The Unsullied are the elite slave infantry trained and sold in the slaver city of Astapor, renowned for their discipline and loyalty. The rich and powerful in the Free Cities like to employ them as guards.
They were eunuchs who were given a drink which deadened all sensation, causing them to feel neither pain nor pleasure. They were denied all possessions, even a name to call their own. They fight in rank, obey all orders, and feel no fear.
During her journey through Slaver's Bay, Daenerys Targaryen tricked the rulers of Astapor into selling her all eight thousand Unsullied held in the city at the time. She instantly ordered them to slaughter their former masters, and kept the dragon she had promised as payment for herself. Later, she freed the Unsullied and gave them the opportunity to leave, but they stayed with her and became the core of her new army.