A Certain Monster
There are thousands of pillars emerging from one place to another, it disappears after being cleared and getting all sort of treasures and gifts in excahnge, but there's seven of them placed in a different category. Though, they're said to be just the same pillars, it has a demon living inside, making it their own sanctuary. It hasn't been cleared yet, but it gives unlimited resources and treasures, it has an entrance and exit. Though, only a few actually used the latter.
So even if it provides, only the higher ranking adventurers dares to give it a chance.
But it's not the demons that scares them, because those pillars aren't the demon's home, but rather their jail. So, it's not the owner that's scary, but rather the monsters within that guards it.
After years of research and a few thousand sacrifices, there's finally a guide provided on how to deal with a sanctuary. One might think: So, it's finally safe?
No.
One certain monster keeps them grounded.