The Dead Lands of Athas
Esylk's Last Resort
Sold by an elven trader named Esylk Siltdancer, she guarantees this potent toxin will prevent the imbiber from returning from the dead as an undead creature. The poison is lethal, and will kill any who ingest it, prohibiting its use as a preventative measure. It is incredibly bitter, and could not be hidden in someone’s food.
Although, as it is named, the poison is meant to be taken as a last resort to ensure that they not haunt the place of their death. She further states that it may be possible to administer the toxin upon someone who has just been killed by an undead creature, but in that case she cannot guarantee its success — only that it would be less likely to occur.
What is in the poison to keep someone from rising, Esylk is unwilling to divulge, claiming that is a tribal secret. Some speculate that there is nothing special about the fast-acting poison, other than it killing its victim before any undead curse can be afflicted.