A more specialized field of assassins, adders are skilled in making and administering a large selection of poisons, thanks in part to their absolute mastery of the basics of transmutation and conjuration magic. Adders tend to be fairly secretive and demure and come from a variety of backgrounds, most involving crime though not all. Just as their namesake suggests, they are quick, venomous, and love watching their prey squirm.

Your archetype grants you features at 3rd level and then again at 9th, 13th, and 17th level.

Serpent's Venom

Starting at 3rd level, you can magically create weak poisons with little to no materials or time. You can use a bonus action granted by your Cunning Action to conjure a single vial of weak poison and, as part of its creation, you may apply it to a piece of ammunition or a weapon you are wielding. A creature that ingests or is struck by the vial or a weapon or ammunition it was applied to must make a Constitution saving throw against your potency save DC or take 1d6 poison damage. The vial lasts a minute before vanishing and the potency of the substance fades the moment after it's ingested or strikes a creature. The efficacy of the vials grows as you gain levels in this class, increasing by 1d6 when you reach 5th level (2d6), 11th level (3d6), and 15th level (4d6).

Potency Save DC: 8 + your proficiency bonus + your intelligence modifier

Adder's Expertise

When you choose this archetype at 3rd level, you gain proficiency with the poisoner's kit and the Sleight of Hand skill if you don't already have them. Your proficiency bonus is doubled for any ability check you make that uses either of them.

In addition, you can create more potent and specialized poisons and acids given enough time and materials. During a short or long rest, you can attempt to make a vial of poison, needing only half the amount of time and gold in materials as would be required to purchase a substance of similar quality. You can readily make basic poison without any difficulty. However, if you encounter an unfamiliar poison, you can attempt an Intelligence (Investigation) Check (DC determined by the DM based on the potency of the substance). If you succeed, you extrapolate the components necessary to create the substance and learn how to recreate it.

Secret Ingredient

Starting at 9th level, you have advantage on Dexterity (Sleight of Hand) checks to sneakily administer or apply poison to another substance. 

Poison Retardant

By 13th level, you have been around and handled poison for so long, you have grown inured to the effects of the substances. You gain resistance to poison damage and are immune to being poisoned. 

Infectious Touch

When you reach 17th level, your touch itself can be infused with harmful magic to severely injure a creature. As an action, you can touch a creature and force it to make a Constitution saving throw against your potency save DC. On a failure, the target takes 8d6 poison damage as a disgusting concoction is directly injected into their body. Additionally, if the target fails, you may also apply the effects of a poison you have on your person to the creature as the contents are conjured into your palm as part of the action. The creature automatically fails the save for the poison assuming there is one.