From A Wiki of Ice and Fire
The strangler is a rare poison, which makes the person unable to breathe. The knowledge of how to make the poison is known to the alchemists of Lys, the Faceless Men of Braavos and the Maesters of the Citadel. The Asshai'i have a name for the plant and the Alchemists of Lys a name for the crystal but to the maesters it is known as the strangler.
The poison is made from plants that are only found on islands in the Jade Sea. The leaves of the plant are picked and aged, then soaked in a wash of limes, sugar water and rare spices from the Summer Islands. The leaves are then discarded, but the liquid is kept and thickened with ash and allowed to crystallize. It turns a deep purple color.
- Maester Cressen attempted to assassinate Melisandre using the strangler but she survived after drinking almost the whole poisoned cup of wine. When Cressen drank the remaining half a swallow from the same cup he died.
- Lady Olenna Redwyne used strangler crystals, disguised as amethysts adorning a silver hairnet worn by Sansa Stark, to poison King Joffrey Baratheon. The hairnet was given to Sansa by Ser Dontos Hollard under orders of Petyr Baelish.