The death that started the battle between Houses Lannister and Stark, Jon Arryn's death happens off screen in episode one, but it 's a major death in Game of Thrones nonetheless. After Jon's death, Catelyn Stark receives a message from her sister, Lysa, accusing the Lannisters of killing her husband. It's revealed in later seasons that Lysa poisoned her own husband at the request of Petyr Baelish.
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