Sack of Astapor

The Sack of Astapor is an event in Slaver's Bay in which Daenerys Targaryen overthrew the Good Masters of Astapor and liberated all the slaves in the city.

The incident was orchestrated by Daenerys, who had previously been faced with the issue of using a slave army to conquer a realm in which slavery had been outlawed and reviled for centuries. Having observed the suffering of Astapor's slaves, Daenerys devised a planned to acquire all of Astapor's Unsullied and turn on the Good Masters. She offered one of her dragons in exchange for the Unsullied, as it was the only thing she (or anyone else) possessed that could possibly be exchanged for them.

Daenerys sprung her trap after handing over her dragon. After making sure that the Unsullied were indeed loyal to her as their new master, she ordered them to kill the Good Masters as well as anyone in Astapor who held a whip. She also instructed them to spare all children and strike of the chains of any slave they came across.

The Good Masters were powerless to stop the situation, as the remaining household guards and slave soldiers were inferior in every way to the Unsullied. Furthermore, the dragon was no help: Kraznys mo Nakloz had assumed that the dragon would, like a slave, accept its new master. Daenerys reminded him that dragons were not slaves before he was immolated.

The Unsullied performed as promised, the Good Masters were slain and the slaves freed. Daenerys freed the Unsullied as well, immediately after the Sack was completed. They opted to continue their service to her as free men and departed the smoking ruin of Astapor with her.

In the books
In the A Song of Ice and Fire novels, the Sack begins in the Plaza of Punishment - the novels' equivalent to the Walk of Punishment - because the Plaza of Pride is too small to contain all the Unsullied. Besides the 8,000 Unsullied, Daenerys' bloodriders as well as the rest of her khalasar take part in the sack of the city (they might have in the TV series as well- the Sack itself is depicted off-screen). Daenerys also orders her forces to spare all children below the age of twelve (the TV version shortens this to just have her say "harm no child" without specifying an age).