Second Sons

The Second Sons are a mercenary company operating in Essos. They fight for coin and the highest bidder. They are noted for their professionalism and ruthlessness in pursuit of a contract.

Season 3
The episode Second Sons appears to be named for them.

In the books
In the Song of Ice and Fire novels the Second Sons are one of a number of mecenary companies operating in Essos, alongside the Stormcrows and the Golden Company. They are so-called because it is common for the second sons of lords and merchants (who would otherwise inherit nothing) to seek service in their ranks for glory and gold. Aegor Rivers, known as Bittersteel, served in the Second Sons for a year before leaving to found the Golden Company.

Prior to the novels, the Second Sons had recently fallen into disrepute due to the rise of a particularly vicious commander, Mero. Mero has led the company in pursuing contracts no-one else would touch, and the company has become almost as ill-omened as the infamous Brave Companions as a result. At the time the books commence, the Second Sons are in the employ of the city of Yunkai in Slaver's Bay.