From A Wiki of Ice and Fire
The Ghiscari Empire, also known as the Old Empire of Ghis, was one of the oldest known nations and cultures in the world. It reigned supreme over much of Essos. Its founding and golden age predate that of its successor, the Valyrian Freehold, by thousands of years. The Empire took the harpy emblem of Ghis. It depicted a fanged woman with leathery wings for arms, the legs of an eagle, and the tail of scorpion, clutching a thunderbolt in her talons.
The Ghiscari Empire reigned supreme over much of Essos while the Valyrian Freehold was still young and savage. It rose from the ancient city of Ghis in Ghiscar and grew into a military, economic and cultural powerhouse.
At its height Ghis fielded highly disciplined legions of lockstep soldiers that were reputed to be nearly unbeatable on the battlefield. The architecture of Ghis was dominated by massive brick towers and pyramids built using slave labor. Its symbol was the harpy. The Ghiscari expanded as far as Sothoryos, founding Gorosh as a penal colony.
The Ghiscari Empire fell over 5,000 years ago, after the Valyrians discovered dragons and quickly rose to power, the Ghiscari wanted the power of dragons for their own and went to war with the Valyrians. Five great wars were fought between Ghiscar and the Freehold; wars which Valyria won with its dragons. The Ghiscari did win some skirmishes as there is a large ancient tapestry in the heart of the Great Pyramid in Meereen showing the last survivors of a defeated Valyrian army passing beneath the yoke and being chained.  In the last great war the Valyarians smashed the Ghiscari legions and used their dragons to burn Ghis and its Great Pyramids to the ground. After Ghis was burned, the Valyrians sowed the fields with salt and sulphur to prevent any surviving inhabitants from returning.
With the destruction of Ghis, the Ghiscari Empire collapsed and most of the original Ghiscari bloodline was wiped out. The surviving populations of the Empire rebuilt what they could from the wreckage. The outlying colonies of Slaver's Bay rose to prominence: Astapor, Yunkai and Meereen. These cities established themselves as a hub of the global slave trade and became extremely wealthy. The island city of New Ghis was also founded. 
Known Ghiscari cities
- Hazdahn Mo
- Vaes Mejhah (Dothraki name)
- Vaes Efe (Dothraki name)
Ghiscari have dense, dark amber skin and wiry hair with red highlights. Men of the Slaver Cities are known to wear their hairs teased, oiled, and twisted into fantastic shapes. They have wear cloaks of yellow with copper disks sewn on. Their tunics are embroidered linen. Below the waist they wear pleated linen skirts and sandals. The richest classes, who are primarily made up of slavers, wear the tokar and are fond of rich foods, such as dogs, octopus stew and duck eggs.
The Ghiscari culture, gods and tongue were largely forgotten over time and the Ghiscari adopted the language of their Valyrian conquerors. Today they speak a dialect of High Valyrian. The crumbling walls, stepped pyramids, and harpy statues are all that remain of the empire's glory days.
The Unsullied tactics are considered a remnant of Old Ghis. They are trained to fight in the same disciplined lock-step manner and are considered the legions of Old Ghis come again.
The harpy was adopted as symbol by all the slaver cities and their silver coins are stamped with a harpy. The Good Masters of Astapor, the Wise Masters of Yunkai and the Great Masters of Meereen have been training slaves for generations, much as their predecessors did, and are experts at it.