Sons of the Harpy

The Sons of the Harpy are an underground resistance movement fighting the rule of Daenerys Targaryen over Meereen. They take their name from 'the Harpy', the traditional heraldic symbol of the Ghiscari cities of Slaver's Bay and the Ghiscari Empire, and conceal their identities wearing masks representing it.

Background
The Sons of the Harpy are formed after Daenerys Targaryen conquers Meereen as part of her campaign to break the slave trade in Slaver's Bay.

Season 5
A member of the Sons of the Harpy slits the throat of White Rat, an Unsullied, when the latter approaches a prostitute and pays her to be embraced by her. The assassin, who was in league with the prostitute, leaves his mask at the brothel as a reivindication of the Sons' responsibility.

In the books
In the A Song of Ice and Fire novels, the Sons of the Harpy are also a resistance movement of Ghiscari with ties to the Great Masters and the slaver families of Meereen. They target freedmen, Unsullied, and "shavepates" - Meereenese that have accepted the rule of Daenerys Targaryen and have shaved off their traditional Ghiscari hairdos as a sign they have abandoned the customs of the 'old' Meereen.