The Crystal Empire (which sprawled across most of northern Equestria at the time, from what would later be the port of Vanhoover in the west to the future Sovyet of Stalliongrad in the east, reaching almost as far south as the Cantervale) was founded about 800 years prior to the Discordant Era by Bucephalus the Great (who, although history from the era is rather fragmentary, appears to have been one of the few pre-Sisters ascended alicorns), having personally conquered most of northern Equestria at the time.
Sadly, much like the great historical conqueror of whom he is a shameless expy, he died of severe magically-aggravated pneumonia at a relatively young age, secondary to his many battlefield injuries. More fortunately, he had an extremely competent nephew and general who managed to hold the Crystal Empire together after his death and found its ruling dynasty - at least until the Discordant Era, in which only its central hieropolis with its geomantic architecture was able to hold out against the draconequus's chaos.
After the Discordant Era, said city was unable to put the Empire back together again in the face of the disinterest on the part of the lands it hadn't managed to protect... and was, in any case, all too soon to face its own internal struggle with the early rumblings of the Sombran Coup.