Saving Serara arc
During the travel to Susukino, in order of rescue Serara, Shiroe's Party enter in the Depths of Palm. There they realize that parts of the dungeon is cracking, something they note was not in the game.
When they arrive in the final zone Shiroe, Naotsugu and Akatsuki encounters Rat-Maso, concluding that it must be the reason the dungeon is falling apart. They decide to take the risk and destroy it clearing a way to the end of the dungeon.