Son and heir to Lord Boremund Baratheon, born at Storm's End in 89 AC, and Lord of the stormlands when the Dance of the Dragons broke the realm in 129 AC. A great brutish lord by all accounts, slow-witted and quick-tempered, more given to the lance than the council chamber, he kept his father's old quarrel with Princess Rhaenyra and her line but had taken no firm side until Aegon II's master of coin Lord Tyland Lannister sent Prince Aemond and Ser Criston Cole to him at Storm's End. The blacks' offer came at the same time in the person of Prince Lucerys Velaryon, who came alone upon his dragon Arrax to bid Borros recall his oath to Rhaenyra: Borros mocked him for a boy and a coward, set Aemond on his trail when Lucerys took flight in the storm, and watched from his battlements as Vhagar tore Arrax and the boy from the sky over Shipbreaker Bay. Thereafter he was for the greens, betrothed his eldest daughter Cassandra to King Aegon II himself, and brought the storm host north toward King's Landing too late to do the green cause any good.
In late 130 AC he marched at last on the kingsroad with four thousand men against the Lord of Riverrun and the river host, and met them two days short of the city upon a muddy stretch of road between a forest and a hill. Heavy in the saddle, wounded thrice over before noon, Lord Borros killed twelve knights in the press, cut down Lord Roland Darry and Lord Jorah Mallister, and at the last met young Lord Kermit Tully axe to morningstar across the mud, refusing the boy's offer of quarter. The Lord of Storm's End died there in 131 AC of a blow that broke his helm and his skull together, leaving his lady Elenda Caron great with child at Storm's End and his line in the hands of the four daughters they would after call the Four Storms; the boy she bore him seven days later, named Royce after her own father in defiance of the dead lord's wish that he be called Aegon, succeeded the murdered greens at Storm's End in his cradle.