The Valyrian Freehold
Shepherds who found dragons in a volcano and spent five thousand years using them, until the volcano answered.
The fuller tale
It was not an empire and had no king, which its citizens never tired of explaining: every freeholder with land had a voice, and two lords were elected annually to hold what authority there was. This was democracy of a kind, conducted over the largest slave population in the history of the world, and enforced from the backs of dragons.
The Freehold broke the Old Empire of Ghis in five wars and salted the ruins; it broke the Rhoynar; it planted its colonies from Volantis to Lorath and drove the great roads that still run straighter than anything built since. Its steel is still the best steel there is, and nobody alive knows how it was made. Its capital sat on the Fourteen Flames and mined them with slaves in numbers no chronicle bothered to count.
In one night it ended: the mountains burst, the peninsula shattered, and every dragonlord within reach died with their dragons. What caused it, nobody knows and everybody has a theory. The Smoking Sea is still there, and the sailors who go looking do not, as a rule, come back.
The ground they held
What became of it ÔÇö if an ending is written, it is here. Spoilers.
The chronicle records no ending here ÔÇö or none the maesters have set down.