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.
After Queen Daenerys sets up her base at Dragonstone castle, Ellaria Sand attends a meeting with Daenerys, Tyrion Lannister, Princess Yara Greyjoy, and Lady Olenna Tyrell in the Chamber of the Painted Table. Ellaria and Yara advocate attacking King's Landing. However, Dany is unwilling to lay waste to the capital and create a bloodbath. When Tyrion confronts Ellaria about her role in the death of his niece Princess Myrcella Baratheon, Ellaria counters that Tyrion cost her lover Oberyn Martell his life. Daenerys ends the argument by warning Ellaria to respect her Hand. Ellaria and Lady Olenna reluctantly acquiesce to Tyrion's plan to use the Martell and Tyrell forces to lay siege to King's Landing. Tyrion's plan involves using Yara's portion of the Iron Fleet to transport the Dornish and Tyrell forces across the Narrow Sea to King's Landing.
While traveling with Yara's Iron Fleet, Ellaria shares drinks with Princess Yara and her brother Theon Greyjoy. She compares the Ironborn liquor unfavorably to Dornish wine. That night, their fleet is ambushed by Yara and Theon's uncle Euron Greyjoy, who commands a larger faction of the Iron Fleet. Euron boards Yara's ship and his men overwhelm Yara's forces. Ellaria retreats below the deck with Tyene but Obara and Nymeria are killed by Euron. Ellaria pleads for a quick death, but is quickly dismissed and taken prisoner by Harrag. Euron and his men capture Ellaria, Tyene, and Yara and sail away in Euron's ship, en route to King's Landing, leaving Yara's fleet to burn in the sea.
In the aftermath of their capture, Euron takes Ellaria, along with Tyene and Yara, to King's Landing to present as his gift to Cersei. He parades them through the streets of the capital, and the crowd take great delight in pelting them with insults and refuse. Euron throws Ellaria and Tyene in chains at the foot of the Iron Throne as a token of his loyalty. Ellaria spits hatefully at Cersei's feet, though she is clearly horrified at the sight of Gregor Clegane looming over them, even more monstrous than when he killed her lover.
Later that same day, in the Red Keep's dungeons, Cersei has Ellaria and Tyene chained up on opposite sides of their cell, gagged. Cersei taunts Ellaria by sadistically recalling the Red Viper's savage death to Ellaria and how Ellaria screamed at the sight of it. She later provokes Ellaria's sorrow by recalling Oberyn's fearsome skill with a spear, and how that eventually didn't stop Gregor from killing him, as well as inferring that Oberyn brought his death on himself by taunting Gregor at the last instead of just leaving him to die. She then remembers raising Myrcella, and the fact that Ellaria murdered her. Cersei angrily demands to know why Ellaria murdered her only daughter, but the Dornish woman only grins mockingly at her in return.
However, Ellaria's mocking smile fades when Cersei turns her attention to Tyene, praising her daughter's "perfect Dornish beauty" and musing that she must be Ellaria's favourite child by her late paramour. Realising that Cersei intends to kill Tyene in retaliation for Myrcella's death, Ellaria desperately tries to beg for her daughter's life, but with the gag muffling her, her pleas are unintelligible, as Cersei mocks. Cersei notes that she thought for a long time how to kill Ellaria; she thought of having Gregor crush Ellaria's skull like Oberyn's but while poetic, it would be too swift a death. Cersei then adds that she thought about having Gregor crush Tyene's skull, but tauntingly remarks it wouldn't be right to have Tyene's "pretty face crack apart like a duck's egg" as she removes the girl's gag and kisses her full on the lips. Ellaria's eyes go wide with horror as she realises what has happened.
As Tyene desperately calls for her mother before Qyburn regags her, Cersei explains that Qyburn figured out it was the Long Farewell Ellaria used to kill Myrcella. As a heartbroken Ellaria sinks down, sobbing, Cersei has Qyburn confirm that the poison means certain death, but how long it takes to kill depends on the strength of the victim's constitution. Cersei then triumphantly tells Ellaria that she will watch her daughter die in their cell, and then she will be forced to watch Tyene's "beautiful face crumble into bone and dust", and that they will keep Ellaria alive to watch that even if they have to force-feed her to do it. Before leaving, Cersei orders the guards to frequently change the torches so Ellaria doesn't miss a moment of her daughter's death. The moment Cersei is gone, Ellaria frantically tries to reach Tyene, but with their chains too short, she cannot reach her daughter to hold or comfort her in her last hours.
Before the death of her paramour Oberyn, Ellaria was shown to be an adventrous, carefree, and spirited woman who shared her lover's tastes and zest for life. She was also rather peaceful as shown when she initially tried to dissuade Oberyn from getting into a fight with two Lannister men at Littlefinger's brothel until one of them made a lascivious remark about her, implying that she must also be a whore, and a prejudiced remark about Oberyn and Dornishmen in general, even though she too is Dornish. Ellaria is also initially courteous to Tywin and Cersei at their first meeting but was rather quickly vexed when Cersei hypocritically mocked her bastard surname. When Oberyn went to fight the Mountain, Ellaria expressed shock that he had chosen to fight such a man, and pleaded that he not leave her alone in the world, and she reacted with horror when Gregor Clegane killed him. This shows that she and Oberyn had a genuinely loving relationship. She displays equally strong affection for her eldest daughter, Tyene, and treats Oberyn's two oldest daughters (Obara and Nymeria) with respect, despite them not being her own children.
Since Oberyn's death, however, her personality has completely darkened. Despite the fact that it was ultimately Oberyn's hubris that got him killed, Ellaria has not only inherited his more vengeful side; she has transcended it to a whole new level. This started off as a considerably aggressive attitude towards anyone who crossed her, especially Jaime Lannister. She behaved in an aggressive and spiteful way, and when Doran treated with Jaime, expressed disgust that he was even considering breaking bread with Jaime. Beforehand, she was enraged that he hadn't declared war on the Iron Throne to avenge Oberyn. She became pitiless and dangerous, suggesting that they send Myrcella back to Cersei piece by piece, and later killing her in front of Jaime, all so she could mount a war against House Lannister.
This being said, Ellaria isn't so blinded by her hatred and revenge that she isn't willing to make alliances as shown when she invites Olenna Tyrell, a member of a family that have been rivals of the Martells for centuries, if it meant that they could take their revenge on Cersei and her family for the damage that they caused to both their families. Still, for all intents and purposes, Ellaria fails to see that she has proven herself no better than the Lannisters by murdering an innocent child and then Oberyn's own brother in cold blood, despite Doran and Trystane having treated her like one of their own family.
Though aware of her baseborn origins, Ellaria's status as a part of Dornish nobility has also hindered her views slightly, such as when she asks Theon Greyjoy to pour her wine despite him legally being Balon Greyjoy's heir to the Iron Islands (which he himself gave up for Yara), and only stops when Yara tells her that Theon is not her servant. Her relaxed attitude towards sexual relations is reflected when she offers Theon to join her and Yara for a threesome, even though Theon and Yara are siblings.