"Old Ghis & Slaver's Bay" is part of the Histories & Lore, a special feature in the Blu-ray of Season 3 of Game of Thrones. It is narrated by Iain Glen as Ser Jorah Mormont, who provides a description of Old Ghis and Slaver's Bay.
Valyria was not the first to conquer the world. In the Dawn of Days, the city of Ghis opened it's gates and poured forth it's 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 civilisations.
Five thousand years ago, Valyrian shepherds stumbled on strange eggs, and within three 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 burned down, the streets and buildings turned to ash and the very fields sewn with salf, 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 it's turn. While the dragon-lords 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 Dothraki rose to pillage most of the continent at will. But gold-laden Astapor, Yunkai and Meereen continue as they ahve for thousands of years: untouched. For even the horse-lords 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.