Game of Thrones had tons of iconic duels to the death, but perhaps one of the greatest that stands out in memory to this day is that between Oberyn Martell and the Mountain. Featuring a cool fight with a deeply emotional tenor to it, the mechanics of the fight and its repercussions on the story resonated throughout the show. However, despite all that, the books give the duel a deeper layer than the show provided. A Song of Ice and Fire reveals that there was a detail to the poison Oberyn used that makes the fight even deeper. And while the detail did not find its way into the dialogue of the show, it might just stand out as one of the coolest details about the series coolest character.
Oberyn Martell proved a quick fan favorite in his introduction to Game of Thrones, and it's easy to see why. The prince of Dorne had a seductive and exotic appeal as much resulting from his wit as his deadly abilities in combat. Known as the Red Viper for his use of poisons in duels, gained from his years studying at the Citadel along with the maesters who make up the most intelligent figures in the world, Oberyn brought the full extent of his talents to the arena in his duel with the Mountain that stands out as one of the series' best fights. Armed with a spear to keep out of his gargantuan opponent's reach, and wielding it with the agility that left him unscathed through much of the match, Oberyn only lost when he neglected to strike a killing blow while trying to force the Mountain's confession.
The emotionality is part of what defined the fight, as Oberyn only agreed to the duel in the first place out of a sense of vengeance for his lost sister Elia, whom the Mountain brutally murdered. Throughout their duel, Oberyn demanded that the Mountain confess, and though he won several wounds against his opponent, he stopped short of a coup de grace to stand over the fallen behemoth. And his arrogance proved his undoing as the Mountain's final moments allowed him to seize Oberyn and bash the prince's head open. However, it turned out that Oberyn's spear was poisoned so that the Mountain was left in critical condition thereafter. In fact, a detail from the books that did not make it into the show reveals the true depths of Oberyn's arrogance.
Using his knowledge of poisons, Oberyn did not just treat his speartip with manticore poison but specifically engineered the manticore poison so that it would prolong the suffering of its victim. A manticore was used in a nearly successful assassination plot against Daenerys Targaryen over in Essos, where it was revealed that the venom would kill its victim the moment it made its way to the heart. But Oberyn thickened the poison he used to avoid instantly ending the suffering he wanted the Mountain to endure.
Had Oberyn not altered the poison he used, he would have beaten the Mountain after landing only a single attack. This adds further depth to his arrogance in their duel, as it means that Oberyn's impassioned demands for a confession were not a mere fit of overwhelming emotion on the prince's part. Rather, they were part of his plan all along as he never went into the duel expecting to win it as instantly as he could have. Since his poison did not kill the Mountain instantly, the knight was left to be resurrected later as a chilling and powerful sentry for Cersei Lannister.
Even outside of his failure to fulfill his vengeance, Oberyn's death ended up having extreme consequences in the context of the show. It spurred on his paramour Ellaria Sand and his daughters the Sand Snakes into violent conflict, even going so far as to murder major figures like Doran Martell and Myrcella Lannister. And to think it all could have been avoided if the Red Viper applied his skills toward efficiency rather than vengeance.