The strangler is a rare poison which makes the victim 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 - though it is not something discussed beyond the confines of the order. The Asshai'i have a name for the plant from which its made, and the alchemists of Lys a name for the crystal, but to the maesters it is known as the strangler. 
The process to make the poison is slow and difficult, the necessaries costly and hard to acquire. 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.
Dissolved in wine, the strangler makes the muscle of the victim's throat clench tighter than a fist, shutting down the windpipe. It is said the victim's face turns as purple as the poison crystals.
A Clash of Kings
Maester Cressen attempts to assassinate Melisandre using the strangler, but she survives after drinking almost the whole poisoned cup of wine. When Cressen drinks the remaining half a swallow from the same cup, he dies.
A Storm of Swords
Lady Olenna Redwyne uses strangler crystals, disguised as amethysts adorning a silver hairnet worn by Sansa Stark, to poison King Joffrey Baratheon at his royal wedding. The hairnet was given to Sansa by Ser Dontos Hollard under orders of Petyr Baelish.
—the ghost of High Heart