"Lady Cassandra Baratheon, later known as Cassandra Swann, is one of the principal antagonists of the story. The eldest daughter of Lord Borros Baratheon and Lady Elenda Caron, she is the sister of Maris, Ellyn, Floris, and of Royce, the future Lord of Storm’s End. She later becomes the second wife of Lord Robert Swann, by whom she has two children.
At the outbreak of the Civil War, Prince Aemond Targaryen selects Cassandra as his betrothed in order to secure the support of House Baratheon for the Greens, promising marriage once the Blacks are defeated. Ambitious and certain of her own worth, Cassandra interprets the proposal as a mark of high favor and envisions a brilliant future at court. In truth, Aemond’s choice is almost arbitrary, as he remains indifferent to her and sees the betrothal as nothing more than a political tool.
Her dreams are shattered when the news of Aemond sand Alys Rivers’ marriege is discovered, publicly humiliating her and her family and fracturing Baratheon loyalty to the Greens, dealing a serious blow to Aegon II’s cause. Mocked across the realm and consumed by jealousy toward a woman she considers beneath her, Cassandra falls into bitterness and despair. Her situation worsens further when her father, shortly before his death, marries her off not to a prestigious match, but to Lord Robert Swann, a man forty years her senior with no influence at court.
Away from the capital and forced to watch her younger sisters achieve the status and the prestige she was denied, Cassandra’s resentment toward the royal family steadily festers. After the Greens’ defeat and Queen Rhaenyra’s accession, that resentment hardens into open hatred, not only toward the Targaryens, but also toward those who supported them. Seeking purpose, she turns to a radical and punitive faction of the Faith of the Seven, which condemns the Targaryens and their incestuous customs as an affront to the gods.
This new cult was led by Dowager Queen Alicent herself, who survived the fire that destroyed her family and now lives in apparent seclusion, while secretly nurturing a deep desire for vengeance, even against her own surviving children, whom she has come to view as traitors and many are its supports among the Clergy of the Faith.
Together, Cassandra, Alicent, and their followers begin plotting to overthrow Queen Rhaenyra and place House Baratheon, hom they consider the realm’s rightful rulers, upon the Iron Throne. Their schemes fuel unrest in Dorne and contribute to the chain of events that leads to Prince Daemon Targaryen’s capture and the outbreak of war. However, rather than bringing down the Targaryens, the conflict ends in their victory and the successfull integration of Dorne into the realm.
Undeterred by this failure, Cassandra decides to act alone, and convinces her stepson Roderick, with whom she has begun a secret, illicit relationship, to murder his father, making her finally free to leave the Stormlands for the Capital. Where she plans to return to court, now as a widow seeking a new husband, presenting herself as a loyal subject while secretly plotting to destroy the royal family from within.
All the while, she is aided by Alicent, who has by then faked her own death and moves unseen through the hidden passages of the Red Keep, disguised as a septa, watching, listening, and haunting her children like a living ghost.
Yet Cassandra’s schemes will not go unnoticed as Baela Targaryen, who had just returned from Dorne, started to grow suspicious and decides to investigates, ultimately uncovering the her real motives.
Cassandra will be killed by the princess before the conspiracy can be set in motion.”