Personal theory
Rudy is going to push himself during the festival and collapse, the Knight is going to make a fountain during this, and since everyone is all there everyone (plot relevant) gets pulled into the dark world, Rudy gets healed by Ralsei or Susie and Noelle starts thinking that dark worlds are just like Heaven, and so wants to make more of them so no one will have to suffer with reality ever again
This is probably going to be what causes the roaring