The Endless Blue
The titular Endless Blue, known in scientific circles as etherspace. A dimension parallel to our own, where reality follows a different set of rules. An infinite expanse of blue “sky”, brightly lit by mobile spheres of glowing particles rather than a single central source, and mirrored by The Void. Large cloud-like formations of varying density are the only other natural features. Many forms of magic and technology work differently or fail to work at all in etherspace, including most ship propulsion systems. There is a general “up” and “down” but gravity also flows in currents that vessels can ride with special sails that are unfurled while in etherspace. It appears to extend into mist and light in every direction, and no one has ever found an edge to it; theoretically, anyone who falls off a ship will fall forever. By shifting into etherspace, travel between worlds is made possible, and it is the only known means of traversing the vast distances between them in a reasonable amount of time (i.e. days or weeks rather than centuries).
Etherspace is linked to both realspace and the Void, but the exact manner in which it does so follows no known patterns. The clouds represent actual worlds and features in realspace, but locations and distances have no logical correlation between the two dimensions; a world physically closer to Earth may take weeks longer to reach in etherspace than one on the other side of the galaxy. The most common method of civilized travel involves crossing through Fairy Rings on inhabited worlds or a station in the Void friendly to the Fey to reach a port in the Blue and take a ship to any other port, but it’s also possible for vessels to “dive” directly into etherspace from any place in the Void. Ships in etherspace navigate between arcane beacons that have been anchored at ports and other waypoints near inhabited worlds or points of interest in the Void; most ships must drop into the Void at regular intervals to get their bearings from the stars. Most beacons are “open,” detectable by anyone with the right equipment, but every race has secret locations whose beacons are encoded and known only by trusted allies or military/government personnel.
Rumors persist of strange creatures and ghost ships flitting among the clouds, and once in a while a ship will vanish without a trace. The Legion also has a presence there, small fleets and individual raiders hunting for easy prey. Their territory is clearly marked, and though they aren’t openly at war at the moment, small skirmishes break out constantly.