A long time ago, in a galaxy which was, then, not so far away, there was a race of people known as "Ethereals". They built great works, they crafted great arts, they mastered great sciences, and they fought great wars. All their kind are long since gone from this life, and little of their own, direct work remains to this day... but what does remain shows a people deeply committed to ensuring peace, prosperity, safety and happiness for all beings, everywhere they could.
—Doctor Oltranis Yvain-Thoran, an archaeologist and scientist who studied the Ethereals
The Ethereals were an ancient species originally from another galaxy, the Cradle, originating on the planet Aetherius. They were a long-lived people, enduring for hundreds of millennia and sharing their home galaxy in harmony and friendship with the other members of the United League, their galaxy's governing alliance. However, after the War of the Ancients, the Ethereals and their comrades were forced to evacuate their home and move to other galaxies, spreading out so far across the stars that for the remaining millennia of their existence, fewer and fewer of the League's members could still make any sort of contact with the others, and worse yet, fewer and fewer of the races yet lived, and if they did, many changed; the Zekrosians and Vatopians, who had long shared a government, eventually merged to form the Zekrotopian race, while the Seralys diminished into some obscurity and the Valyrakyr and Duranesh became warlike and, from the point of view of billions whose friends and family members they slaughtered, evil.
When the Ethereals came to this galaxy, members of three races came with them; the Eternals, the Progenitors, and the Kalendin. However, the Eternals built themselves a new homeworld, shutting themselves away from galactic affairs and eventually dying out, while the other two gradually declined and regressed some ways in technology and science, both coming to die out after the Ethereals had, at some point following the Celestial-Rakata War. As for the Ethereals themselves, they determined not to isolate as the Eternals had, not to go on pretending their doom was not inevitable, as the Progenitors had, and not to deliberately regress themselves in hopes of survival, as the Kalendin had. Instead, they selected Humans from far away, elsewhere in the galaxy, and seeded them on nearby worlds, performing experiments, modifications, enhancements, alterations, all gradually building towards leaving their legacy, their culture, their abilities, all that they were save most of their remaining knowledge of the sciences, even including their reincarnating "souls", to these new Ethereal Humans.
As the Ethereals' numbers dwindled, initially still numbering in the billions, slowly declining to a single billion, then hundreds of millions, then tens of millions, the survivors decided it was time to finally settle down, together, in what would become their final resting place; after giving their last gifts to their "children", the Ethereal Humans (notably including alterations to their planets, star systems and even the region of space around them), the Ethereals gathered together on the world they named Vada Sevulchem, a name translating as "(The) Last Home (for death)". Here, the last of their great ships were dismantled, replaced with a network of gateways capable of teleporting people and things to pre-determined destinations, including not only every continent of the massive planet, but also the other worlds and all the moons of the star system. Here, they transformed each world and its moons to suit a particular purpose, with the main world, where almost all of them permanently lived (and, eventually, died) into a masterwork of their sciences; the last great work of the Ethereals was to be this, the star system Vada Tandrara, whose name means "(The) Home of (Those Who Are) Dead". Here, the plants, animals, and even minerals of their original homeworld were recreated, among monuments to their mastery of both science and art. Each secondary world was similarly remade in the image of the homeworld of each Ethereal "race", from a new Astraia to a new Caerion. One day, the system was discovered by the Ethereal Humans, who wondered and marvelled at what their progenitors had created, yet, they could not risk large-scale study and exploration of the ancient race's last known remnants, as exposure of it to the greater galaxy would be too great a risk. So, by the time of the Battle of Yavin, millennia later, not one of the system's worlds, not even a moon, had been so much as one quarter charted.