This is a minor event in the game.
How to Trigger this Event
After making it to the foggy area in the Flesh Walls world and hit the calm kaibutsu; avoid it for as long as you can.
Within the Flesh Walls world there is a foggy and peaceful area with a calm Kaibutsu walking around. Once you hit this kaibutsu with your Iron Pipe and quickly make your way back to the teleporter you will find yourself in a very corrupted and bloody version of the area you were just in. The Red Demons are now there as well as the now insane kaibutsu. You are trapped in that area unless you wake up or use the Arm effect.
- This is the only calm place the rust monsters are seen invading until the endgame.
- It is possible this place represents a spread of a virus, due to the fact that it is calm and healthy-like until hitting the Kaibutsu, in which it becomes rusted and unhealthy-like.