Fate/Grand Order: Tokugawa Restoration Labyrinth - Ooku

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.

Headquarters has fallen eerily silent, now almost empty following an attack from the outside. Master is called urgently to the Control Room, and along with the single Servant who managed to escape harm, Rayshifts to determine the cause. They arrive in mid-seventeenth century Japan—the Edo Period. In the deepest reaches of Edo Castle, the epicenter of the issue, the silent abnormality still grows. The castle's Ooku, a realm detached from the rest of the world, has transformed into a bottomless labyrinth brimming with sinister mystery. The party roams the endless paths of the luxurious osuzurouka, the "corridor of bells," in search of an exit.