Just a few days ago, when Niusel gave them the house... under the indication from Neve Tower, they were also given the "key" to enter the local nightmare.
Before going to sleep, lock the door from the inside and tie a cloth around your left ankle—that was the key to enter the "Sporogenesis Mill".
Yes.
The local nightmare of this underground city is called Sporogenesis Mill.
It is a Twisted Tier nightmare, and its content leans towards horror puzzles. And it's the kind of horror puzzle with death traps—you might be walking and suddenly fall into a pit and die, or you might be investigating something, accidentally take a few extra glances and then suddenly kick the bucket.
The worst part is that dying three times in this dungeon doesn't get you kicked out of the nightmare.
There are special logout points, you must reach that location to exit... That exit point was found by Charge on the second day.
