Both a drug and a poison, Poudre de la Mort Vraie – The Powder of True Death or Powder of the True Death, depending on your translator – is specifically designed for use by vampires. Created by Signalman Reynolds, formely of His Majesty’s British Army, it is extremely potent and addictive to vampires.
Reynolds, whose own wife had been a vampire, spent decades discovering how to create a substance that vampires would be attracted to. The actual formula for the creation of the grey powder is unknown, although it includes the dust of ground up mummies and assorted alchemical treatments.
Its effect on vampires is exactly what Reynolds intended: it has a euphoric rush, but leaves the vampire overwhlemed with the guilt of all their actions (both as a human and as a vampire) and not at all hungry for blood. The idea is that the vampire will be tortured by both remorse and starvation as they slowly wither away to nothing – the taste of blood will lead a vampire who has consumed Poudre de la Mort Vraie retch and vomit.
A large enough dose of the untainted blood of another vampire will throw off the effects of the drug, and a small enough dose can be outlasted – after a few days, normal vampiric instincts will reassert themselves.