- Cersei Lannister: "Qyburn here is the cleverest man I know. Clever enough to learn what poison you used to murder Myrcella. The Long Goodbye, was that it?"
- Qyburn: "The Long Farewell."
- Cersei Lannister: "That's the one. How long does the poison take?"
- Qyburn: "Difficult to say. Hours, days, it depends on the subject's constitution."
- Cersei Lannister: "But death is certain?"
- Qyburn: "Oh yes, your grace. Quite certain."
- — Cersei Lannister and Qyburn, after the former poisons Tyene Sand.[src]
The Long Farewell is a poison originating from Asshai. As its name implies, it takes some time to take effect, but it is highly lethal. One single drop making contact with the skin is enough to bring about death. Symptoms include bleeding from the nose, blurred vision, and nausea. Of all the known poisons, it is the only one with an antidote that has been presented and used successfully.
It is unclear how long it takes for the poison to work. Bronn is imprisoned for hours or even days before he begins showing symptoms, but Myrcella Baratheon succumbs in less than an hour after the poison is given to her by Ellaria Sand. Qyburn later explains that the length of time for the poison to take effect ultimately depends on the person's constitution. (This coupled with the fact that Myrcella was a product of incest could explain why she died much quicker than most people would due to a weakened immune system.)
Tyene Sand is particularly fond of this substance, and coats her twin daggers in it. During a skirmish in the Water Gardens of Dorne, she slices Bronn with one such dagger. Later, after making him beg for it, she gives him the only antidote.
As Jaime, Myrcella, and Trystane leave for King's Landing, Ellaria Sand poisons Myrcella with the Long Farewell through a kiss of good will on the lips. Though both are poisoned, Tyene is able to produce an antidote for Ellaria, which she takes after her nose starts to bleed. Meanwhile, Myrcella dies within minutes in Jaime's arms.
After capturing Ellaria and Tyene, Euron Greyjoy delivers them to Cersei Lannister to face her justice for murdering her daughter. Cersei poisons Tyene with the Long Farewell through a kiss, just as Ellaria had done to Myrcella. She briefly misnames it as the "Long Goodbye", but is corrected by Qyburn. She then tells Ellaria that she intends to keep her alive so she can watch her daughter inevitably succumb to the poison and rot away, even if they have to force-feed Ellaria.