As I was preparing for a one shot, a player of mine asked about making poisons. I thought "Oh, this should be simple. They have access to nearly any kind of resources and level 20 characters, so I'll see what the rules are in the DMG and Xanathar's and let the player know what he can do." Alas, there are next to no rules for actually creating poisons, assuming that everything is going to be left up to the DM. This is not necessarily a bad thing, but having enough DM experience to know how hard checks should be while still allowing for players to make things that are useful is something not everyone has. Although this is a fairly brief outline, and I don't really break down material costs and crafting time, something that would be more important for a campaign, here is the system I developed for crafting poisons. I borrowed degrees of success (in this case, only applied in degrees of failure) from other systems to express how the consequences for failing work, but this is not outside the traditional D&D concept of failures that fall far outside of the range for success having more severe penalties.
The types of poisons and the strength of the save and effects impacts how difficult it is to safely make the poison. For the crafting checks, let’s use a system like this. For making the attempt, the roll uses the character’s intelligence modifier and proficiency bonus with a poisoner’s kit. Application outside of combat usually does not require a check, though in combat it may require a dexterity check if a character is being harried or applying the poison is very dangerous. Here is the standard formula I use for determining poison crafting DCs.
Base crafting DC: 5
Type of poison: These are the standard types of poison in the examples given in various sourcebooks. For a more unique type of poison, see the Umber Hulk Gaze Extract example at the bottom of the post.
Ingested: +2
Injury: +3
Contact: +4
Inhaled: +5
Damage: +1 for 3d4, +2 for 3d6, and +3 for 3d8. 1d4 damage is free if the poison has no other damage dice or status effects.
Since poisons in the core rules tend to be 1 dice, 3 dice, or 7 dice, with the top being 12 dice, I figured that intervals of three brings the numbers in line with the vanilla poison damage dice.
Persistent Application: Persistent application can only be applied to injury type poisons, with DM discretion for other cases. Poisons with persistent application linger on a weapon, applying across consecutive hits. Three hit persistence increases the crafting DC by +1 per 1d4, +2 per 1d6, and +3 per 1d8. Ten hit persistence costs +2 per 1d4, +3 per 1d6, and +4 per 1d8. Ten minutes of persistence (unlimited hits) costs +5 per 1d4, +8 per 1d6, and +10 per 1d8, starting upon application. Recurring damage is very powerful, hence the high cost, but this can be applied in addition to initial damage allowing for first round bursts and sustained damage, if the poisoner is willing to risk the crafting penalty. Recurring damage prevents an additional condition from being applied.
DC: Poisons have a base constitution save DC of 10, which applies to saving for half damage and saves against any conditions. This DC can be increased by +1 per rank increase, so reaching DC 15 would cost five points.
Conditions: All conditions have a duration of one minute by default. See the section on extending the duration below for more details.
Poisoned/Sleep: +2 to DC
Poison damage over time: +1 for 1d4, +2 for 1d6, +3 for 1d8, third die is free. Damage is taken every round for the duration, allowing a save for half damage. The duration of this effect cannot be increased. It also does not count as damage for the purpose of death saves, though it can render a target unconscious and force them to make saves.
Charmed (further damage breaks): +3 to DC
Stunned condition (save attempt every turn): +3 to DC
Paralyzed condition (save attempt every minute): +4 to DC
Death: +10 to DC if death occurs at the end of the duration, +15 if death is instant. Making a poison this dangerous is very difficult because it’s almost impossible to practice, so the DC is high but the payoff is also high. This type of save also doubles the base cost of the vector, making ingested cost +4 DC and inhaled cost +10 DC, because it becomes much more hazardous to handle.
Increasing the duration increases the DC cost by 1 per step, from one minute to ten minutes, from ten minutes to one hour, from one hour to eight hours, from eight hours to a day, from a day to a week, and from a week to recurring endlessly (saves may end the effect). Increasing the duration of an effect may be subject to DM approval- a sleep poison that lasts indefinitely might need to be magical in nature, falling under the magical poison rules below, and other status effects may not be able to be increased at all.
Crafting: When crafting a poison, success follows a baseline, but failure is somewhat relative. For success, reaching the DC of the poison is enough- going beyond confers no benefits. Making a more difficult poison has some degree of risk. Failing by one degree (a result of 1 to 5 below the crafting DC) means that the poison is lost and the materials are ruined. If attempting a very difficult poison, a lesser poison may be created instead, perhaps with entirely different effects than intended. The poison created by this possibility still seems like a success to the character in this case, unless the poison is tested (if it was crafted as a large enough batch). If the poison is completely impotent, the character is almost certainly aware of that outcome if they are proficient with poisoner’s kits. Two degrees of failure (6 or more below the DC) results in the poisoner being subjected to the penalties of the poison, with three degrees of failure (11 or more below the DC) providing disadvantage on the save. Attempting a very difficult poison may kill or afflict an amateur. A poisoner who repeatedly fails checks may take permanent penalties because of repeated exposure, depending on the DM’s rulings.
