Optional Variant Rule: Immunity Score

When properly explored, poison stands apart from other conditions and sources of damage in myriad ways. It covers a broad spectrum of sources, from purely physical to magical; it also covers an array of ways of being applied and how it works on a creature’s body. Unfortunately, the basic rules offer little to exploring this nuance in a game, preferring to make creatures of all types equally immune to this aspect of the game. The following optional rule corrects this by introducing an easy-to-use mechanism by which immunity to poison can be overcome using a sufficiently potent poison.

• Immunity Score. All creatures have a new ability score, referred to as their Immunity score, which replaces their previous immunity or resistance to poison. For most creatures, this score is 0, reflecting no innate resistance or immunity to poisons. Creatures who were immune have a base Immunity score equal to their challenge rating or character level; those who were only resistant have a base Immunity score equal to half their challenge rating or character level (rounded down). This results in creatures who are immune to some, but not all, poisons. Poisons with a potency value greater than a creature’s Immunity score have the potential to overcome their immunity.

• Bonus to Immunity. A creature can attain a bonus to their Immunity score in several ways. Spells that grant immunity to poison now increase the Immunity score by an amount equal to their spell level. For example, a character benefiting from Heroes’ Feast gains a +6 bonus to their Immunity score. Magic items that grant immunity to poison likewise increase the bearer’s Immunity score based on their rarity, beginning with uncommon granting a +2 through legendary granting a +5.

• Immunity versus Potency. When a poison is used against a creature, the game master should first compare the potency value of the poison to the creature’s Immunity score. If the creature’s Immunity score is greater than or equal to the potency value of the poison, it has no effect. However, if the potency value of the poison exceeds the creature’s Immunity score, then the poison works as usual, regardless of the creature’s previous immunity.

• Poison Resilience. Certain creatures previously immune to poison may be given this additional trait to reflect their superior poison resilience. The creature may make all saving throws against poisons with advantage, even if another source gives them disadvantage. Additionally, when the creature is subject to a saving throw against a poison, it instead takes no damage if it succeeds on the saving throw, and only half damage if it fails. Secondary effects still work normally on a failed saving throw.

The game master is encouraged to use their discretion regarding when to apply this trait. Not all creatures immune to poison should be given this ability, as they shouldn’t stand above others in terms of their immunity. Giving this trait to player characters whose class features (such as those playing a monk) provide immunity to poison is recommended, as those are often core and iconic features that one would be remiss to fully take away.

Source: DM Lair - Poisons Explored

Undead and Constructs.
Undead and constructs are slight outliers in the context of the various types of creatures. Because poisons are typically designed to work on living things, undead and constructs remain entirely immune to most poisons, regardless of potency. However, poisons that affect them can still be created; they just have to be specially made and can only affect the target creature type.