Ooku is an interlude chapter of Cosmos in the Lostbelt, taking place between Lostbelts 3 and 4. Nearly everyone at Chaldea has mysteriously vanished, leaving only the Master, Mash, and Sion behind. Discovering the source to be 17th-century Japan, the protagonist and the only remaining Servant, Sessyoin Kiara, are rayshifted to Edo Castle. Alongside Kasuga-no-Tsubone and Pārvatī, they must navigate the Ooku chambers, which have been turned into a massive labyrinth. Originally released as a limited-time story event, Ooku was later rereleased as a Main Interlude available to all players upon completion of Lostbelt 3 and the SE.RA.PH Main Interlude chapter.