Jorah Mormont:Valyria was not the first to conquer the world. In the Dawn of Days, the city of Ghis opened its gates and poured forth its legions across the continent of Essos. With their lockstep discipline and absolute obedience, they ground entire nations beneath their boots and planted the Harpy in every corner of the known world. What they didn't destroy, they chained. Slavery is as old as man, but until the Ghiscari, it was never an art.
The slave lords grew rich and fat as pyramids were raised around them. Pleasure houses were filled and fighting pits were opened. Nobody remembers if the waters around Ghis had names before the Empire, but ever since they have been known only as "Slaver's Bay" and the "Gulf of Grief". Of Ghis, however, nothing remains but ruins - where end all great civilizations.
Five thousand years ago, Valyrian shepherds stumbled on strange eggs, and within a few generations an upstart Valyrian Freehold rose across the sea. Five times did the Ghiscari contend with Valyria, and five times did they go down in defeat. For the Freehold had dragons, and the Empire had none. The best of their legions burned, the others broke. The brick walls of Ghis were pulled down, the streets and buildings turned to ash and the very fields sown with salt, sulfur and skulls.
Yet the Empire was not wholely destroyed. Astapor, Yunkai and Meereen - once lowly colonies along Slaver's Bay - survived, and even thrived. Valyria had watched the Ghiscari grow rich and powerful off the backs of conquered peoples, and now the self-styled Freehold wanted its turn. While the dragonlords brought the world to heel, the slave markets of their former enemy never lacked for flesh.
Lamenting the lost empire, the descendants of Old Ghis grew rich and fat. The Doom fell on Valyria and the Dothrakirose to pillage most of the continent at will. But gold-laden Astapor, Yunkai and Meereen continue as they have for thousands of years: untouched. For even the horselords understand what the Ghiscari taught Valyria so long ago: what good are slaves without slavers?
Jorah states that "Slaver's Bay", as well as the "Gulf of Grief", were always known by these names (in their local languages) even during the time of the old Ghiscari Empire five thousand years ago. Previously, it wasn't clear if they only became known by these names after the fall of Valyria four hundred years ago.