ser gregor is chosen as cersei's champion for tyrion's trial by combat, and fights oberyn martell, tyrion's champion who wants to kill gregor as revenge for the murder of his sister, elia martell. oberyn inflicts several serious injuries on clegane with his poisoned spear, but gregor eventually kills oberyn by crushing his skull, while admitting that he did rape elia, killed her children and enjoyed it, before collapsing from his own injuries. it is later revealed that the mountain has been poisoned with manticore venom, a poison with which oberyn had laced his weapon, and that he is slowly dying. cersei enlists ex-maester qyburn to save him, though qyburn claims that the procedure will "change" clegane.