Fire poison is a weapon made by the Hunters, or Vimaren, for the specific purpose of killing Shimaren.
Fire poison takes the appearance of an intricately-wrought, silvery blade, typically a dagger (or a sword, as Sylphia has demonstrated).
The only way for the poison to begin working is by stabbing its victim in the heart. Once the blade is in the heart, the poison works its way through the bloodstream, devouring the blood. Fire poison was designed to inflict as much tortuous pain as possible, to the point that it causes insanity long before death.
In humans, the poison only causes a slight warming feeling, similar to alcohol.
The wound never fully heals in survivors, even after several millenia, and leaves a silvery mark after the wound has healed. In almost any other case, no scar is visible from an injury.
The only way to counteract fire poison's effects is to drown it in strong blood, typically that of an ancient. However, the only way to truly stop the poison is to drain so much blood that it kills the donor, and extinguishes the poison.