Preface: This is not a backstory for K'aresh, the homeworld of the Ethereals. (What's with these apostrophes, by the way? Are they diacritics to other letters, or do they mark exotic consonants or morphological boundaries?) The idea was that it could be the setting for an expansion, and this would be the lore to go with it. I posted this on Blizzard's forums, but it got little attention there.
Know first that the Ethereals have no true homeworld; they are born of the vortices in the Twisting Nether, and one of the roles of the Consortium its rivals is to seek out and raise the newborns. (Which explains much of their trading culture: they are not more selfish than other races, but they have no bonds of parentage or patriotism.)
K'aresh was built by the Ethereum, working with most of the other Nexus associations. It was a world on which to meet fleshy trade partners, to rent out to them, to be a safe base of operations, to do research on huge scales, and to be a symbol of their power and ingenuity. So while the Orcs felt the loss of their world as a threat to their very survival, to the Ethereals it was above all a blow to their pride.
K'aresh was the world that the Titans would have built if they had the budget. It was made as a Halo (or Ringworld, or Orbital) with about the same land area as Azeroth in a corner of the Nether. A life-bearing surface on the inside of a huge ring of nether-imbued steel with a khorium frame, with a protective layer of thorium underneath. At its center, held by pylons of khorium coated with glistening mithril, was the Sun, a huge mana collector that gave both light and energy to those on the surface.
And there was the cause of its downfall. While building K'aresh, the engineers drained the Netherspace about it of ambient mana, both to power the construction and to make the world safe to live. It left only a handful of accreted clumps of mana, twinkling faintly, floating about the world. They continued the draining for centuries afterwards, and their economy grew around massive, often wasteful, energy consumption. This was strained when the mana was drained from the collector's range, and transmitting it from further afield would have consumed more than it gave.
The Ethereum's experts said the best solution was to cut down on energy use and rely on the slow renewing of mana in the region. Many also suggested using elemental forces on K'aresh itself, which the Ethereals were reluctant to do because it would lead to the growth of an independent ecosystem of both wildlife and elemental beings; they preferred to keep the place under their own control. Unwilling to take these solutions, the leadership went for another source of energy that could feed their rate of consumption, despite a lot of evidence of the dangers it posed, which was the Void. Hey, you're the one seeing political allegories here, not me.
Void energy kept K'aresh lit for decades, but in time the dangers started to show. They ranged from sudden attacks by hostile void beings to subtler corruption on all life on the world, including the Ethereals themselves. Yet even as opposition built up, the Ethereum came to be dominated by a faction that would become known as the void cultists (Yes, the "nothing is sacred" pun works in their language too).
These were Ethereals who saw in the Void more than a source of energy; they enslaved its beings, based their magic upon it, even infused their body with void energy, much like the warlocks of other races with fel magic. And they gradually took over the Ethereum for the simple reason that they were willing to do the dangerous jobs on the Sun's void collectors. Once they organised themselves, they were able to accrue political power by threatening to strike and cut off the energy flow.
It is clear that people willing to take morally dubious decisions despite being repeatedly advised against it will always turn up when needed. They are the true unsung heroes of Warcraft, for without them there would be no franchise, or at least no backstory.
Things came to a head when the void watchers confirmed that an off-the-chart void lord, the one they would later call Dimensius, was manipulating the energy collectors for its own ends. The Ethereum's void cultists blocked any phasing out of the siphoning, believing the lord was just a bigger target to be enslaved and used. At this, the other side took up arms.
The ensuing civil war only worsened the situation as both sides fought over the Sun. No control was possible as one side tried to shut it down and the other tried to redirect the flow to the loyalists. In time the rebels won and the void siphoning was stopped, but it was too late. The flow of energy had created huge conduits to the Void, and Dimensius' legions of voidwalkers were able to run through them and overwhelm K'aresh's defences.
The whole population had to be evacuated, and being Ethereals, they succeeded quite well, most of them flying off into the Twisting Nether on their own power. But the psychological and material loss was beyond count. And their political unity never recovered.
Yet even in K'aresh's darkest hour (literally, with the Sun shut down), the last to leave witnessed a wonder. One of the mana residues in the sky grew greatly in brightness and gave a dim day to the surface. The voidwalkers seemed repelled by it, and a number of Ethereals said they owed their survival to it. This was what they called the "Second Sun of K'aresh".
Afterwards, the void cultists kept the name of the Ethereum, but their continued experiments drove away everyone else. The results can be found in the Netherstorm. The rest of the Ethereal people split up into Nexus associations, of which the Consortium became the biggest and most stable.
And the expansion comes when Dimensius' armies make assaults on other worlds, among them Azeroth. By catching and reversing the portals, the Ethereum finds ways to pierce the veils of Void around K'aresh, thinking they can harness Dimensius's power. The Consortium follows behind, to end the threat of both the Ethereum and Dimensius, and salvage something of the treasures they left behind. They expect to find K'aresh in lifeless fragments, the very metal corrupted away.
Instead they find that three-quarters of the ring are still intact, and voidwalkers only control it in parts. They have been kept at bay by the Second Sun, which has become nothing less than a manifestation of the Light; the Void and the Light are complementary and where one goes the other follows. Beneath the Sun, wildlife and elementals have grown, albeit shaped by the savage struggle against void beings.
The Consortium, as usual, hires adventurers to help its work reclaiming the world. Many dangers lurk, with pinnacle being Dimensius waiting in the now-defunct Sun. (The one you fought in the Netherstorm? A weak avatar.) The Alliance and Horde are pushed to help by the threat of Dimensius. Shamanistic and druidic missions come to help nature settle in a new world. The Burning Legion has arrived by its affinity with the Void. Nesingwary has heard that some of the feral pets have grown to be huge. There will be a flurry of blades, spells, and mouse clicks.