The Ghiscari Empire was one of the oldest civilizations in the known world, which ruled much of the continent of Essos, centered around the present-day region of Slaver's Bay. The Ghiscari Empire was flourishing and building massive pyramids thousands of years before the rise of the Valyrian Freehold, when the Valyrians were only humble shepherds.
Its symbol was a harpy: a woman's head and torso, with bat wings, clawed eagle legs, and a scorpion's tail.
Five thousand years ago, when the Valyrians discovered and tamed dragons and began to conquer outwards, the Ghiscari Empire sought to put a halt to their expansion. The two great rivals vied to be the dominant power in Essos, and in a series of five major wars the massive Ghiscari legions marched against the Valyrians, but each time they were defeated by the Valyrian dragons. Much of the fighting took place in and around the Valyrian peninsula and (what would later become known as) Slaver's Bay.
The Valyrians finally defeated the Ghiscari Empire when their armies and their dragons attacked the Empire's capital city of Ghis. The buildings and streets were burned to ash, and the Valyrians sowed the earth with salt so that nothing would grow again. Five thousand years later, Old Ghis is still a ruin.
The Valyrians conquered the surviving Ghiscari people as well as their outlying colonies throughout the region, including Astapor, Yunkai, and Meereen. For the next five thousand years, the Ghiscari people were ruled by the Valyrian Freehold. Indeed, very few Ghiscari survived the destruction of the Empire, and combined with five thousand years of intermingling with other conquered peoples that the Valyrians incorporated into their colonies, the bloodline of the Ghiscari Empire actually runs very thin in modern-day Slaver's Bay.
However, after the Doom of Valyria four hundred years ago, the local Ghiscari cities east of the Gulf of Grief reasserted their independence, with a handful of aristocrats establishing great wealth and power by ruthlessly exploiting a vast slave population, to the point that the entire geographical region became known as "Slaver's Bay". The city of New Ghis was also founded on an island further south of the bay. Despite being separated from the Ghiscari Empire by five thousand years' worth of racial and cultural intermingling, the ruling elites of these slaver-cities looked back the past glories of the old local empire as a way of re-asserting their independence, latching onto whatever cultural trappings and heritage had survived the centuries. After they regained their independence the slaver-cities of Astapor, Yunkai, and Meereen also began using harpies on their flags (with different variants for each city), emulating the harpy symbol of the old Ghiscari Empire. Even in the present day, many aristocratic slavers from these cities think of the Ghiscari Empire as "their" empire.
In the books
In the A Song of Ice and Fire novels, the Ghiscari Empire was founded many millennia ago by Grazdan the Great. One of - if not the - oldest civilizations in the known world, the Ghiscari Empire's golden age was thousands of years before the rise of the Valyrians five thousand years ago, though the exact age of the Ghiscari Empire has not yet been mentioned.
After the Doom of Valyria, the peoples of Slaver's Bay tried to re-assert their independence by championing their descent and cultural heritage from the old Ghiscari Empire now that their Valyrian overlords were gone. In reality, however, five thousand years of domination by the Valyrians took their toll, and present-day Slaver's Bay only has certain scraps of culture and traditions which date back to the Ghiscari Empire itself. The region's current inhabitants are really a mongrel race, descended from the small surviving Ghiscari population which intermingled with many other enslaved peoples under the Valyrians. Even the present-day languages of Slaver's Bay owe more to High Valyrian than they do to the old Ghiscari language. Much like the local peoples in the Free Cities, once the Valyrians were gone they developed their own romance dialect of High Valyrian; however the "Low Valyrian" of Slaver's Bay is quite different from the dialects in the Free Cities, because it contains many borrowings from the older Ghiscari languages, i.e. "Mhysa" in Slaver's Bay Low Valyrian is a direct borrowing of the word for "mother" in the ancient Ghiscari language. Still, in terms of basic structure, the language is really a derivation of High Valyrian with some Ghiscari influence, and the language of the old empire is largely extinct.
The rivalry between the Valyrian Freehold and the Ghiscari Empire seems to be a narrative echo of the Punic Wars between Rome and Carthage.