Fate/Labyrinth

Manaka Sajyou, an existence connected to Akasha, is unable to experience phenomena like change and surprise, so she wishes to attempt to "dream." She sends her consciousness through the backside of the World, beyond the end of the world, and into different time and space. She ends up in a labyrinth, hosting a variant Holy Grail War, featuring four Servants and Masters attempting to reach the bottom. Manaka possesses the body of a Master who had summoned a female King Arthur. Saber interacts with Manaka, while Manaka and the original Master see each other when looking into a mirror.

(Source: Type-Moon Wikia)