|“||Men call me Darkstar, and I am of the night.||„|
|~ Gerold Dayne, to Myrcella Baratheon|
He's been cut from the TV series adaptation, and he is not expected to appear,as his antagonistic role has been adopted by Ellaria Sand.
A Feast for Crows
Darkstar allied himself with Princess Arianne Martell during her secret attempt to crown Princess Myrcella Baratheon as a new queen of the Seven Kingdoms, using her betrothal with Trystane Martell and the Dornish law where the eldest child inherits, male or female. The idea is to put Myrcella and Tommen and their supporters against each other. Arianne, who thinks her father hates her and wants her brother Quentyn to rule Dorne, and is also unaware of his real plans, hopes that the Dornishmen will rise to defend her claim for a chance to war against the Lannisters. During their travels in the desert on their way to Hellholt, they get ambushed by Areo Hotah at the river Greenblood, who ordered them to surrender in the name of Prince Doran Martell. While Kingsguard Ser Arys Oakheart rode to his death, Ser Gerold attempted to kill Myrcella (for reasons not quite yet known), but failed as Myrcella's horse runs away. Apparently he still managed to only slice off her ear and scar her face. He escaped capture. Maester Caleotte saved Myrcella's life, but she lost an ear and has now a hideous facial scar.
A Dance with Dragons
Ser Balon Swann of the Kingsguard, along with Obara Sand, is currently hunting Darkstar in order to bring him to justice. Myrcella was instructed to blame him for Arys' death as well, when she returns to King's Landing.
The Winds of Winter
While travelling in the stormlands Arianne Martell discusses Darkstar with Daemon Sand, former squire of the late Prince Oberyn Martell. They both agree he should have died long time before, and consider him a traitor.
Myrcella's status and theoriesIn the television series Game of Thrones, Ellaria Sand and the eldest 3 Sand Snakes, who have Gerold's villainous role, tried to attack Myrcella Baratheon in the Water Gardens and besides the attempt fails, due to the foolish idea of doing it in the day in front of Prince Doran, she is murdered later by Ellaria's Dornish poison. Myrcella's body is returned to King's Landing.
In A Song of Ice and Fire, we know that Tyrion Lannister had sent the handmaiden girl Rosamund Lannister with Myrcella, Arys, and Septa Eglantine on the way to Dorne as a way to fool assassins and to have Myrcella with her friend who's also of her age. We also know that, as Arianne planned, Oakheart disguised Rosamund as her cousin Myrcella and gave her fake redspots. The disease is harmless for children, but potentially lethal for adults, so anyone would have stayed away from her long enough for Arianne, Oakheart, and the real Myrcella to leave the city. After Hotah's ambush and Darkstar's treason, the last time we see Myrcella she got a hideous scar and lost an ear. It is known that Darkstar tried to kill Myrcella and the child was found with her wounds after the fights against Areo Hotah ended. After that the only known thing is that Doran instructed Myrcella to blame Darkstar for her wounds and the death of her bodyguard Ser Arys, when the latter was actually killed by Hotah.
While Darkstar actually tried to attack Myrcella, it is theorized by few that someone else might have wounded her at the end, perhaps even by accident in the fight during the night.
Another theory is that Dayne did actually killed Myrcella, who died of her wounds either during the travel back to Sunspear or in the Old Palace of Martell in the city, where Maester Caleotte failed to save her life. If the Lannisters learn this, Doran's preparations in support of House Targaryen would be ruined and their war against the Lannisters and Tyrells would start prematurely. When Ser Balon Swann arrived at Dorne to deliver Ser Gregor Clegane's skull and take Myrcella back to home, he saw her with her scars and she instructed him to hunt down Dayne with Obara. The girl Balon saw in reality might have been Rosamund Lannister, who's now on her way back to King's Landing and will be the only girl Cersei Lannister will see. Myrcella's body remains hidden in Dorne.