When the members of S.E.E.S. fall asleep, they wake inside the Nightmare Hour — a space similar to the Dark Hour from the main Persona 3 game — and must descend through Oneiros, a dungeon accessible through a hollow opening in a great tree. Arche-Persona bosses block the path every twenty floors, with the first two determined by the active party composition. The 60th and 80th floors belong to Fuuka and the Protagonist's Arche-Personas, regardless of who is in the party.