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.
In 1872, the Shadowhunter child Jem Carstairs was exposed to the substance when he was eleven years old when he was injected with large doses for days by the Greater Demon Yanluo as a form of torture. Jem's body became largely dependent on the substance, and no cure was found for his unwanted addiction even years after the attack. For years, Jem was forced to continue using it to keep him alive, despite it also killing him slowly.
Jem took it as infrequently as possible, only resorting to using yin fen when he became too ill from withdrawal, to extend his lifespan. When Jem went a long time without taking the drug, he would sometimes experience attacks or episodes of intense coughing fits, during which he would cough up blood and grow weak. Jem took more of the drug to gain strength when he needed to fight.
In a span of 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 when the heavenly fire burned through him and everything evil in him, 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.
Over a century later, in the year 2000, Raphael Santiago received word of a large shipment of yin fen being smuggled into New York on a cargo ship. He asked for Brother Zachariah's help in stopping it, and Brother Zachariah enlisted help from the New York Institute. They discovered that the werewolves were smuggling it on the same ship that was bringing the recently orphaned Jonathan Wayland to his new home with the Lightwoods.
Brother Zachariah, Raphael, Lily Chen, and Robert Lightwood snuck on board before the ship could dock. There, they — including Jonathan — ended up in a vicious fight against the werewolves on deck; Raphael was injured and the other Shadowhunter that was on board, Catherine Ashdown, was killed. With the remaining werewolves detained, Zachariah and Lily retrieved the box of yin fen from below deck. Zachariah used a rune known only to the Silent Brothers to burn away the harmful magic of it so as to not pollute the waters.
- Jem kept his stock in a silver, enamel box with the lid depicting Kwan Yin, the goddess of mercy and compassion.
- The Chinese word "yin fen" means "silver powder" (銀粉).