All the Kings and Queens of Westeros
A Complete History of the 21 Kings and 2 Queens That Ruled Westeros
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Raised as Robert's son and heir, Joffrey is actually the eldest son of Cersei and Jaime. Unlike his two younger siblings, who are kind and gentle, Joffrey is a cruel, spoiled, sadistic monster from a very young age. He goes back on his word and executes Ned Stark as one of his first acts, forcing Ned's daughter (and Joffrey's intended) Sansa to watch. His reign is marked by his cruelty, as he keeps Sansa captive and torments her, abuses, and accuses his own family members, and rules with a care for his own whims and little else.
Ultimately, his arrogance and cruelty make him easy to manipulate, which leads to his downfall. He is persuaded to drop Sansa in favour of Margaery Tyrell. Then, at their wedding banquet, Joffrey is poisoned and dies almost immediately. Years later, Olenna Tyrell reveals that it was she who poisoned Joffrey.