This is an event in while you witness 9 black haired girls spontaneously exploding one after another and then witness 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.
Getting here is a little different than getting to the flesh walls world normally and takes a bit of chance. First off, the entrance is in the second area of the apartments which you get to by going: Pipes and Phone Poles > Apartments > Isometric Pathway > Apartments > Flesh Walls World
There should be a face by the blood puddle like in the picture 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 getting in 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 you can use the whistle to make a path instead (this method requires strategic thinking). There you we be able to witness the event.