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 enlisted the aid of Entoma Vasilissa Zeta and Mare Bello Fiore to bolster its defense and 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 and were crucified at the entrance. 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 pressed to uncover what was going on in the mine finding traces and evidence along the way that unspeakable magic had taken place to summon demons.

The party finally arrives in the deepest part of the mine where they meet Abyss Demon who declares himself a demon from the abyss, and that he killed his summoner. The group attack with Lakyus attempting to land a blow but the demon appears to be invulnerable to the attack. Lakyus then 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
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. After the adventurers were driven off, much of the mine retains the features due to convenience.

Altar
A prop that was placed in the final part of the mine to give it the impression that a demonic summoning had taken place. The cover story of the bones on the altar were claimed by the dungeon boss to belong to his said summoner after the demon rebelled.

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.