- "Go home, pup, and tell the bitch, your mother, that the Lord of Storm's End is not a dog she can whistle up."
- ―Borros to Lucerys Velaryon
Lord Borros was the head of House Baratheon during the Dance of the Dragons, the civil war between two factions Blacks and Greens, cadet factions of House Targaryen. After the coronation of Aegon II Targaryen, his mother, Alicent Hightower sent her second son Aemond Targaryen to rally support of their faction, the Greens, in order to gain the allegiance of lords that hadn't already declared for Aegon. Rhaenyra Targaryen, the leader of the Blacks, also sent her second son Lucerys Velaryon to try and gain Borros' allegiance. Aemond arrived at Storm's End first on his dragon, Vhagar, and pleaded his brother's case. By the time, Lucerys had arrive on his dragon, Arrax, Aemond was still there. He pleaded his mother's case. However, Borros had already made his decision, and sent Lucerys away, signifying his allegiance to the Greens. Aemond's artificial sapphire eye cleared at Lucerys, causing him to flee Storm's End and Borros. Aemond would kill Lucerys moments later above Shipbreaker Bay.
In the books
At the start of the Dance of the Dragons both the blacks and the greens hoped to win Lord Borros over to their cause. Prince Aemond Targaryen hoped to wed one of his four eligible daughters to win House Baratheon over to the greens, and arrived on dragonback to do so. Prince Lucerys Velaryon arrived soon after to win Borros to the black cause. Eyewitness accounts varied, with some saying Lord Baratheon was embarrassed for Lucerys to find him treating with Aemond (as Borros's father had been a supporter of Princess Rhaenys Targaryen), while others say he relished the prospect of both factions competing for his aid.
Borros refused Lucerys, since unlike Aemond he could wed none of Borros's daughters as he was betrothed to his cousin, Rhaena. Although Borros forbade the kinsmen to fight beneath his roof at Storm's End, he tacitly allowed Aemond to seek combat, knowing the older Vhagar would be able to overcome Arrax, which would lead to the death of Lucerys. As the war began in earnest, Storm's End officially declared for the greens.
Princess Jaehaera, daughter of King Aegon II Targaryen, was sent to Storm's End under the charge of Ser Willis Fell of the Kingsguard when King's Landing fell to Rhaenyra Targaryen and her blacks.
Borros kept the armies of the stormlands out of heavy fighting for most of the war, but he eventually took King's Landing during the Moon of the Three Kings. Borros's army was defeated by the Lads and their rivermen in the battle of the Kingsroad. Borros is said to have killed twelve knights, along with Lord's Darry and Mallister, before falling against Lord Kermit Tully.