Touhou JRPG

After solving the anomaly, Reimu was invited to a tea party at Koumakan. In addition, two members of Hakugyokurou were also invited.
However, Koumakan suddenly shook greatly, and in an instant the structure of Koumakan changed.
What in the world is the meaning of this?

The main character chooses three companions, advances through a dungeon, and defeats a boss at the far end of the dungeon in this RPG.