Basically, Manaka wanted to try dreaming and experience change, surprise, and all the sorts of things that aren’t really associated with someone being hooked up to the Root, so her conscious goes past the backside of the world, beyond the end of the world, to different times, different space, and in the process she sees a dragon waiting for someone for a very long time, then she sees a light shining at the light of the world, stitching the front and back of the world together, and then her conscious gets dragged into some one room apartment that seemed like the whirl of everything, telling her nope, leave it here, and then she finds herself in some labyrinth, apparently possessing someone else’s body and as the summoner of Saber.
So it’s basically Quantum Leap.
Except Saber sees Manaka, Manaka sees the original owner of the body when she looks in the mirror, and the original owner of the body sees Manaka when Manaka looks in the mirror.
Apparently this is one of those variant Grail Wars, with a labyrinth and four Servants instead, and the goal is to get to the bottom of the dungeon.
Same writer and illustrator as Fragments. Rather than a new series this will be more like a story within a story, if you ask me. According to Sakurai herself, It is confirmed to be written in a span of three months, which means it’ll most likely be three chapters/acts long and released as a special volume.