Assassination of Myrcella Baratheon

The Assassination of Myrcella Baratheon was a secret plot orchestrated by Ellaria Sand and the Sand Snakes in an attempt to draw Dorne into conflict with House Baratheon of King's Landing.

Prelude
After King Joffrey Baratheon is poisoned and murdered at the Purple Wedding, Tyrion Lannister is falsely accused and put on trial until he demands a trial by combat. One of the three judges, Prince Oberyn Martell of Dorne, volunteers to be his champion, mainly for a chance at revenge against Ser Gregor Clegane, the Crown's Champion who previously murdered Oberyn's sister, Elia Martell, and her two children during the Sack of King's Landing. Though he puts up a good fight, Oberyn is brutally murdered.

A Vengeful Family
The Dornish, already full of hatred towards the Lannisters for Elia's death, are outraged by Oberyn's murder and the Dornish army implore Prince Doran Martell, Oberyn's older brother and ruler of Dorne, to declare war against the Iron Throne. When he refuses, on the basis that Oberyn was killed in a trial by combat, the Dornish turn to Oberyn's paramour, Ellaria Sand, and his ruthless daughters, the Sand Snakes, all of whom seek to avenge Oberyn. Ellaria hatches a plot to instead make the Lannisters declare war against Dorne by murdering Cersei's daughter, Myrcella Baratheon. They almost kidnap her but fail due to the arrival of Jaime Lannister and Bronn. All are temporarily detained until Doran brokers a deal with the Lannisters to prevent further conflict: he will allow Jaime to take Myrcella back to King's Landing, as long as Trystane Martell, his son and Myrcella's betrothed, accompanies them, which Jaime accepts. Doran also issues Ellaria an ultimatum: if she defies him again, she will die. However, as the Lannister party prepares to leave Dorne, Ellaria poisons Myrcella by kissing her on the lips with a poison lipstick. The poison claims Myrcella's life, while Ellaria drinks up the antidote to save herself.

Aftermath
Though peace seems to have been restored in Dorne, Doran eventually finds out that Ellaria murdered Myrcella despite his warning. Before he can react, he is stabbed to death by Ellaria, who also has Trystane killed and takes over Dorne, due to the Dornish army having grown dissatisfied with Doran's lack of action against the Lannisters.

In the Books
In the novels, there is no parallel plotline to the assassination.

As much as the Snad Snakes hate the Lannisters and wish to get even with them, none of them has any ill intentions toward Myrcella. Ellaria has no intentions of vengeance at all.

In the beginning of the fourth novel, the three Sand Snakes confront Doran, demanding an act of revenge, and tell him their plans. Tyene's plan is the only one that involves Myrcella, and does not include harming her: they will crown Myrcella, who is the legal heir of the Iron Throne according to the Dornish law. That act will provoke the Lannisters and Tyrells to declare war against Dorne. Dornishmen fight best at home, so the Lannisters and the Tyrells come down on them, the Dornish shall bleed them in the passes and bury them beneath the blowing sands, as happened many times before. Doran rejects this plan too, and as a precaution he orders to imprison the elder Sand Snakes to prevent them from igniting a war, and to detain Ellaria and the young Sand Snakes at the Water Gardens.

Doran does not order to imprison Arianne too, and that proves to be a serious mistake. She intends to carry out Tyene's plan. She seduces Ser Arys Oakheart for that purpose and persuades Myrcella to come along, and gathers several followers, among them a questionable knight named Ser Gerold "Darkstar" Dayne. On the way to Hellholt, Darkstar tells Arianne "Crowning the Lannister girl is a hollow gesture. She will never sit the Iron Throne. Nor will you get the war you want. The lion is not so easily provoked". Instead, he advises Arianne to kill Myrcella, and she refuses, stating that she is no murderer of children and Myrcella is under her protection.

Unknown to Arianne, someone informed her father about the plan. Her party is intercepted by Areo Hotah. During the cofusion moments, Darkstar slashes at Myrcella and escapes. Luckily for Myrcella, her horse recoils, otherwise the blow would have severed her head. The slash opens her cheek down to the bone and slices off her right ear. Maester Caleotte saves her life, but he cannot repair her disfigured face.

Arianne is imprisoned for many days. Eventually her father comes to talk. He is furious at Arianne, for the incident may cause the Lannisters to declare war, and Dorne is not prepared yet for open confrontation. Arianne scoffs him for lack of action, and he tells her he has been working at the downfall of Tywin Lannister since the day they told me of Elia and her children.

Cersei, who has no idea about the incident, sends Ser Balon Swann to Dorne to bring the Mountain's skull, and take Myrcella back (and to play his part in her plot to assassinate Trystane). His party does not include Bronn. They arrive at Dorne openly and do not encounter any hostilities. To buy time, Doran sends Myrcella to the Water Gardens and orders his bannermen to delay Ser Balon by arranging games and feasts for his party.

Ser Balon finally arrives at Sunspear and is welcomed by the Martells. Doran has Arianne, Ellaria and the Sand Snakes free. He treats Ser Balon amicably, pretending he has no idea about Cersei's vile plan against his son. He invites Ser Balon to the Water Gardens the next day, where Myrcella is.

Later, Doran has a secret meeting with Ellaria, his daughter and nieces. Ellaria implores the Sand Snakes to abandon their plans of revenge, reasoning that enough blood has been spilt already. Arianne suggests that Myrcella, instructed by her, will tell Ser Balon what Darkstar did to her, and lie about Ser Arys's part in the incident, and ask him to hunt down the man who hurt her. Doran accepts that. The Sand Snakes reconcile with Doran and swear to serve him loyally. He gives them and Arianne various missions: Obara will lead Ser Balon after Darkstar; Nymeria will accompany Myrcella to King's Landing, where she will take Doran's seat at the Small Council; Tyene will go to King's Landing too, and try to get close to the new High Septon.

As Doran expects, Ser Balon believes Myrcella's story. After reporting to King's Landing, he and Obara go to High Hermitage in chase after Darkstar.

Kevan receives Ser Balon's report and tells Cersei, who is still imprisoned and has not performed the walk of atnoement yet, the bad news about Myrcella. Jaime has no idea about this, because he is currently at the Riverlands. Cersei is shocked and saddened to hear her daughter was disfigured. She has no idea who is Darkstar and why he wished to harm Myrcella. She thinks Tyrion is behind the attempt, and that he planned it from the beginning when he sent Myrcella to Dorne. Kevan disagrees with her.

Currently, at the point the novels reached, Myrcella is on her way to King's Landing, disfigured but alive, accompanied by Nymeria. Trystane remains at Sunspear after a tearful parting from her.

It is unknown why Darkstar was interested in igniting a war with the Lannisters by attempting to kill Myrcella, because there is very little information him in the books. There are various fan speculations about his personal motive: Myrcella's death in the TV show may imply that she will die in the books too, but considering how far the show strayed from the books on "The Red Woman", especially about the Dornish plotline - it is uncertain.
 * He is an overzealous Dornish who, like the Sand Snakes and many others, has grown impatient at Doran's seeming hesitance and lack of action to revenge Oberyn's death, and decided to push Dorne into war.
 * He acted as a catspaw for someone who wanted to stir up war between Dorne and the Lannisters (clearly that person is not Tyrion, as Cersei suspects).
 * He was frustrated that his distant relative, the legendary Ser Arthur Dayne, is the only Dayne whom the world remembers (he actually stated that during the journey). It was his pervert idea to enter history books by starting a war.