When Tyrion Lannister got arrested for killing Joffrey at his wedding, Oberyn met him and asked about what had happened during the Sack of King's Landing. Tyrion revealed that Gregor Clegane had indeed killed Elia Martell and her children. Oberyn then decided to fight for Tyrion. During the fight, he severely injured Gregor, but instead of killing him, he asked him to admit what he did to Elia. Gregor made Oberyn slip and killed him.