing out of all know Cracks outside Nazarick, causing entire populations to revert in savage Contaminated Beasts, Enhela revealed himself to be the mastermind behind the crisis, claiming that the power to control the Cracks and the contaminants was borrowed via "divine authority" from Kuyō. The entire plot was all an elaborate ploy to locate the Great Tomb of Nazarick by extending his divine right throughout all the Cracks to determine the one Crack that was in the sought dungeon with the afflicted being not only collateral damage but part of a long-term plan by Kuyō.

According to Enhela he himself has not actually conversed with Kuyō, and only acts upon his faith and self-interpretations of his god, which he believes gave his role as Assistant Priest of the Pontifex Maximus in order to strengthen humanity through using a survival of the fittest tactic. This is done by controlling the Cracks to spread miasma so that its mists infect humanity, turning them into feral and yet stronger creatures before eventually curing the small numbers of them to evolve the human race.

However he changed tactics after he sensed the protagonist was a creation of Kuyō as well. While he was not sure what role the homunculus had to play he knew that his aligning with the Sorcerer Kingdom whose actions were advancing things way too fast as it was already deduced a cure for the miasma and planning to mass-produce it. Because of the protagonist's many number of investigations of Crack phenomena, Enhela believed it was his role to disturb them and cause the simultaneous pollution scenario.

After being refused by the protagonist to join him and bring his faith to Kuyō, Enhela directed his power to taking over Nazarick, forcing all the NPCs to evacuate. During the usurpation, a disembodied voice rang out which was suspected to be Kuyō's. Due to these actions Momonga confirms that Kuyō is going against Nazarick's interest. Enhela having hacked into the master source of the dungeon gained full control and gave them an ultimatum, either the protagonist joined him and used his own divine right or else Nazarick would be destroyed.

Despite Kuyō being a World Enemy, Momonga thinks all is not lost. Since Kuyō has yet to appear, Momonga suspects that the entity is either unwilling to show itself or is unable to do so, thus giving some hope to the denizens of Nazarick as they prepare to launch a raid to retake their home.

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 Devourer is an existence that exceeds the Six Great Gods, who can't even compare even if they tried. From his testimony the Devourer is an entity that is above the likes of not just the gods of his homeland, but the Eight Greed Kings and Thirteen Heroes.

The Devourer is able to grant the faithful powers such as "divine right" which enables a user to manipulate and control the Cracks in the New World. Another ability granted is the balance-breaking "hacking" ability which can override the master source of a dungeon and allow a user to assume total control.

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.
 * In Mass for the Dead, the World Enemy is said to be in full control over the Cracks, save for one which exists in the Throne Room.