Needs review
Source: Book of Vile Darkness

Description: After prolonged exposure to a poison or toxin, you have rendered yourself immune to it.

Prerequisites:

The character is immune to one specific poison (chosen by the DM or the character's player), whether available as a blade poison, the venom of a specific creature, or one other toxin. The character also gains a +1 circumstance bonus on saving throws against other poisons.

Special

A character may take this feat multiple times, choosing a different poison each time. The +1 bonus against other poisons doesn't stack with itself, because the circumstances of each poison immunity are essentially the same.