Dead man's blood was like poison to vampires, and it comes from, obviously, dead people. This is an essential tool when hunting vampires. A hunter could kill a vampire without this, but it creates enough pain to incapacitate a vampire.
Use
Once in the vampire's system they can no longer move and lose all their strength. It also hurts them, which is why Gordon Walker uses it to torture Lenore.
Although, dead man's blood does wear off, and the vampire will be able to regain his or her strength. So far, there is only one known vampire able to still move after being injected with it: the Alpha Vampire, their father, the strongest and oldest vampire in existence.