(Will add when I come up)

Demons and Their Realm

• The last contact between human and the demon was a long time ago. Stories about warlocks summoning demons to make contracts are considered as nonsense.
• Black magic is, actually, demon’s magic power borrowed to a human in exchange for life force. Because human magic and demon magic mix and cause a strong rejection response, warlocks easliy go mad or don't live long. It was an old method of harvesting life force. But dungeons are safer and yield more life force, so it is no longer used and demons have stopped responding to warlocks' summoning attempts.
• Demon’s magic power is fundamentally different from the small amount of magic that each human possesses. Therefore humans instinctively feel a sense of wrongness and revulsion about it. The reason the residents of Middlesea felt discomfort when they first encountered the Dungeon Core is for exactly this reason.
• The demon realm has far more advanced magitech than the human world.

I'm the type who can't be satisfied until I've worked out a setting with real plausibility, which is why even something like this exists. The reason the setting related to demons is so detailed: this is written when I was really passionate about writing this AU. I poured every bit of my attention into keeping things coherent. But since the inhabitants of Middlesea never will know about all of this, I told myself I'd spill it out as a side story. And then I never finished the writing.

The Middlesea Dungeon

• Wild animals and livestock that could not flee by jumping into the sea were also affected by the Dungeon Core. They transformed into a diverse variety of monsters, most changing into monster types similar to their original forms. However, unlike the former humans, they are comparatively weaker and lack intelligence.
  • Just in case, those that ran wild and any dangerous types were herded into one area and dealt with by those with strong combat abilities. They were apparently quite a lot of trouble.
• Even within the dungeon, basic self-sufficiency is possible through the power of the Core. While resources that must be brought from outside have become difficult to obtain, inhabitants are substituting them with resources generated naturally within the dungeon.