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There are places in the dungeon where at times the walls of reality bleed together. These are areas that might seem normal, but are actually fully traversal, walls, floors, ceilings.
On the other side, lay what seems to a twisted and silent universe of shafts and hallways.
Some terrible creatures inhabit this realm, such as the Bagman (troll stats), or the Knockers, twisted humanoids of various sizes that lure in people by knocking on walls to get unsuspecting marks close enough to pull them into the backrooms.
Knockers can shift in and out of the walls of in this dimension, seemingly teleporting, using this to their advantage to terrorize marks so they can absorb their fear.
Knockers will rarely actually kill a target, preferring to let them go mad, and starve.