After the Great Tomb of Nazarick has somehow been transported to the New World, Momonga and Demiurge went together outside to investigate. Ainz Ooal Gown (known as Momonga at the time) and Demiurge were amazed by the beauty of this world and the two pondered if they may have some sort of purpose for being here. At that moment, Momonga lightheartedly mused that the reason could be for them to take over.
Although that was merely a joke by Momonga, Demiurge took his words as literal. Following the day when Momonga rechristened himself as Ainz Ooal Gown, Demiurge informed the denizens of Nazarick on what their king has told him. Everyone gladly accepted it and swore to make Ainz Ooal Gown's dream a reality.
Much time has passed, Ainz Ooal Gown and Narberal Gamma had traveled to the New World and posed as Momon and Nabe of the Adventurer team, Darkness. They had built a phenomenal reputation, earning the respect of this world's inhabitants. Meanwhile, Demiurge has concocted a plan to further reach that goal and chose the Re-Estize Kingdom as the perfect tool.
First Stage of the Plan as Precursor
Met with approval from Demiurge, Sebas was to rescue Tuare while she is still alive and have her swiftly evacuate to Nazarick. Demiurge appointed Solution to assist the butler in rescuing Tuare and healing her. Demiurge does not forget to warn Sebas that if the latter dare aid any of the humans who trod Ainz's name into the dirt, he will kill the butler himself this time around. Considering the life-or-death situation Tuare is in, Demiurge informs Sebas the different ways of how if he rather than the Eight Fingers, would have handle her as a hostage.
After Sebas and Solution parted ways with him, Demiurge explains to the rest present that they must capture people from several locations who might possess intelligence, dividing them into seven groups for which place they will be allocated to assault at respectively. Unlike the other NPCs, Demiurge place Shalltear on standby for mobile support duty out of fear that her [Blood Frenzy] skill will jeopardize their operation. He tells them that they will only be involved in his operation up to the first phase of the plan where they will return to Nazarick after completing it to reduce the chances of information leaking outside.
Hence, Demiurge cannot reveal the details of phase two to them, much less the details of the entire plan. He also made sure to inform all his collaborators of this operation to spare one human (presumably Climb) with help from Entoma who use her illusion spell to show them what the warrior look like. Let alone that, he warns his collaborators ahead of time that they are not permitted to leave their operation areas without his permission. When any Guardian including himself questions them, the latter will immediately put up their hands or the equivalent appendages and refrain from suspicious movements. Any such movements will be met with death, for safety's sake in precaution towards the unseen foe who used an unknown World Item to brainwash Shalltear.
Additionally, Demiurge informs them that Sebas without a World Item to protect himself will serve as bait for such unseen foe to lure them out. Meanwhile, Albedo will be standing by in the Throne Room for precisely that reason to watch out for whatever may happen to Sebas. To add on, those who cannot use the [Message] spell must be more careful and not act on their own. Since he will be overseeing the entire operation, Demiurge requires everyone to contact him if they have any questions. In addition to that, if an emergency arises where they cannot contact him, they must instead contact Mare who will direct the operation in his stead as the latter have already know about everything, including the demon's plan. After that was settled, Demiurge sends Nazarick's forces to attack several locations, including a new location added at the last minute upon being informed by Renner.
Once the first stage of the plan is completed, they will immediately proceed to the second stage, which holds the most critical and importance part of their operations coined as Gehenna. Every stage of this operation seems to be targeted and happen at the capital city of the Re-Estize Kingdom, Re-Estize. During a private discussion with Ainz, Demiurge revealed that the vast armies of demons being used to attack the Royal Capital were none other than summoned monsters conjured forth by his loyal servants, the Evil Lords.
Sebas Tian and Solution Epsilon had been living in the capital, acting as spies for Ainz and were working undercover to gather information. During that time, Sebas discovered a young slave girl named Tuareninya Veyron and took her in. This drew the attention of Eight Fingers and after a series of events which leads to the capture of their subordinates, the organization decided to kidnap the Tuareninya to lure Sebas out to be killed for his defiance against them. Ironically, their latest scheme proved to be defiance against Sebas's master, Ainz Ooal Gown who commanded his Floor Guardians to hunt the criminals down like the animals they were.
Sebas was joined alongside another party who also were also determined to bring Eight Finger down. The party comprised of Brain Unglaus, Climb, and Lockmeier. Together, they confronted Eight Finger's security forces, Six Arms. Sebas single-handedly killed five of them while Climb managed to defeat the remaining one. The team successfully rescued Tuareninya and crippled the criminal organization.
