Venomous Shadow

Roguish Archetype

Unlike the common cutthroat, your weapons are mainly a method to deliver the real threat: A toxic coat that will soon make the wound from the slash or arrow point seem insignificant by comparison. Whether to cause grave harm among enemies, or providing your companions with an advantage - there surely is a recipe that will get the job done.

Adventurous Brewer

At 3rd level, you gain the Brewer of Poison feat if you don't already have it.

Starting at 3rd level, when brewing poison using the Brewer of Poison feat, instead of creating one dose of Basic Poison, you create 1d4 doses.
You can choose to instead brew one dose of any poison that has a cost no higher than your Rogue level times 50. If you acquire one or more fitting ingredients for a specific poison, you can brew that poison if it has a cost no higher than your Rogue level times 100. The DC to craft any poison other than Basic Poison is 20.

Due to the requirement of properly storing these poisons, you can only keep a number of doses equal to your Proficiency bonus with you. Any further poisons that aren't Basic Poison lose their potency.

Related: List of Poisons

Steady Hands

Also starting at 3rd level, you can apply poison to a weapon or ammunition as a bonus action. Also, you can coat twice as many pieces of ammunition with a single dose of poison.

Improved Potency

At 9th level, you have experimented enough to create poisons with a personal note, making them even more potent in the process. The DC to ignore effects of the poisons you brew becomes 8 + your Intelligence modifier + your Proficiency bonus, unless the poison's regular DC is already higher.

Antidote

Starting at 9th level, you can create a matching antidote for any poison you brew, using the Brewer of Poisons feat. The DC to craft any antidote is 10.

Antidotes have to be ingested to affect a creature. Ingesting an antidote immediately lifts the Poisoned condition, as well as any effect of the poison it is meant to combat, and further prevents the creature from being affected by that poison for the next 24 hours.

Paralytic Essence

Starting at 13th level, all your poisons slightly inhibit the movements of your targets. You and others have advantage on attack rolls made against creatures suffering the Poisoned condition from a poison you brewed.

The Secret Ingredient

Starting at 17th level, you can deal Sneak Attack damage to any creature you affect with any of your brewed poisons, even outside of combat, and without advantage on the creature.

Also, whenever you cause Sneak Attack damage with a poison, poisoned weapon, or piece of ammunition, you can choose that the damage type of your Sneak Attack damage is Poison.