Yin fen, sometimes referred to as silver powder, is a highly addictive warlock drug and demonic poison, concocted and purchased in warlock dens. It is found mostly in the form of a powder and can either be dissolved in water or inhaled.
The drug slowly kills the user, as well as drains them of color, from the eyes, hair, and skin. It, however, also supplies energy, acting as a stimulant for werewolves, and a sort of energizer, to some extent, for humans.
An overdose gives a burning, scorching sensation in the veins and tears at the mind, making the user delirious with hallucinations and wild dreams. Meanwhile, withdrawal from it makes the addict abruptly weak and causes terrible pain, consistently pushing the dependent body towards death. It is said that the stronger the user is, particularly for Shadowhunters or Downworlders, the less the pain is during withdrawal.
Jem Carstairs was exposed to the substance when he was eleven years old and was injected with large doses for days by the Greater Demon Yanluo. Even after he was rescued, a cure for Jem, whose body had become dependent on the substance, was not found until some hundred years later.
During the "illness", Jem was forced to continue using it to keep him alive, despite it also killing him slowly. He took it as infrequently as possible, to extend his lifespan, and took more when he needed to fight to give him strength. When Jem has gone a long time without taking the drug, he would sometimes experience attacks or episodes of intense coughing fits, or "bad spells" as Charlotte had put it, during which he would cough up blood and grow weak. After approximately six years, Jem came close to death several times. It was his Silent Brotherhood that halted his dying process and kept him locked in his new state for more than a century. After 130 years in this state, Jem was cured from the infliction, freeing Jem from the yin fen in his body, his dependence on it, and the runes that kept Jem in the Brotherhood, safely reverting him back to his human state.
Axel Mortmain, who intended to derive Jem of his supply, ordered the packless werewolves who worked for him in 1878 to take the drug, at least six to seven times a day, to keep them working tirelessly. They only realized too late that the drug was lethal when their numbers began to dwindle.
- Jem kept his stock in a silver, enamel box with the lid depicting Kwan Yin, the goddess of mercy and compassion.