Oberyn made his debut in the eight episode of Season 4, when he arrived at King’s Landing for the sake of Joffrey Baratheon and Margaery Tyrell’s wedding.
However, the real reason for his coming was vengeance against the House of Lannister for raping his sister and murdering her along with her children.
The Red Viper soon finds himself needed by Tyrion Lannister, who is to be judged for allegedly killing Joffrey. He becomes Tyrion’s champion, which gives him the opportunity to exact retribution on the man directly responsible for his sister’s death – Gregor ‘The Mountain’ Clegane.
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Needless to say, this doesn’t end well, as Oberyn ends up with his head crushed by Clegane’s giant hands.