Aegor Rivers, far better known as Bittersteel, was one of the Great Bastards of King Aegon IV Targaryen, born in King's Landing in 172 AC to the king's fifth mistress, Lady Barba Bracken. He came into the world a fortnight before Queen Naerys nearly died bearing the king another child, and when Barba's father pressed Aegon to set the queen aside and wed his daughter, Prince Aemon the Dragonknight and Prince Daeron forced both mistress and bastard from court. So Aegor was raised among his mother's kin at Stone Hedge in the riverlands, a Bracken to the bone. Half-Targaryen, he had the purple eyes of his father's line but the black hair of the Brackens, and grew tall and lean and hard as a panther, with a close-cropped beard and a face that never smiled. Angry all his life, he had little use for anything beyond war, and he reserved his deepest hatred for his half-brother Brynden Rivers, called Bloodraven, whose mother Melissa Blackwood had supplanted Barba as the king's favorite and who won the love of their beautiful half-sister Shiera Seastar, whom Aegor desired for himself. To bind him closer, Daemon Blackfyre betrothed his eldest daughter Calla to Aegor, and ever Bittersteel urged Daemon to press his claim to the Iron Throne.
When Daemon raised the black dragon in the First Blackfyre Rebellion of 196 AC, Aegor commanded the right of his host at the Battle of the Redgrass Field. As Daemon and his twin sons fell beneath the arrows of Bloodraven's Raven's Teeth, Bittersteel snatched up the Valyrian steel sword Blackfyre, rallied the breaking ranks, and led a mad charge straight into the archers. He and Bloodraven met in the heart of the melee, and in the duel that followed Aegor put out his half-brother's eye, yet the day was already lost and he was driven from the field. He fled across the narrow sea to Tyrosh with Daemon's widow Rohanne, the surviving Blackfyre children, the sword Blackfyre, and hundreds of now-landless lords and knights, nursing his hatred and his vow to set a son of Daemon upon the Iron Throne.
In exile Aegor served a year with the Second Sons, but as the scattered loyalists drifted into rival companies he founded the Golden Company to bind the exiles together, and it swiftly became the most renowned and disciplined sellsword brotherhood of the Free Cities, said never to have broken a contract. Its war cry, "Beneath the gold, the bitter steel," honors him still. Bittersteel kept the company and the sword Blackfyre out of the Second Blackfyre Rebellion in 211 AC, but in 219 AC he and Haegon I launched the Third, in which he and Bloodraven dueled a second time; Aegor was taken in chains, yet his ship to the Wall was intercepted and he won free to crown Daemon III and fight on. He led the Fourth Blackfyre Rebellion onto Massey's Hook in 236 AC, only to be broken at the Battle of Wendwater Bridge. Bittersteel died at sixty-nine in 241 AC in the Disputed Lands, sword in hand and defiance on his lips, and at his command the Golden Company boiled the flesh from his skull, dipped it in gold, and bore it before them, that he might one day return to the conquest of Westeros.