How to Get It
Visiting Pholus and attacking him with the Nail bat is the fastest way to trigger this event. The other, more
infamous way is to kill Trisha and then visit Pholus, which will trigger the same reaction in him. His face will turn red and pink with a distorted appearance, and he will begin to jerkily chase after Usotsuki.
When Usotsuki flees, the once-empty halls will be filled with long-haired girls rocking back and forth. Even if she runs away with the Rabbit effect and gets ahead of him, Pholus will automatically enter right behind her in whichever hall she has entered.
If he catches her, he will transport her to an area resembling the inside of the holes Usotsuki can look through in the Marble Hall's endless hallway.
If you manage to escape the Marble Hall during this event, you'll be taken to the coffin passage in the White Plains.
Theories and Observations
The Corpse Blitz event has been likened to triggering a Kaibutsu in the fangame .flow, as passive Kaibutsu can be made to mutate and chase the player in much the same way as Pholus.
Killing Trisha to activate the event has made players come to the conclusion that Pholus is a rather protective older brother or relative to Trisha, and that the two had a strong emotional bond.