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  === Season 4 ===
  Oberyn arrives in King's Landing with his paramour, Ellaria Sand, to attend Joffrey's wedding in his brother's stead, and his meeting with Tyrion makes it clear that he has actually come to take revenge against the Lannisters for their role in the deaths of his sister, nephew and niece. At the wedding, Joffrey dies after being poisoned, and Tywin initially suspects Oberyn of having a hand in the murder since Oberyn has a past with poison chemistry, while Oberyn denies involvement and accuses Tywin of ordering Gregor Clegane to rape and murder Elia. The two reach a settlement when Tywin promises Oberyn a meeting with Clegane in exchange for Oberyn serving as one of the three judges at Tyrion's trial. At the trial, Oberyn implies that he is not convinced of Tyrion's guilt, and openly questions Cersei's testimony and asks Shae why Tyrion would tell her about all of his plans to murder Joffrey if he was the perpetrator.
  When Tyrion demands a trial by combat and Gregor Clegane is chosen as Cersei's champion, Oberyn volunteers to fight for Tyrion, proclaiming that he will exact his vengeance, starting with Ser Gregor. Martell valiantly fights Clegane, his superior speed making up for Clegane's size, and manages to wound him in the shoulder and the leg, flooring him. Refusing to kill him immediately, Oberyn furiously demands that Clegane admit to raping and killing Elia and her children, and that the order came from Tywin himself. Distracted for a moment, Oberyn is floored by Clegane, who – in a manner self-admittedly paraphrasing the murder of Elia – knocks out his teeth, straddles him and slowly gouges out his eyes, admitting to the rape and murder of Elia before crushing his skull. Tyrion is subsequently sentenced to death, but Oberyn's objective of vengeance was not in vain, since his spear is revealed to have been laced with the deadly venom of the manticore, which is slowly killing Clegane.