Pedro Pascal as Oberyn Martell
You might recognize him from... The Adjustment Bureau, Homeland and Graceland

Westeros better watch out because... not only is Oberyn a born leader with a killer set of moves and a killer nickname — "The Red Viper" —  but he's in the mood for revenge. He blames the Lannisters for killing his sweet sister Elia during the Sack of King's Landing. Welcome to the We Hate the Lannisters Club, Oberyn!