Ser Aegor Rivers, known as Bittersteel, was one of Aegon IV’s Great Bastards by a lady of House Blackwood. Though famed for his prowess as a warrior, Rivers was unhappy with his lot in life as a bastard (even a royal one), and joined Daemon Blackfyre in rebelling against King Daeron II. At the Redgrass Field, he rallied Blackfyre’s forces for one last charge following his death, and crossed swords with his other half-brother Ser Brynden Rivers (Bloodraven) and which led to the loss of one of Bloodraven’s eyes.
Afterwards, Bittersteel went into exile in the Free Cities with Blackfyre’s surviving sons. They plotted a return, and Bittersteel took part in at least one further Blackfyre Rebellion. He also founded the Golden Company, the most famous and powerful of the sellsword companies of the Free Cities. It is not known whether he has any descendants.