Oberyelia is the first child of Oberyn Martell. She was brought into this word from a whirlwind affair between him and a fisherman's wife in Essos. When she was very young, her mother was killed by the man she married, and Oberyelia was taken to Dorne to live out her days as a princess. She never expected to become Queen of anything, much less Westeros. She wanted to travel like her father- have fun with her life. But Ellaria Sand, a woman she sees as a mother, asks a huge favor of her. Go marry the King in the North, Robb Stark, to secure an alliance between the north and Dorne. Together, they can take the Lannisters off of the Iron Throne.
So she does it. But things are not as easy or simple as they may seem.