The Mist arc was born many years ago when I would play with my toy spaceships. I tried to imagine a form of faster than light travel that didn't come with a lot of the troubling paradoxes that employing hyperspace or other such devices would. So I imagined something a little more material. Namely, I imagined that spaceships could, at the command of their captains, disintegrate into a fine, white mist, and reintegrate wherever else in the universe this mist was present. It didn't need to be connected in any physical way, it just had to be there. Because this travel was by force of will of a human mind, it introduced a lot of philosophical peculiarities, along with the physical ones.
Hence, this arc is geared towards exploring those peculiarities, formally starting with the human discovery of a completely mist planet, progressing with the slow adaptation of the human race, with its previous mindsets and philosophies, to this new substance, and ending in the distant future in a setting I have yet to fully work out.
The nature of this arc is so orthogonal to the Manifold arc that its integration into it is actually, counterintuitively, very easy. So, more so than the Magi arc, the Mist arc can really be thought of as a huge story arc for the Manifold universe.