Fate/Grand Order: Tokugawa Restoration Labyrinth - Ooku

This is the prelude to Cosmos in the Lostbelt No.4.
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.