Probably one of the most brutal, tragic, and gorey scenes from Game of Thrones.

Gregor fights Oberyn at Tyrion's trial by combat, clad in a heavy plated armor and wielding a large broad sword in one hand. Despite his immense size and strength, Gregor is no match for Oberyn's swift and agile fighting style. Oberyn manages to knock Gregor's helmet off early into the duel and proceeds ​to slowly cut him down, all the while taunting him and demanding he admit to the rape and murder of Elia Martell and her children, chanting ever more loudly "you raped her; you murdered her; you killed her children."

Oberyn eventually severs ​Clegane's hamstring, bringing the giant to his knees. Oberyn presses his advantage by jumping and thrusting his spear into Gregor's chest, severely injuring the gigantic man and seemingly sealing his defeat. Thinking his opponent is fully incapacitated, Oberyn removes his spear from Gregor's chest and circles him, refusing to land a killing blow until Gregor admits that he killed Elia and reveals who orchestrated it, directing his gaze accusingly at Tywin. However, in his hubris, Oberyn is unexpectedly caught off guard when Gregor trips and pulls him to the ground, grasping him around the neck and slamming his fist squarely into his face, instantly knocking out several of his teeth.

Berserk with fury at nearly being killed by his opponent, Gregor pins Oberyn to the ground and proceeds to gouge out his eyeballs, making him scream in agony, as he roars for all to hear: "Elia Martell! I killed her children! Then I raped her! Then I smashed her head... in like this!." He then crushes Oberyn's skull, making his paramour Ellaria Sand scream in horror. Gregor collapses beside his fallen opponent from the wounds he sustained as Tywin announces that, according to the rules of trial by combat, Oberyn's death has officially sealed the fate of Tyrion and sentences him to death.

Despite his exceedingly gruesome death, Oberyn nonetheless manages to get some posthumous revenge. Before the duel, Oberyn had his spears coated with manticore venom, one of the deadliest poisons in the known world. Gregor's wounds start putrefying, emitting an overpowering stench. As a final torture, the venom forces him to remain conscious during the whole ordeal, even as his flesh dies and peels off his body. Pycelle regretfully tells Cersei that Gregor is beyond any hope of healing.