Who they were
The Shen'dralar were a secretive Highborne society, and their name means Those Who Remain Hidden. They were Queen Azshara's most trusted arcanists, and their job was her library: the magical tomes she did not want anybody else reading.
Under Prince Tortheldrin they built Eldre'Thalas in the southern jungles of Kalimdor - the place the rest of the world now calls Dire Maul.
They should not have survived
The Sundering broke the continent. Eldre'Thalas did not break, because its arcanists wove a great spell to shield the city from it.
That single decision is the hinge of everything that follows. A city of arcane scholars came through the end of the world intact, with its library, its walls and its habits - and without the thing all of it ran on. The Well of Eternity was gone.
The basement
Highborne immortality came from the Well. With the Well destroyed, the Shen'dralar were facing the end of being what they were.
Tortheldrin's answer was to find a replacement, and the replacement was a demon. Immol'thar was imprisoned inside pylons in the west wing of Eldre'Thalas, and the prince siphoned the demon's power and handed it to his followers.
It worked better than it had any right to. The wiki's own phrasing is that the demon's power proved more potent than the Well of Eternity had been.
It was also addictive. A people who had lost one dependency acquired a worse one, from a source with a will of its own, held in a cell they had built themselves.
The part nobody tells you about
Immol'thar's energy could not be stretched forever. There were too many Shen'dralar drawing on it.
Tortheldrin solved the arithmetic. He and his loyalists began murdering their fellow Highborne - not as a purge of enemies, but as demand management. Fewer elves meant more power each, and the ones who remained could draw on Immol'thar indefinitely.
It worked. That is the horrible part. The survivors got their magic, and it cost them their civilisation, and for centuries the outside world knew none of it because the gate was shut.
Why this matters to Forever
Two of the four new regions grow out of this city.
Shen'dralas in Kalimdor is built on the thread directly: the Highborne of Eldre'Thalas, the Valley of Bones, and the centaur tribes that have held the land around Desolace since forever. Classic set this story up in Dire Maul and never resolved it.
Zephras Isle is the other half, and the connection is in the names. The Skyborne call themselves shen'dorei, the hidden people. They are exiles from this same city - people who left rather than stay for whatever Tortheldrin was building. One group of Highborne hid behind a warded gate in a jungle. The other hid in the sky.
What is not known
Whether Eldre'Thalas itself is being reworked, or whether Shen'dralas simply sits beside the Dire Maul that already exists. Blizzard has not said, and it decides whether the story gets an ending or a neighbour.
Also open: the level range of Shen'dralas, whether it holds a dungeon, and what any of it has to do with the centaur. See Shen'dralas for what is confirmed about the zone itself.