Momonga and others launch a trade-based asset acquisition plan to obtain resources to revitalize and strengthen Nazarick. However, information arrives that the adventurer team- "Blue Roses" will be going to the mine secured by Demiurge.
Ever since the Catastrophe, the threat of Chaos Beasts have deeply affected the human nations. A consequence of this disaster is the rising prices of ores needed for weapons and armors. The Great Tomb of Nazarick still recovering from the calamity sought to find a stable source of income to supply the funds needed to repair the dungeon. Demiurge seeking to capitalize supply and demand of the ore market located an abandoned mine and began an operation to extract ores with the intent to sell the materiel to the human nations. Unfortunately, the operation came under the danger of being exposed by adventurers led by Blue Roses seeking a new ore source for the Re-Estize Kingdom. After being informed by their spies in E-Rantel, rather than retreat and abandon their claim on the mine, Demiurge enacted a plan called the "Dungeon Mine Project" to turn the mine into a "dungeon of death" to make the mines unsuitable for human occupation.
Momonga gave his support of the plan suggested by the Floor Guardian, providing the demon with a troop of Skeletons as a start. It was urged to minimize the use of resources from Nazarick and so Demiurge enlisted the aid of a few NPCs from Nazarick. The monsters that already inhabited the mines such as Hanging Spiders were subjugated by Entoma Vasilissa Zeta to be the dungeon's minions. Mare Bello Fiore changed the internal structure of the tunnels to create locations where monsters could ambush intruders in the tunnels. A fork in the mine was created to eventually separate the adventurers to further weaken them. Demiurge wished to ensure that the mine developed a worldview that would have an impact on society when word got out of the horrors within. An elaborate cover story was created, having the mine being a place where a demon summoning that went horribly wrong occurred. Abyss Demon provided the altar of which this summoning occurred and Demiurge's demons would create a demonic atmosphere. Additionally, throughout the tunnels, they planted evidence of demonic forces as work in the form of foreign summoning circles, tomes, and corpses to make it convincing.
At the mine, Demiurge planned to minimize the resources Nazarick needed to convert the mine into a dungeon. To do so he enlisted the aid of Entoma Vasilissa Zeta, who used her abilities to subjugate the insect monsters present in the mine as their minions. Mare was contracted to assist in altering the internal structure of the mine to Demiurge’s specifications. Abyss Demon provided his aid by creating the perfect cover story for the mine, as the den of a demon and provided the necessary props to make it look convincing. He was also installed as the nominal boss of the dungeon and waited alongside his master for the adventurers to arrive.
When the initial adventurers came to the mine, they were the first to be claimed by the mine. Blue Roses leading the next batch of adventurers came to find their corpses crucified at the mouth of the entrance. The group was horrified by the display, but those in Blue Roses noticed that this work was not done by mere monsters and hinted some sort of intelligence. They enter, press forward slaying skeletons and hanging spiders in their path. Lakyus though notices that the color of the walls were different between the parts that have passed the age and those that do not. It indicated that the mine was altered in some way. Evileye suggested that earth type monsters could be responsible like Stone Eaters or maybe Quagoa, though Gagaran doubted that even they could pull something off in such a short time. Gagaran abruptly hits a wall revealing a Hanging Spider and kills it. The discovery causes them concern as insects do not normally follow undead.
Coming to a fork in the tunnel which is not on the map. Certain that some intelligence had taken over the mines, Blue Roses takes the old tunnel, believing it to lead deeper into mines while the other adventurers take the newer route which they believe to be safer. Traveling through the tunnel, Gagaran becomes uneasy and Evileye discovered the scent of blood. A blood character is found written on a wall. Deeming it to be no trap the group investigates it to be part of some ritual. An imp suddenly attacks them and the party quickly kills them. The appearance of the demon causes Blue Roses to be concerned as its appearance in a mine should not be. Finding a drawing in blood on the ground with an animal sacrifice, they are attacked by another imp. After dispatching the creature they turn back to their findings and determine the geometric shape of the symbol to be part of a demonic summoning ritual. Additionally, they discover bodies with their internal organs removed and a book with foreign words, the same as the one on the walls and floor. It is believed to be a magic book specializing in summoning demons. This book, Evileye surmises allows one to summon a demon beyond their usual summoning abilities. They speculate on who would do such a thing, from an underground organization to devil worshipers. It is believed that the mine was used as a testing ground for their magical abilities and that they played with forces beyond their understanding, resulting in the infestation of demons.
