https://www.cnet.com/news/game-of-thrones-already-lost-its-greatest-hero-oberyn-martell/

Oberyn -- nicknamed the Red Viper -- came into King's Landing like a whirlwind at the start of season 4. The prince from the southern Dorne region seemed exotic, mysterious and dangerous in his first scene, which saw him stabbing a Lannister goon through the hand and pinning him to a table after being dissed.

From here, we learn Oberyn seeks vengeance for the death of his sister Elia and her children at the hands of Gregor Clegane, the horrifying unit better known as The Mountain and one of smug jerk Tywin Lannister's bannermen.