- "Dissolved in wine, it makes the muscles of a man's throat clench tighter than any fist. The victim's face turns purple as the little crystal seed from which his death was grown, and so they call it the strangler."
- ―Oberyn Martell
The poison is dealt in the form of amethyst-colored crystals which can be crushed and dissolved into beverages for the targeted victim to drink.
- Maester Cressen attempted to assassinate Melisandre by offering her a cup of wine containing the strangler. As an old man and more concerned about Stannis's well-being, Cressen knowingly drank from the poisoned cup as a sign of good faith, in the hope that this would trick the Red Priestess into accepting his offer. To Cressen's surprise, Melisandre's magical abilities apparently protected her from the effects of the deadly poison, while Cressen himself collapsed and died.
- King Joffrey Baratheon was assassinated by Olenna Tyrell using the strangler.
Maester Cressen uses the strangler in a failed attempt to assassinate Melisandre, offering her the same cup he is drinking from. He dies from the poison's effects, but Melisandre is apparently protected by her magical powers.
Tyrion is subsequently arrested on the false charge that he was behind the poisoning. Speaking to him in his cell in the dungeons, Podrick Payne tells Tyrion that a man he didn't know approached him and offered Podrick a bribe in the form of a knighthood if he testified against Tyrion, and said he saw him buy a poison called "the strangler". Podrick declined the offer.
During the Sack of Highgarden, Olenna again asks Jaime how he plans to kill her. Jaime tells her of Cersei’s idea of having her whipped and beheaded, or flayed alive and hanged, but he talked her out of those ideas. He then produces a small vial and empties its contents into one of the glasses of wine, giving it to Olenna who then drinks it after Jaime confirms that it will be a painless death. Olenna reflects on the horrible way that Joffrey died, and the gruesome details that the poison caused; she admits that part was unintentional on her part, as she had never seen the strangler work in person before. Jaime is shocked to learn Olenna was the one who poisoned his son.
In the books
The poison is always referred to as "the strangler" with a lowercase "s".
The TV series did not explain it at the time, but the poison used to kill Joffrey is the same kind that Maester Cressen used two seasons earlier in an attempt to kill Melisandre. Half a swallow of wine spiked with the strangler was sufficient to kill Cressen, while Melisandre finished almost an entire cup of poisoned wine and was unaffected, apparently due to her magical powers.
The strangler is known to the Maesters of the Citadel in Oldtown, the shadowbinders of Asshai, the alchemists of Lys, and the Faceless Men of Braavos. It is made from the leaves of a plant found in the Jade Sea of Essos, which is then pickled in jars and mixed with herbs from the Summer Islands. The leaves are subsequently discarded while the remaining liquid is allowed to crystallize, turning a deep blue color. It is known by different names to each of the different groups that use it in different countries, but the Maesters of the Citadel know it as "the strangler".
It isn't clear if there is any known antidote to the strangler. When Joffrey collapses, Pycelle shouts for someone to help him back to his chambers to fetch his potions, but Pycelle did not have time to diagnose Joffrey and determine that the strangler was the specific poison used on him; he might have just hoped that it was one of the poisons he did have an antidote for.
Given that Olenna would have never allowed any harm happen to her granddaughter, logically there has to be an antidote, perhaps given to Margaery in advance as a precaution in case she drinks the poisoned wine, too.