Plague Warden

Some rangers devote themselves to protecting society from the ravages of contagious disease and poisonous pollutants. Known as plague wardens, such rangers use their martial prowess to fight against disease spreading monsters and those who would poison the world, augmenting their fighting with miasma absorbed from the air. They use their skills to cure those affected by toxins and brew poisons to wield against their enemies.

Miasma Infusion

At 3rd level, you gain the ability to infuse your weapons and magic with noxious miasma. On your turn when you hit a creature with a weapon or deal poison or necrotic damage to a creature, you can cause the creature to take an extra 1d6 poison damage. If the creature is immune to poison damage, they instead take 1d4 necrotic damage, or 1d6 if it is one of your favored enemies. You cannot use this feature more than once on your turn.

Plague Doctor

Also at 3rd level, you become preternaturally aware of the presence and nature of toxins. You gain proficiency in the medicine skill, in herbalism kits or in poisoner's kits, and you can also cast detect poison and disease as a ranger spell without a spell slot or components. You can use this feature to cast the spell times equal to your proficiency bonus per long rest.

Toxin Tolerance

Starting at level 7, you gain resistance to poison and necrotic damage, and you have advantage on saving throws against poison and disease. You are also immune to the effects of any stench or harmful smell, like stink cloud.

Infectious Corruption

At 11th level, you know how to spread miasma from foe to foe, increasing in power as it spreads. When you deal damage to a creature using miasma infusion, you can also deal 1d8 poison damage to up to two other creatures of your choice within 15 feet of the target. If a creature is immune to poison damage, it takes 1d6 necrotic damage instead, or 1d8 damage if it is one of your favorite enemies.

Fuming Wounds

Starting at level 15, your wounds and injuries give off noxious fumes that you direct. When a creature hits you with a melee attack, you can use your reaction to expose the attacker to these noxious fumes. The attacker must succeed on a constitution save with the same DC as your spellcasting save DC. If it fails, it is poisoned until the end of your next turn. If it can't be poisoned, its speed is reduced by 10 feet for the duration.