Tyene Sand

Tyene Sand is a recurring character in the fifth season, though she was alluded to in Season 4. She is played by Rosabell Laurenti Sellers and debuts in "Sons of the Harpy".

Background
Tyene Sand is the third of the eight bastard daughters of Prince Oberyn Martell, known as the "Sand Snakes". She is also his eldest daughter by his paramour, Ellaria Sand. Tyene fights using duel-wielded daggers, though her other signature weapon is her skill with poisons - a trait she inherited from her father. Like her father, she is known to coat her blades with poisons, making a single scratch lethal.

Tyene can feign being soft-spoken and childlike, but this is just an act to keep her enemies off guard, as she is just as fierce as her older sisters.

Tyene is the youngest of the three adult Sand Snakes, and more impetuous than her older sisters Obara and Nymeria. Where Obara will face threats head-on (confident in her martial prowess), and Nymeria will stand back and analyze the situation to make a precise strike at her enemy's weak points, Tyene tends to heedlessly charge ahead. This is exemplified by her choice of daggers as her main hand-held weapon: they are quick on the draw, allowing her to lash out quickly, before even her older sisters can restrain her.

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

Season 5
When Ellaria Sand goes to the Water Gardens to scold Doran for his inaction to Oberyn's murder, she tells the Prince that Dorne must go to war against the Lannisters, and points out that the Sand Snakes, who have the love of their people, agree with her, and that they will avenge their father if Doran does not.

Tyene, Nymeria, and Obara meet with Ellaria to discuss rallying Dorne to go to war against House Lannister. When the conversation shifts to Myrcella Baratheon, Obara reveals information she received from a merchant captain that Jaime Lannister has been smuggled into Dorne. Ellaria insists that they must stop Jaime from reaching Myrcella. They are presented with a choice: side with Doran and have peace, or side with Ellaria and have war. Tyene, Nymeria, and Obara agree to side with Ellaria.

In the books
"Tyene Sand" in the TV series seems to basically be a condensation into one character of two of Oberyn's daughters from the novels: Tyene Sand and Elia Sand.

In the A Song of Ice and Fire novels, Tyene is Oberyn's third daughter, whom he fathered on a septa from the Reach. As a result of having an Andal mother, unlike the other Sand Snakes she does not have classic Dornish physical features. Instead has a pale complexion with golden hair, blue eyes, and an almost otherworldly innocence about her. She dresses in elegant white and blue silks, and appears as chaste as the Maiden of the Seven. However, her gentle appearance is just an act: while maintaining a sweet and pious persona, she is regarded as the most treacherous of the Sand Snakes - to her enemies; she is fiercely loyal to her own family. Tyene shares her father's great knowledge of poisons. She keeps numerous poisons hidden in her clothing at all times, including several poison-coated small blades and needles, from which even a single, unnoticed pin-prick can kill.

Elia Sand, meanwhile, is Oberyn's fifth daughter. Ellaria Sand is the mother of the four youngest of the eight Sand Snakes, and Elia is her eldest daughter. The younger four Sand Snakes barely appear the fourth and fifth novels (when the other Sand Snakes are introduced) due to their young ages: Elia Sand is only fourteen in the fifth novel, and the Loreza the youngest is only six. However, a preview chapter was released from the beginning of the sixth novel, The Winds of Winter, by which time Elia Sand is just old enough to begin accompanying the older Sand Snakes. As a result Elia Sand is very impetuous and reckless, wanting to make a name for herself. She is a skilled horse rider and fond of jousting, and her preferred weapon is a lance for mounted combat. Overeager to earn the fame and renown of the older Sand Snakes and her father before them, Elia charges around on her horse announcing "I'm Lady Lance!" to all she meets hoping that she will earn it as a nickname, the way her father became known as "the Red Viper", even though Elia has yet to see any combat. Both Tyene and Elia are companions of their cousin Arianne Martell, Doran's daughter and heir.

The TV series's version of Tyene Sand therefore seems to be a condensation of book-Tyene and book-Elia Sand into one character. This may have been due to worries about time constraints: given that there are already three Sand Snakes, "Elia Sand" might never appear in the TV show itself (though she has been mentioned by name in dialogue during Season 4). Similarly, during Season 2 the producers were seriously worried that Shireen Baratheon would never appear in future seasons - and in House Tyrell, Loras and Margaery actually have two older brothers in the novels (Willas and Garlan) but they were also apparently cut for time. Elia Sand, however, is the only one of Ellaria's children to ever appear prominently in the novels so far: indeed, the few scenes between Ellaria and Elia are the only presentation of a Woman of Color having a mother-daughter relationship in any of the novels. Otherwise it is only mentioned that Ellaria has daughters with Oberyn. The writers may have weighed this into this decision to combine the characters, wanting to actually show Ellaria having a mother-daughter relationship on-screen.

Like book-Tyene, TV-Tyene is stated to be very skilled with poisons, her signature weapon. The difference is that, apparently to make her a bit more "active", the TV show changed this from subtley killing enemies with a scratch from a poisoned ring, to coating her daggers with poisons so she can take part in melee combat (though this does echo how her father coated his spear in poisons). Like book-Elia Sand, TV-Tyene is the eldest of Ellaria's daughters, but the youngest of the adult Sand Snakes - Ellaria's younger daughters are only small children, but Oberyn had older daughters from prior relationships. The actors have said that TV-Tyene has a youthful tendency to charge recklessly into danger without thinking, which also matches the impetuous nature of book-Elia Sand, trying to prove herself to her older sisters.

One result of combining the two characters is that book-Tyene and book-Elia have different physical appearances due to having different mothers from different ethnicities. Book-Elia has the darker features of Dornishmen, while book-Tyene has classic Andal features of blonde hair, blue eyes, and pale skin. Given that TV-Tyene is supposed to be Ellaria's daughter, she logically wouldn't look nearly as Andal/Northern European, so the TV show cast dark-haired Italian actress Rosabell Laurenti Sellers (just as book-Elia Sand looks more like Ellaria because she is her daughter). While this is closer to Ellaria in appearance than book-Tyene, Westeros.org did point out that the actress still doesn't look quite like TV-Oberyn and TV-Ellaria (i.e. they could have cast a Latina or South Asian actress). Even so, this is not outside the realm of possibility when it comes to Dorne, which is a very multiethnic society, instead of having a rigidly uniform appearance. For example, Oberyn's squire Daemon Sand is a bastard of House Allyrion, who are classic olive-skinned "Salty Dornishmen" from the river valleys and coasts, but he has bright blue eyes and light sandy brown hair - apparently taking after the appearance of his non-Allyrion parent. Even the "Stony Dornishmen" of the mountains on Dorne's western border are basically Andal in appearance, blonde-haired and blue-eyed. Therefore it is not implausible that TV-Tyene simply inherited a bit more in appearance from one of Oberyn's or Ellaria's parents (or even grandparents), who happened to be Andal (i.e. one of Ellaria's parents might have been a Stony Dornishman from House Dayne or House Yronwood).