Effective Against
VampireDead man's blood is blood drawn from a human corpse. To vampires, it acts as a powerful toxin — essentially a sedative that weakens and disables them on contact with their bloodstream. It is the primary method for incapacitating a vampire long enough to perform a beheading, which is otherwise extremely dangerous to attempt on a fully alert, superhumanly fast and strong vampire.
Usage
Dead man's blood is typically applied to arrows, darts, syringes, or blade edges. When introduced to a vampire's system — whether by injection, a cut with a coated weapon, or even ingestion — it causes immediate weakness, disorientation, and physical pain. The effect is temporary, lasting minutes to perhaps an hour depending on the dose, but it is sufficient to subdue a vampire for decapitation.
Acquisition
Acquiring dead man's blood requires access to a fresh corpse. Hunters have obtained it from morgues, funeral homes, and — in more desperate circumstances — by raiding graves. The blood must be relatively fresh; how long it remains effective after extraction is not precisely established, but hunters prefer recent specimens.
In Supernatural
Dead man's blood was introduced in Season 1, Episode 20, "Dead Man's Blood," where John Winchester used it against a vampire nest. It remained the standard pre-fight preparation for vampire encounters throughout the series. The concept is not drawn from traditional folklore but is an invention of the show that has since been adopted by other supernatural fiction.