The Poisoner’s Kit tool offers a lot of opportunities for narrative interaction in D&D.
The mechanical implementation falls a bit flat. By the rules, the Poisoner’s Kit doesn’t do anything. The rules text gives you proficiency to create poisons. But, there’s no DCs for poison creation in the Player’s Handbook (PHB), Dungeon Master’s Guide (DMG), or the expanded tool rules in Xanathar’s Guide to Everything (XGE).
The DMG does speak on poisons:
Harvesting. If you’ve felled the right monster (there’s 4 listed), you can make a DC 20 Intelligence (Nature) check to harvest their poison. If you have the Poisoner’s Kit, you can use that instead, but you don’t get to stack it with your Nature bonus. Without more, a Poisoner’s Kit is a tough sell for a proficiency investment vs. a skill with broader application.
Crafting. For crafting purposes, the DMG points us to the crafting rules in the PHB. Those crafting rules are not based on Difficulty Class (DC), but give a time per cost. Your proficiency bonus is never taken into account.
In the spirit of 5th Edition, the implementation is left to the Dungeon Master. While the Poisoner’s Kit isn’t one of the provided examples, XGE tells us to make judgment calls:
So, how do we do it?
The 30/30 Rule
We’re not lost. The PHB guidelines use cost as a reference. We can balance on the same principles. If we divide all the costs by 30, we get a reasonable DC spread that allows players to reliably create poisons at lower levels. Here’s our DCs:
We cap the DCs at 30, which is the ceiling for a “nearly impossible” task according to the DMG guidelines:
Without capping some of the most exorbitant costs, the DCs quickly run into the unreachable range. A DC 30 is in the realm of usability, but only when considering an ability score modifier in addition to your proficiency bonus. So, how do we balance it with regular 5e expectations?
Make it an Intelligence check. Borrow from the DMG rules on harvesting poison. It makes even more sense that you’d require specialized knowledge to know how to craft a poison.
When crafting a poison, consider a few things beyond the roll. You need to balance these factors to ensure that poisons are useful, but not stockpile abuse.
Cost of materials. You might determine that one is warranted in roll-to-craft scenario that skips the RAW timelines. The XGE crafting guidelines don’t require a reagent expense for non-magic items. While there are reagent costs for magic items, you will need to scale them based on cost/DC.
Material expense. Are these materials expended when you attempt to brew a poison, or can you reuse reagents until you achieve a successful brew?
Duration of a crafting attempt. How long does it take to try and brew a poison? An hour? Can you do it during a short rest? 8 hours? Can you do it at the end of a long rest? 24 hours? Do you want to follow the magic crafting guidelines?
Spoliation. How soon after brewing must a poison be used?