Mine Dungeon

Mine Dungeon (鉱山ダンジョン) is the tentative name for a mine under the rule of the Great Tomb of Nazarick in Mass for the Dead. To prevent incursion by humans it was temporarily converted into a Dungeon by Demiurge.

Background
Originally a mine belonging to the Re-Estize Kingdom but was abandoned and later became inhabited by various monsters. After the Catastrophe, the mine was found by Demiurge, who under the orders of Momonga began extracting the ores within. The ores were planned to be sold into the New World, as the human nations were now in a perpetual state of war with the appearance of the Chaos Beasts. The ores being converted as a source of income would then be used to fund the repairs for the damages the Great Tomb of Nazarick suffered during the Catastrophe.

Mass for the Dead Arc
The protagonist and Slimeko working as adventurers in E-Rantel attended a meeting of other high-ranked adventurers called by the guildmaster. At the meeting, the two see Blue Roses present forewarning something serious is about to occur. Pluton Ainzach arrives to call the meeting into order to discuss the state of the world and the rising prices of ores for weaponry and armors. Under a royal edict, the King requested the aid of the Adventurer's Guild to reclaim a mine north of E-Rantel to gain a new source of metals. Though the guild was supposed to maintain neutrality in national affairs, the guildmaster deemed the state of affairs in the world to be an exception and the necessity to acquire new resources imperative. The protagonist and Slimeko realizing that the mine the adventurers would be sent to reclaim was the one Demiurge was stationed at, reported to Momonga of the developments. Momonga having been delivered the news called a meeting of the Floor Guardians to address this issue. The group did not wish to relinquish control of the mine, nor did they wish to attract unneeded attention to themselves. Demiurge had predicted a scenario like this and proposed a plan. Calling it his “Mine Dungeon Project,” the demon proposed to turn the mine into a “dungeon of death” in order to not only wipe out the adventurers but also dissuade any future investigations once word of the deadly hazards of the mine would spread to the human nations. As the adventurers would require time to prepare to reach the mine, Momonga approved the idea and gave Demiurge a troop of skeletons for his use and ordered him to proceed.

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 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 body began to emit 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.

It was deemed a huge victory for Nazarick, who could use the mine without fear of discovery. A few weeks later E-Rantel got word that an emissary from another nation was selling cheap ores. The emissary, Lupusregina Beta, introduced herself to the fortress city, and the Kingdom managed to acquire the needed ores unknown that the ores were coming from Nazarick.

Layout and Features
The mine was radically altered by Nazarick and converted into a dungeon. Mare transformed the tunnel into an intricate shape to lower its visibility to force adventurers to spend more energy on detection. A forked tunnel was also included with the intent to separate the adventurers to commit them to longer searches to eventually exhaust their stamina. Mare used his power to superficially make it appear that the tunnels went in all directions. Along the tunnels, there were places that allowed monsters to hide away in order to ambush them in a trap. Additionally, traces were intentionally left behind by Demiurge to show that the mine was a site of demonic summonings, using foreign characters traced in blood and corpses of would-be summoners and tomes detailing in demons.

Trivia

 * Demiurge while creating the dungeon in the mine, had intended to create it as a site that would impact a worldview on the New World, so much it would inspire fear and terror to prevent further attempts at exploration.