This is an event in which the player witnesses 9 Black-Haired Girl NPCs exploding one after another, followed by viscera splitting out of the stomach of a large black-haired girl on the wall. After witnessing this event, your vision goes blurry and you wake up.It takes place in the Flesh Walls World, accessed through an alternate entrance. You will need the iron pipe effect to reach this event, and potentially the ghost effect to get past the Kaibutsu in the Isometric Pathway.
Getting here requires a different route than the one normally used to access the Flesh Walls World, and takes a bit of chance. The entrance is located in the second area of the Apartments, which is reached by following the Pipe World entrance to the Apartments' first area and, through there, traveling to the right across the Isometric Pathway.
There should be a face above the blood puddle in the hallway of this area, which you will need to hit with your pipe to open. If the entrance is not there, you might need to wake up and try again.
After entering you'll notice a familiar black-haired girl you could only see before staring into the giant gap between the two areas. Keep going right to the next room. The next room will be full of black-haired girls blocking your path; either kill them with the pipe to make a path or use the whistle to make a path instead (this method requires strategic thinking). Then you we be able to witness the event.