Raised In Dungeon Volume 5
Road to Oxwadeshire Academy
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The Satan's worshipers have an ordo called The Ordo of Seven Devils; they are all high priests of Daemon Servus who lead the organization. Each of them possessed the Book of Legis, which allowed them to use demonic mana outside the dungeon.
Once a year, they have a day called the black sabbath, on which they do nothing strenuous and eat nothing. And on the black sabbath, the High Priests of the Black Church gathered.
In a very dark room, there was a very large coffin, about four times bigger than a human body. Underneath the coffin was a red light, which was the room's light source, but it was dim, not bright enough to make you see the room entirely.
Ten people were surrounding the coffin, sitting on a silver chair with a carved snake and goat that was 3 meters high; they were the high priests who gathered after five years of separation around the world.