Sons of the Harpy

The Sons of the Harpy are an underground insurgency fighting the rule of Daenerys Targaryen over Meereen. They take their name from the Harpy, the traditional heraldic symbol of the slave-masters in the Ghiscari cities of Slaver's Bay, and the Ghiscari Empire before them. They conceal their identities wearing masks that look like harpies.

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 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. The murderer is later caught by Daario Naharis, who finds him hiding in a wall. Daenerys states the man should have a trial for his crimes, as this is the new law. However, Mossador believes the man does not deserve a fair trial, which leads him to finding his prison cell and killing the harpy.

Known victims

 * White Rat
 * Ser Barristan Selmy

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.

Their membership seems to be drawn from those who benefited from the old regime, particularly the old aristocratic families of the Great Masters, but Daenerys cannot confirm which former slave-master families are supporting the Sons of the Harpy and which ones are not. She strongly suspects the House of Pahl, among others, but the main problem is not even knowing who they are: as an insurgency, they are conducting a "shadow war" in the alleys of Meereen, ambushing Daenerys's soldiers when they are alone or in small groups, then fleeing back into the night to avoid retaliation.

Unlike in the show, the Sons of the Harpy never act on broad daylight. So far in the books, Grey Worm and Barristan Selmy never had any encounter with them.