Lethal Poison (3.5e Feat)
From D&D Wiki
Lethal Poison [Monstrous]
Your poison becomes so potent, that it can cause death.
Prerequisite: Poison attack that deals ability damage, Con 15, Alternative Poison
Benefit: Choose one of the poison attacks to which you have applied the Alternate Poison feat. The initial and secondary damage for this poison become Constitution damage.