Magical Poisons: Magical poisons are poisons that are not inherently organic by function. An poisoner who lacks proficiency in arcana has disadvantage on creating poisons which are magical in nature, either because of their unusual ingredients or a particular enchantment that changes the poison’s properties. If the poisoner has proficiency, this penalty does not apply, and if they are proficient and have a value of +6 or higher in the arcana skill, they instead gain advantage when crafting magical poisons, as their skill sets synergize. In addition to becoming magical because of their unusual ingredients (DM discretion), poisons can also become magical because of the following effects:
Changed damage type: Instead of dealing poison damage, the poison deals elemental damage of an appropriate time- a poison made from the blood of an Efreeti (no easy task, given that their bodies disappear when they die) might gain fire damage instead of poison damage. Generally speaking, this effect only costs more if it is an unusual type of damage (like Psychic) or the effect is an enchantment on the poison itself. DMs can decide if this cost applies and how much it should be.
Changed save attribute: Changing the attribute for a save is very powerful, and a cost of +5 or more is probably applicable. For +10 to the DC, the poison is insidious and targets the weakest save. This change applies if making the potion backfires, which can make it safer to make some poisons, or more dangerous if the poison is insidious.
Examples:
Purple Worm Poison: The strongest damage poison in 5E's core rules, Purple Worm Poison does an impressive 12d6 damage and has a constitution save DC of 19. The DC for crafting this poison would be DC 8 for an injury poison, +8 for the damage dice, and then an additional +9 for the DC increase, for an impressive value of 25. This is obviously very difficult, but a level 20 character with 11 rogue levels with expertise and a maxed intelligence score (or a headband of intellect) could reliably make this poison, provided they can find the appropriate worms and deal with them. As this is the strongest damage poison in the core rules, I feel that this DC is mostly appropriate, as a very skilled rogue who specialized in poisons could easily make this poison, but most others would have to deal with the incredible risk it poses to its creator.
Poison, Basic: The most basic poison in the game, it lives up to its name with a meager 1d4 damage and a terrible DC 10 constitution save to half damage. It only lasts for a minute once applied, meaning that it almost always has to be applied in combat or immediately before. This poison has a low DC of 8 to make, though an argument could be made for 7 because of the limitations. Although it’s an injury poison with no additional cost from damage or DC, it does very minimal damage and only lasts for one minute exposed to the air, a penalty that may reduce the DC of crafting. The only reason why someone wouldn’t make this without proficiency is the risk of injuring themselves making it. This is basically like coating your sword with some toxic herb; you don’t need to be a poison expert to make or use it.
Instant Death Food Poison: This poison can instantly kill someone who eats or drinks it. Detection of the poison depends on how skillfully it is applied. This poison has a constitution save, DC 15. This poison is a custom example. It has the incredibly high cost of DC 29, because the vector costs 4 (doubled because of the death effect), the instant death effect costs 15, and the DC increase costs 5. This poison is craftable, given a decent level of skill, but not without risk.
Umber Hulk Gaze Extract: This magical poison requires extract from the eyes of an umber hulk. While the umber hulk does not, strictly speaking, need to be dead, it certainly helps the chances of whoever is extracting the poison. This poison does no additional damage, but forces a DC 15 Charisma saving throw to all creatures who can see the poison when their turn starts to prevent the effects of an umber hulk’s gaze. Unlike an umber hulk, a target does not have the option to avert their eyes unless they know in advance the poison is applied to an item. The poison must be applied to a flat surface with at least enough room for a one inch diameter circle, and then exposed to the target or targets when the poison is to take effect. The poison loses its enchantments after one use, though powerful poisoners can increase the persistence of the poison with enchantments. This poison is an example of an unusual poison. Not only is its effect not an orthodox poison effect, but its component is magical in origin, meaning that it falls under the rules for magical poisons. The DC for crafting this poison is unusual, as is the vector. Even applying this poison is more difficult than normal- if the poisoner looks at the circle after it is complete, they are subjected to the effect! The DC for crafting this poison would be a DC 17 check, with a +4 modifier for the unusual vector, a +5 modifier for the save DC increase, and a +3 modifier for the status effect type. Changing the save does not incur an additional DC cost increase in this case, as it is a property of the materials used. To extend the persistence of the poison when applied, the DC increases by +2 for three round persistence and +6 for ten round persistence, for a total of 19 or 23. Applying this poison is difficult, however, and incurs a dexterity check plus poisoner’s kit proficiency bonus, DC 15 since the check must be made without looking at the application. Completing this check takes one minute, and as such it should not be attempted in combat. Failing this check means that the circle is made improperly and has no effect, and as with crafting the poison, failing too significantly applies the poison to oneself. Once the poison is applied, the item to which it has been applied can be safely sheathed, stowed, or concealed. A cloth veil and preparations may be necessary for concealment on armor, unusual items, or shields.
This is an example of a more complicated poison, one which has significant alterations from the way poisons "usually" work in D&D. It's entirely feasible, and not even that difficult to craft, but may require your party to hunt down umber hulks or find a merchant specializing in exotic goods in order to find the materials. Additionally, it is more complicated to create and apply than a usual poison.
I hope that this guide is somewhat helpful for those of you with rogues dreaming of assassination attempts, herbalists gone evil, and various other masters of skullduggery in your parties.