Meanwhile, Mare and Entoma had being tasked to infiltrate the home belonging to one of the Eight Finger's high ranking leaders. After eliminating all of the guards, Mare incapacitated her and Entoma's quarry. Mare departed with the leader and anything valuable leaving Entoma to stay and finish off what remained.
This leads to Entoma have her encounter with Blue Roses who nearly had killed the combat maid if Demiurge hadn't intervened and saved her. Demiurge, under the guise of Jaldabaoth, made short work of the Adventurers, killing two of them in the process.
Coinciding with that battle, Darkness had just arrived at the Royal Capital to help suppressed Eight Fingers. Momon sensed Demiurge in battle and understood this to be part of his Floor Guardian's plans. He helped the remaining Adventurer, Evileye from the demon in order to throw off suspicion and allow Jaldabaoth to escape. After that, the Flames of Gehenna started to appear. This was when the Operation has begun.
Some time ago, Demiurge became aware of both the rise in criminal activity and the corruption within the kingdom's nobility. He saw them as an opportunity for Operation Gehenna and Tuare's kidnapping was the perfect excuse to carry it out.
Operation Gehenna was to have an army of demons invade the capital. The goal was to carry out a series of violent and destructive acts that will force the Kingdom's Royal Army to intervene. When the kingdom's forces will prove unable to fight off the demons, Momon will enter the battle and successfully repel the invasion.
Darkness and Evileye met up with Princess Renner's forces. A plan has been made for a majority of the Kingdom's army and the Adventurers to fight off and stall the demons. While this happened, a small team consisting of Momon, Nabe, and Evileye will enter the heart of the disturbance and defeat the demon's leader, Jaldabaoth.
The Re-Estize Kingdom struck back. Lead by Blue Roses, Gazef and members of the Royal family, the re-inspired Royal Army and Adventurers managed to wipe out most of the demons. Momon, Nabe, and Evileye attacked Jaldabaoth and his maids and won after Momon brought Jaldabaoth to his knees. Before Momon could deal a finishing blow, Jaldabaoth bargained with the Adventurer. The demon revealed that he has an additional army standing by who will attack the capital in case of his defeat but if Momon spared his life, the demons will leave Re-Estize. Momon agreed and the invasion ended.
While Re-Estize celebrated their victory, the forces of Nazarick resumed their plans. The leader Mare and Entoma captured, Hilma, was sent to Kyouhukou to be tortured. Hilma was mentally and physically broken by the torture that she betrayed her comrades by giving up all of their information to her captors.
This combined with the devastating loss of Six Arms, Nazarick easily infiltrated and took over all of Eight Fingers' operations.
Operation Gehenna was a complete success. Nazarick was able to successfully give rise to the newly-founded existence of the Demon King, Jaldabaoth in Re-Estize for people to acknowledge as a threat. Everyone lay the blame on Jaldabaoth and his armies of demons that were responsible for creating the chaos in the capital and for the disappearance of their citizens that were unknowingly kidnapped by Nazarick. The Nazarick denizens involved in this operation were never found out by the public thanks to Jaldabaoth and his demon armies using themselves as a scapegoat to cover their tracks.
As a result, people came to acceptably believed that Jaldabaoth ventured into the capital to retrieve a dangerous item. They viewed such item to be responsible for the summoning of Jaldabaoth and its forces in the capital. After discovering the item at one of the Eight Fingers' warehouse districts, this also lead people like Climb to lay the blame on the criminal organization for bringing it to the city.
Due to the Royal's Army's inability to repel the invasion and the nobles who cowardly hid away from the battle, public opinion towards them were dangerously low. Instead, the citizens praised Momon for bravely saving the Re-Estize Kingdom from ruin. By the time of Ainz Ooal Gown's take over of E-Rantel, many of its citizens and Adventurers would rather stay alongside Momon rather than have faith for the kingdom that abandoned them.
Due to the success of this operation, Nazarick was able to stealthily obtain all the wealth and good in the capital's warehouse district unnoticed without problem. At the same time, the forces of Nazarick has subjugated Eight Finger's chain of command. After enduring repeated torture like their comarade, Hilma, the leaders willingly submit to their new masters. With that, Nazarick used Eight Fingers to carry out their underground activities to further undermine the Re-Estize Kingdom and conquer it.