in "a game of thrones", tywin lannister sends him to raid the riverlands in retaliation for catelyn stark's abduction of tyrion. beric dondarrion is sent to arrest gregor, but gregor ambushes and kills him. he and his men continue raiding the riverlands throughout "a clash of kings". when tyrion lannister demands a trial by combat, the accuser cersei lannister chooses gregor as her champion. elia's brother oberyn martell volunteers as tyrion's champion to expose gregor as a murderer. oberyn wounds gregor with a poisoned spear, but the mountain is ultimately victorious, admitting to murdering elia and aegon before crushing oberyn's skull. gregor succumbs to the poison and his body is given to qyburn for experimentation. following cersei's walk of atonement, she gains a bodyguard named ser robert strong who is of similar enormous stature as gregor, and whose body is completely covered in armor.