King Stannis’s new banner represents the fiery heart of the Lord of Light, R’hllor, whose religion Stannis is supporting. While Stannis is not particularly religious, the red priestess Melisandre of Asshai has promised him her support and has converted many residents of Dragonstone—including Queen Selyse—to the worship of R’hllor.
It was by her magic that Stannis’s brother, King Renly, and the castellan of Storm’s End, Ser Cortnay Penrose, were murdered. She has also proclaimed that Stannis is Azor Ahai reborn, and has given him a magical sword which she claims to be the Red Sword of Heroes, Lightbringer. The sword did not help him win the Iron Throne, however, and his hopes were shattered at King’s Landing by the western alliance of the Houses Lannister and Tyrell. Stannis barely managed to flee with his life.
After much turmoil and indecision, and the near-fateful decision of sacrificing his bastard nephew Edric Storm to make a stone dragon come to life, Stannis was finally directed towards a clear path by his new Hand Davos Seaworth, whom he had raised to lordship. Stannis has moved to the Wall to fight the Others and to gather new supporters, and arrived in time to save the Wall being breached by Mance Rayder and his wildlings. Garrisons loyal to him remained in control of Storm’s End and Dragonstone, although Dragonstone later falls when stormed by King Tommen’s forces. Storm’s End spends some time besieged by Mace Tyrell, until Queen Margaery is imprisoned by the Faith, resulting in Lord Tyrell breaking the siege and rushing to King’s Landing to rescue his daughter.