Union of Eldar

In post-War Thera, a group of Elves, mostly of what would become the Adamant and Vagabond kindreds, came together and created something new: Cities on the surface of the World. The Titans had their sky cities, but those were more like assemblages of individual manors and estates than actual civic entities, as well as being highly specialised. The cities of the Union of Eldar were different, created to provide all the needs and many of the wants of an entire extended community. They were the first example of the high elven drive to civilise.
In addition to the cities of lowland Thera, the Union's architects created platform settlements for their Featherfolk allies. They employed the sonva as messengers, seeking to gather and unite the kindreds under the Union. In addition, the Union took on many of the nosva and trained them as spies. They built a realm that they intended would last forever, but even their greatest cities were crushed by the fury of the Cataclysm, resulting in the deaths of millions.
In the aftermath of the Cataclysm, the surviving elves of the Union committed to the path of their creator, H'ya Hivish (Raiser-of-Cities), and made the spread of civilisation their legacy. In the Adamant philosophy, cities are anchors which increase the harmony of their surroundings and residents. It was this thinking that informed the continuing push to resume the unification of the elven race and the creation of the Union's successor civilisation, the Regime.
The Regime and its successors have always had a romantic view of the Union of Eldar, viewing it as a shining beacon of ancient order and sophistication. This overlooks a number of texts which suggest that its immediate successors formed the Regime to avoid the mistakes of the Union, or that their high-handed attitude alienated the other elves, the Eladrin, and both the Dragonborn and the Orcs, or that the featherfolk who served the Union were unwilling slaves rather than willing allies.