Boon of Poison Mastery

Boon of Poison Mastery
Feat
Prerequisite: Level 19

You gain the following benefits.
Antitoxic.
You have Immunity to Poison damage and the Poisoned condition.
Perfect Poisoner.
Once per turn, when you roll dice to determine Poison damage a creature takes from your attack, spell, or feature, you can instead use the highest number possible for each die.

Source: Forgotten Realms: Heroes of Faerûn (FRHoF), page 43