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 whose antidote has been presented and used successfully.
It is unclear how long it takes the poison to take effect: Bronn was imprisoned for hours or even days before he received the antidote, but in case of Myrcella - it starts taking effect while Ellaria was standing at the docks, no more than several minutes.
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. Myrcella dies 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.