The protagonist and Slimeko watching from the sidelines quietly report the thoughts of Blue Roses to Demiurge, who becomes pleased that the clues they left have created the proper cover. Demuirge, however, does not wish to allow the adventurers to rest and orders Entoma to send her minions to attack. Blue Roses and their allies are then assailed by Hanging Spiders. Then suddenly hear a cry for help from the other party, and rush to find one of the other adventurers being attacked by a strong looking imp. They fail to save the man and the demon escapes. Further, they find another ensnared in the grip of a Slime. The women are unable to help the poor man as the creature is able to melt their armor and watched helplessly as he is digested. They are distraught at their inability to save their comrades, and Evileye believes that it was the enemy's intention to lure them into the mine and eroded not just their strength but resolve. The agents of Nazarick, try to dissuade the group from continuing further, but the party despite the risks believes they owe it to their comrades to continue the mission. Plus even if they fail to find a new source of ores the threat of the demons is all the more reason to continue on to prevent them from escaping into the outside world. The group recommends using Lakyus's special move to level any strong enemies.
Demiurge learning that the adventurer still intend to come, sends out Entoma to greet them. Abyss Demon surprised by the developments readies for his eventual part in the plan. The adventurers reach a dead end but the twin assassins find it to be a hidden passage, where Entoma is waiting. The women have difficulty fight Entoma to the point that Lakyus is willing to slay her using her technique but is stopped by Evileye who wants to test her trump card [Vermin Bane]. The spell is blocked by one of Entoma's spider minions and is instantly killed. Seeing that its magic specializing in killing insects, the maid retreats, but not before warning of them of the Abyss Demon. Gagaran attempts to chase her only for a piece of bedrock to fall and block her. The revelation of a powerful demon waiting for them causes Lakyus to waver but her friends encourage her to continue and face the monster.
The party finally arrives in the deepest part of the mine, where an elaborate altar decorated with a human corpse is found. Abyss Demon announces his appearance declaring himself a demon from the abyss, and that he killed his summoner to prevent himself from being tied to a lowly creature. The group suspected as much ready for battle and attack. Lakyus attempts to land a blow but the demon appears to be invulnerable to the attack. Seeing the Abyss Demon being immune Lakyus activates her ultimate move: [Dark Blade Mega Impact] and manages to defeat the demon causing him to collapse on the ground. The group celebrates their victory, but before they could determine how to dispose of the corpse, Abyss Demon's emits a poisonous gas that begins to flood the chamber and the mines. The adventurers realizing the danger evacuate the mines to safety on the outside. The outcome of the mine reclamation campaign for the Kingdom ended in failure. The demons may have been killed but the poisonous gas made it impossible to extract the ores. However, the adventurers took solace in the fact they wiped out a major den of demons.
Back in Nazarick, Abyss Demon was teleported back to the dungeon where his wounds were attended by Demiurge. The plan was a success in Momonga's eyes as Nazarick secured the mine from further human intrusion under the cover it being a hazard for human activity. Thus mining operations resumed without interference. Weeks after the failed mine reclamation mission, an emissary from a foreign country, Lupusregina Beta, arrived in E-Rantel with cheap ores to sell. Thus the Re-Estize Kingdom was able to acquire the needed metal for its defenses whilst Nazarick was able to acquire a stable source of funds for its restoration.