Tyene Sand

Tyene Sand is a major character in the fifth season, although she had already been alluded to in the fourth season. She will be played by Rosabell Laurenti Sellers.

Background
Tyene Sand is the third of the eight bastard daughters of Prince Oberyn Martell. She's also the dauther of an unknown septa.

Season 4
Oberyn mentions his eight bastard daughters to Queen Cersei Lannister.

In the books
In the A Song of Ice and Fire novels, Tyene is Oberyn's daughter whom he fathered on a septa. She is regarded as treacherous, while maintaining a sweet and pious persona. She is fair, with golden hair, blue eyes and an almost otherworldly innocence about her. She has soft pale hands but they are as deadly as her sister Obara's callused ones, if not more so. She shares her father's knowledge of poisons and it is her chosen weapon.

When Tyene was in the cradle her mother read to her from the Seven-Pointed Star. Her cousin Princess Arianne Martell once crossed the Mander River with three of the Sand Snakes to visit Tyene's mother - vaguely implying that her mother is from the Reach.

Tyene and Arianne once fled Sunspear to try to marry Arianne to Willas Tyrell at Highgarden, but they were caught by Tyene's father, Oberyn Martell, at Vaith.

Tyene in the TV series is a condensation of several aspects of the different Sand Snakes. In the books she was the daughter of a septa from elsewhere in Westeros, and thus the only one of the Sand Snakes who doesn't have classic "salty Dornishman" dark features like the rest of House Martell. Instead, the TV series has changed it so that Tyene is a daughter of Oberyn and Ellaria - a "salty Dornishman" and a "sandy Dornishman", both of them with very dark features. Even so, they have still cast an ethnically Italian actress who does not have classic dark salty Dornish features.

Also, the casting report says that Tyene makes deadly use of daggers, while book-Tyene never uses combat weapons - instead preferring to surreptitiously poison her enemies.