She is the paramour of Prince Oberyn Martell of Dorne and a bastard of House Uller. "Sand" is the surname used for noble-born bastards in Dorne, much like Snow in the North.
Ellaria is the mother of the four youngest bastard daughters of Prince Oberyn Martell: Elia, Obella, Dorea and Loreza Sand. They are known as the Sand Snakes.
After their arrival in King's Landing, Ellaria and her lover, Prince Oberyn Martell, head to one of Littlefinger's brothels. Oberyn examines three whores while Ellaria lies on the bed, asking her which one she would like. She says that she doesn't want a timid one and he eventually finds one to her liking. When Olyvar calls Ellaria "my lady", she calls him out for using a lie to address her and tells him to call her what she is: a bastard. When Oberyn hears men singing the Rains of Castamere in the next room, Ellaria tries to stop him but can only watch as Oberyn takes out his rage on the Lannister men-at-arms. She proceeds to passionately kiss him as Tyrion Lannister and Bronn enter the room seeking Oberyn.
Ellaria and Oberyn attend Joffrey's wedding, where she is introduced to Tywin Lannister and Cersei Lannister. Cersei subtly mocks her bastard status, instigating a debate over the cultural differences between the Dornish and the rest of Westeros. She later witnesses the poisoning of King Joffrey.
In the days following Joffrey's death, Ellaria and Oberyn again visit the brothel. On this occasion, Ellaria is serviced by Marei and another woman; Ellaria steals a kiss from Olyvar, who apologizes and says he simply hasn't developed the taste for women. Ellaria soothes Oberyn's anger at the suggestion that she is an "acquired taste", by reminding him that this means he has Olyvar to himself. Ellaria leaves when Lord Tywin unexpectedly shows up and asks to meet privately with Oberyn, but says that Oberyn need only call if he needs her.
During Tyrion's trial by combat, Ellaria, in spite of having heard tales of the Mountain for years, is somewhat disturbed to see exactly how large he really is, expressing with disbelief, "you're going to fight that?!". Ellaria watches with Tyrion from the sidelines and is delighted when it looks like Oberyn has won. Her delight turns to shock and horror when the Mountain gains the upper hand on Oberyn and crushes his skull, at which she can do nothing but scream.
At the Water Gardens, Ellaria, still in mourning clothes, observes Trystane Martell and Myrcella Baratheon from a distance. She approaches Prince Doran Martell, but her path is initially blocked by Captain Areo Hotah. Ellaria threatens Areo before the captain is ordered by Doran to stand down. Angrily, Ellaria scolds Doran for his apparent inaction at the news of Oberyn's death. Doran plans to bury and mourn his beloved brother, but Ellaria and Oberyn's daughters want the Dornish people to take up arms and avenge their fallen prince. She also voices her contempt for Myrcella and wants to harm her in order to send Cersei Lannister a message. Doran strictly forbids this, stating that under his rule, they will not mutilate little girls for revenge. Ellaria voices doubts that Doran will remain in power if he continues to do nothing and leaves.
She later meets with Obara, Nymeria and her daughter Tyene, telling them that Doran will mourn for Oberyn but won't do anything. As Obara points out that they have no army without the support of the Prince to challenge the Lannisters Ellaria says that they won't need an army because they have Myrcella. Nymeria informs her that there's perhaps a problem, presenting her the merchant captain who smuggled Jamie Lannister to Dorne and tried to sell this information to Obara, just to end buried in the sand. Ellaria realises that Jamie has come for Myrcella and asks the three which way they choose, Doran's or hers. Tyene is the first one declaring for her, followed by Nymeria and then Obara, who makes her point by telling how she first met Oberyn, when she made her choice, and then she uses her spear to kill the captain.
Ellaria and the Sand Snakes return to the Water Gardens to kidnap Myrcella and use her for their revenge against the Lannisters. They swear, by the words of House Martell, to do what is needed to avenge Oberyn. However, Prince Doran had forseen their attempt to take Myrcella and his guard, led by Aero Hotah, arrests the Sand Snakes and Ellaria for their treachery.
Ellaria is later brought before Doran to receive Jaime, though she is openly hostile towards him, calling him and Kingslayer and refusing to drink a toast to Tommen as the new King. On her way out, she insults Doran, who in turn threatens to kill her if she speaks to him like that again. Later, Doran issues Ellaria an ultimatum: pledge allegiance to him, or die. She tearfully complies. She later visits Jaime in his quarters and makes amends with him, knowing that neither he nor Myrcella played any part in Oberyn's death, but expresses her knowledge that Myrcella is not Jaime's niece, but his daughter. Before leaving, she mocks him for not being born centuries ago into the Targaryen family, in which position his incest would have been accepted.
As the Sand Snakes say goodbye to the expedition back to King's Landing, Ellaria kisses Myrcella on the lips. As the ship is away in the horizon, Ellaria wipes blood from her nose and takes an antidote for the poison coated on her lips, which has just taken Myrcella's life on the boat.
Ellaria takes a stroll with Doran in the Water Gardens, with Areo Hotah and her daughter Tyene following. As she aids her prince to his seat, they discuss how her paramour Oberyn would have been a terrible ruler, and Doran a terrible adventurer. After Doran tells Tyene what a brilliant woman her mother is, flattering Ellaria, Maester Caleotte arrives with a message informing him that Myrcella was poisoned. As Doran immediately realizes what happened, Ellaria stabs him in the chest with a concealed knife and pulls him out of his chair while Tyene dispatches Areo and Caleotte. Dissatisfied with Doran's decisions, the other guards simply stand and watch. Ellaria reprimands the prince for allowing the Lannisters to get away with the deaths of both his siblings. Slowly bleeding to death, Doran begs for his son's life. Ellaria refuses, as Trystane Martell's death is also a part of her coup, already having sent Obara and Nymeria to ambush and kill him.
Jaime Lannister later receives word in King's Landing that Ellaria and the Sand Snakes have taken over Dorne and will likely soon declare war on the Lannisters.
Following the Destruction of the Great Sept of Baelor, Ellaria and the Sand Snakes extend an invitation for Olenna Tyrell to meet them at the Water Gardens. Upon their meeting, Lady Olenna reminds the Sands of the fate of Lyonel Tyrell when he came to Dorne, prompting Ellaria to assure her that she has nothing to fear from them, even though they practically murdered their own family to seize power in Dorne. Ellaria insists that they must work together now for survival, as Queen Cersei has declared war on both of them. Olenna objects, saying that survival is not what she is after due to the loss of her son and grandchildren. Ellaria offers Olenna her heart's desire, which she identifies as "justice and vengeance." She then summons Varys, who further clarifies that he and Ellaria are offering "fire and blood", signifying that Ellaria has declared for Daenerys Targaryen after hearing of her recent alliance with Theon Greyjoy and Yara Greyjoy.
When Daenerys's assembled armada sails for Westeros some time later, ships of Dorne are seen among them, supplied by Ellaria. However, Ellaria remains unaware that Tyrion Lannister, who was Myrcella's uncle, is now Daenerys's Hand and will likely demand justice for his niece's unjust murder.