Even the most powerful creatures have a fatal bane, but are you willing to pay the price for it?
--Tyrik Skaeliren, Scholar of the Occult
Vampiric Poison is an extremely rare mixture of garlic, holy water, and holy wafers, mixed with a specially treated type of hemlock. The hemlock is actually killed through smoking and then a raise dead spell is cast on the hemlock, by a druid (if this is done in Ravenloft, the druid requires a powers check because of the dark nature of the ritual).The undead hemlock is not necessary, but it does increase the effectiveness (the vampire gets a -2 penalty to its saving throw). However, dead hemlock is necessary in the ingredients. For a herbalist to create this poison (once he/she actually knows the ingredients) he or she must roll a natural 1 on their proficiency check. The herbalist attempting this must also have an intelligence of 16, and a wisdom of at least 12, this is because the recipe is complicated and the directions consist of somewhat arcane methods, not known to anyone during this age (with the exception of Pyoor Twohundredsummers and Naranus Drannorian).
This type of poison is incredibly rare, it should be treated as an artifact in terms of rarity. The poison puts vampire (any type) into a coma for 12 turns if it fails its saving throw after ingesting it. The vampire goes into a coma for 6 turns if the poison is applied to an object that pierces its skin. When the vampire goes into a coma it only turns into gaseous form just before daylight and returns to its coffin (or otherwise). If the coffin has been properly eliminated the vampire suffers the effects of the sunlight. While at night (or if the vampire is old enough to be immune to the effects of the sun, during the day) the vampire, while under the effects of the poison, it cannot revert to any other forms due to its abilities.
This poison was rediscovered by a half-elf in the northern part of Nova Vaasa, named Naranus Drannorian. He was skilled with both spell and sword, and was trying to stop a blood stealing creature in a small town, with the aid of his companions. Fearing the worst, that a vampire was preying on the town's populace, he created a poison he thought would work on the undead menace. The creature turned out to be a human trying to summon a fiend using a blood pool. However, later on, when Naranus found himself trying to undo a vampire, the poison worked as planned.
Before Naranus discovered this poison, the creator of the Amulet of Ravenkind, a priest in the prime-material Barovia, had another vision, previous to creating the amulet, of the undead drinking the life's blood from the entire land and pledging fealty to a fallen lord. With this vision in mind, the priest created this poison, designed to put the undead beasts into a mortal coma, so the warriors of light, justice, and virtue may perform the right rituals in order to slay them. This poison, and its recipe is hidden somewhere in the domain of Barovia. The only person who knows this is Pyoor Twohundredsummers, leader of the Keepers of the Black Feather (see Domains of Dread). Naranus has kept his discovery completely secret as well, knowing the repercussions of such knowledge being spread around the demiplane.
This poison should be given artifact status in every way. It should be an important item in a campaign against these powerful undead beings. The actual apprehension of one vial of this material should be the subject of a long and difficult adventure. The apprehension of the recipe for this poison should be a mini campaign on its own. DM's, be careful with how you use this poison in your game. It's power is so great that it can make for a great addition to a dramatic campaign, or it could be the very downfall of your campaign.