The Aeturnum Elves

Millennia after The Great Diminishing opened the door for mortal races to become the dominant species of the world, it was the elves who were the first to claw their way out of the primordial muck, set down roots, and begin building towards something greater.
The apex of elven civilization was Aeturnum; the Eternal People. While it was the nature of the kingdoms of men that would come later to squabble and climb over each other to achieve their heights, Aeturnum was a culmination of millennia of elven collaboration, debate, and unity. It was a civilization whose glory lasted for thousands of years and can still be found even to this day, though its shine has vastly diminished since its heyday. At its zenith, Aeturnum culture stretched from the far north of Horizon into the heartlands of Requiem and the western reaches of Kentro. And while the kingdoms that arose within the Sylvanlands were ethnically distinct, Aeturnum still held a strong influence with their wood elf brethren.
Unfortunately, much of what is remembered of Aeturnum is viewed through a human-centric lens. Even the name "Aeturnum" is an Antecean derived word. The elves that made up the Aeturnum empire referred to themselves as the Nur. The reason? A great calamity once befell the united kingdoms of elves and splintered them. Whatever this calamity was, the scholars of men can only speculate as the elves refuse to say . Whatever it was, it meant the elves were already fractured and divided when the kingdoms of men decided Archaios wasn't big enough for the both of them.
Even with their power a mere ghost of what it once was, the elves did not go quietly into the night. While eventually humanity overthrew them and cemented their place as the most influential race of Archaios, the elves yet lingered in their far-flung strongholds. But because of their isolation, they grew culturally distinct over time. Now, the city of Mir'Heled in the mountains of Horizon claims to be all that is left of the "true" Aeturnum Empire, while cities like Dor'loss in Requiem have "deviated" from the old ways.
After Antecea's collapse, Aeturnum saw a brief renaissance when the disappearance of their old foes allowed them to take back large swathes of their old lands. Unfortunately, it proved to be a Fool's Spring, as The Second Abyssal War decimated the tenuous control they had seized back and again allowed the returning kingdoms of men to force them back into their distant homes.

Culture

The customs and traditions of the Núr'uir people is steeped in their close connectivity to the magical Weave. Over the millennia, the elves have perfected the craft of magical automation and now enjoy a collectivist society where, for the most part, they are able to pursue higher callings and specializations while their mundane needs are tended to. The elves of Aeturnum do still utilize a currency system with which to trade goods and services, which take the form of crystalline shards that derive their attributed value from a centralized bank.
Magic is an inseparable aspect of elven life. Almost every house in Mir'Heled is managed by a host of conjured familiars, constructs, and hommunculi. Magic allows for instant communication and allows for mills and other agricultural devices and services to run themselves. Magically imbued tattoos give elven bladesingers additional powers in battle or provide elven nobility with whimsical abilities. Waste is cleared away via magic and music and color constantly streams through the streets thanks to innate enchantments.
Much of the Núr'uir's society revolves around the cultivation and expansion of knowledge. Before their empire fell, the elves venerated an entire Pantheon of gods and patrons (casting them aside when they did not intervene in their empire's decline), but now, they deify a singular being: Sae'l He'ru. It is the belief of the elves that when they die, their souls merge with Sae'l, but only if they have contributed something worthwhile to the zeitgeist of elven lore and knowledge. Because of this, most elves of Aeturnum dedicate their lives to cultivating as much knowledge as possible throughout their long lifetimes.

Peregrinations

In order to facilitate this never-ending quest for new knowledge, each member of Aeturnum society is expected to undertake a pilgrimage of sorts upon completion of their general education and mandatory service within the city's garrison, the Sentinels. They are charged with venturing out into the wider world to find something new. These journeys often take anywhere from a single year to decades, while some elves deign to never return at all. Those that do return always return with a fascinating new revelation or piece of technology from the everchanging world that continues to evolve beyond the timeless walls of Mir'Heled.

Governance

Mir'Heled is steeped in deep-rooted tradition. For untold millennia, the Nur'uir have governed themselves by bloodline monarchy. Because of their long lifespans and dedication to tradition, dynasties can last for thousands of years. As such, the Nur'uir have only had a handful of different bloodlines ruling the kingdom. The regent also keeps the council of a number of prominent family heads, usually no more than 6. Should a king or queen ever pass without an heir, it is this council who decides on a new ruler from among their number.

The Elves and the Former

It is clear from elven texts that The Former had taken a shine to early elves and were responsible for their initial elevation from the other mortal species still getting their bearings and figuring out stuff like making fires and growing plants and what not. 
It's unclear what exactly the Former sought to gain from their mentorship of the elves; many suspect they wished to raise them up to take on the mantle of stewards of the earth. Wood elves in particular seemed to have taken that mantle to heart while the high elves seemed to grow more independent as they developed. Regardless, their relationship with the Former can be highly credited with their success as the first civilizations of mortals, and the Former's disappearance can likely also be credited (at least partially) with the elves decline.

High Elves vs Wood Elves vs Dark Elves

The ethnical distinction between the latter two dates back to the very beginnings of elven evolution in their modern forms. The elves that would become "high" elves were the more ambitious of the two; creating expansive kingdoms and utilizing arcane magic to grow their influence over the land.
Meawhile, those that would become wood elves followed more closely in the Former's footsteps and sought balance with nature. They found their homes in far-flung wilderness and vast forests and created kingdoms that inhabited a delicate balance with the natural world around them. Their kingdoms grew slowly and deliberately like the trees their homes were hewn from, while the kingdoms of high elves constantly sought new ways to expand their own knowledge and arcane power, and by proxy, their influence over the natural world. 
As for the dark elves, their distinguishing event would occur much, much later. It is unknown what first caused the rift, only that in the mid-third millennium before Antecea, a civil war broke out in Aeturnum, that resulted in the losing side being banished to the Underdark to "never again walk in the light of the sun." It's likely that the winning side believed this exile to be little more than an extravagant death sentence, but much to their surprise and chagrin, the banished elves survived and eventually came to thrive in their new subterranean homes, nestling in along the dwarves that remained in their underground realms and the other outcast denizens of the Underdark.