Old Ghis is a ruined city, located in the region of Ghiscar on the southern coast of Essos. It was once the ancient capital of the Ghiscari Empire, but was destroyed by the Valyrian Freehold with the use of dragons.
In the books
In the A Song of Ice and Fire novels, Old Ghis led five great wars against Valyria before being destroyed once and for all. Frustrated by the city's constant uprisings, the Valyrians pulled down the city's walls, burned the buildings to ashes and cinders and poisoned the surrounding lands with salt, sulphur and skulls to prevent it from rising again.
The city and the Ghiscari Empire were founded by Grazdan the Great, a mighty leader. Its symbol, and that of the Empire it led, was that of a harpy clutching a thunderbolt.
Old Ghis may have been inspired by Carthage, that was likewise at the head of a powerful nation and opposed the Roman Republic but was defeated and leveled in retribution.
- Old Ghis at A Wiki of Ice and Fire (spoilers from the books)