"Don’t you think it’s wrong for you to hang about here?"
Oneiros is a dungeon comprised of 80 floors that strongly resembles Tartarus from Persona 3. In Persona 3 Em, S.E.E.S. are taken there after falling asleep, and they must complete 20 floors of exploration and defeat an Arche persona boss (Arche-Penthesilea, Arche-Polydeuces, Arche-Lucia and Arche-Orpheus respectively) before being taken out of Oneiros.
Oneiros has distinctive designs for all of its different blocks that are lifted from Persona 3. However unlike Tartarus, Oneiros includes both regular chests and golden chests that contain special items.