I've gone 'round and 'round on this, but here's where I am right now. This certainly owes a debt to the SotC RAW, which very efficiently breaks down the role of poisons in the narrative. What I really wanted, though, was a consistent, reasonable way for PCs to make poison themselves, something that SotC, and the pulp genre, frowns upon when it comes to heroes.
Poison is treated something like a character, but with only three skills -- Power, Endurance, and Stealth -- and a minimum of one aspect, which defines its nature. The skills don't have any real relation to one another, and don't need to be arranged in a pyramid or anything like that. Long-term processes that mostly just serve the plot, such as a prince slowly poisoning the king over a period of months in an attempt to usurp the throne, shouldn't follow any of these rules.
Power measures the poison's potency. There are two basic types of poisons: poisons which affect the body (call them physical poisons -- the ones that kill, paralyze, nauseate, etc.) and poisons which affect the mind (mental poisons -- hallucinogens, mostly). Power is the poison's attack skill, opposed by Endurance for physical poisons and Resolve for mental ones.
- Mediocre and Average poisons are limited to Minor consequences, Fair and Good to Minor or Moderate consequences, and Great and up can deal any degree of consequence.
- Of course, if the target has already taken a Minor consequence and can't take another, a Mediocre poison can deal a Moderate consequence, and so on.
- Minor consequences from poisons don't go away at the end of a scene. Only a skill roll can remove them. Moderate and Severe consequences dealt by poisons are treated normally (i.e., with a skill roll or by becoming/modifying an aspect).
- On a successful attack, the poison deals a consequence of the appropriate type (physical or mental).
Endurance is how long the poison sticks around -- in game terms, we'll define this first as how many attempted attacks it makes, including the one it makes upon contact, and second as how long it lasts. At the beginning of each exchange (or scene, if out of combat), roll the poison's Endurance against the target's Endurance. If the poison succeeds, it makes an attack. If it fails, it doesn't.
- Deadly poisons (i.e., those that just do damage) attack as many times as they are able, dealing as many consequences as possible, until out of attacks.
- Poisons with other effects, such as paralysis or sickness, only attack until they deal one consequence, but that consequence lasts for a number of scenes equal to their Endurance. Or until the poison has served its purpose or gets boring -- whichever comes first.
- The number of attacks a poison can make it limited by its Endurance + 1. Thus, a poison with Mediocre (+0) Endurance attacks only once -- when it first makes contact with the target -- while one with Good (+3) Endurance would attempt up to four attacks (once at contact, and then once per scene for another three scenes).
- If a poison is neutralized (see below) before it can make all of its attacks, it loses any attacks it hasn't made.
Stealth is how difficult the poison is to detect. Some poisons, like contact poison on a blade, won't bother with Stealth at all, but for others, such as the proverbial Mickey, not being detected is of utmost importance.
- Stealth is opposed by whichever of the PC's skills is relevant in the situation. If a character sniffs his wine before drinking it to see if it's poisoned, it's a contest of Stealth vs. Investigation. If a physician is examining a corpse to see what killed it, it's Stealth vs. Physik.
The poison's aspect defines its nature. Usually, the name is good enough, assuming that's backed up by a few sentences describing how the poison works.
- Woorari: This sophisticated plant toxin that severely relaxes the victim's muscles, to the point of paralysis. Large doses can even mimic death. It has no taste, but smells vaguely of cinnamon, and is commonly administered by arrow or blade.
- Nux Vomica: This deadly poison causes severe muscular spasms, internal bleeding, and, more often than not, death. Victims have been strangled by their own tightly closed throats, if their hearts don't explode from over-exertion first. It has a very strong, bitter taste, difficult to conceal, but is most effective when ingested.
- Dreaming Moon: The Fae carefully guard the secrets of this magical poison's formulation, which involves a clear spring, a full moon, and a drake's liver, among other ingredients. Victims of Dreaming Moon cease to interact with the world around them in favor of vivid hallucinations. The nature of these hallucinations depends on the victim and are impossible to predict, but its use is widespread among Fae mystics.
The poison's aspect will also probably give clues as to its skills. For example, based on the above descriptions, I can stat out these three poisons pretty easily.
- Woorari: Great Power (full paralysis is pretty severe), Good Endurance (lasts long enough to simulate death, and has enough attacks to have a good chance of taking effect), Fair Stealth (it's not too difficult to detect).
- Nux Vomica: Superb Power (it causes uncontrollable spasms and internal bleeding -- that's downright superb), Average Endurance (it's a fast-acting poison), Mediocre Stealth (its distinctive bitter taste make it easy to detect -- of course, by then it might be too late)
- Dreaming Moon: Good Power (mild, but effective), Great Endurance (those mystics only use the good shit), Average Stealth (drake's liver isn't especially subtle).
The bigger question for players, though, is how to make a poison -- assuming you have all the necessary ingredients, of course.
Add the three skill values together to get its quality. That's the target number of your Physik roll. Thus, Woorari requires a Legendary + 1 effort, Nux Vomica requires a Fantastic effort, and Dreaming Moon requires a Legendary effort.
Seem difficult? You bet your sweet bippy. But a good laboratory can help immensely. Add the quality of the lab to your roll for your result. For example, if you're working in a Good (+3) lab, you need only make a Superb Physik effort to concoct Dreaming Moon.
(Still, it's clear that only a well-trained apothecary is going to attempt something like that.)
Poisons with two modus operandi -- that is, two natures, or two aspects, such as a damaging hallucinogen -- are also possible, but this requires adding a second Power skill to measure the secondary effect. Make one Power (Deadly) and Power (Visions), tack on a separate aspect (such as "Deadly Nightshade" and "Nightshade Visions"), and you're good to go. Of course, this will increase the overall quality of the poison, but nobody ever said this would be easy.
The time required to brew a poison starts at A Few Minutes, modified by one step up the table per point of difficulty. E.g., a Legendary poison would normally require a week's worth of work.
Shifts obtained on the roll can be spent on increasing the quality of any of the poison's three skills (at 1 shift/+1), adding an aspect (at 1 shift/aspect), or reducing the time required (at 1 shift/step on the time increments table). So if you were attempting to make Nux Vomica (a Fantastic poison) and ended up with an Epic Physik effort (due, perhaps, to a high skill value, a well-stocked lab, and some good luck), you could make it even deadlier by increasing its Power and Endurance by one each, make it harder to detect by increasing its Stealth from Mediocre to Fair, take only a few hours to brew it instead of the whole day, or anything in between.
Failing the Physik roll can mean a few different things. Just as with Craft, you can take extra time to get it done right -- up to four time increments for a total bonus of as much as +4. You can end up creating a weaker poison by subtracting the margin of error from the poison's skills (for example, knocking Nux Vomica's Power and Endurance down to Good and Mediocre respectively, to make your Good Physik effort a success), or the GM can offer you a Fate Point to think there's nothing wrong with it, as if compelling an aspect (or maybe he just compels one of your aspects -- either way). You won't find out you messed up until it doesn't work, of course, but that's how it goes sometimes.
Neutralizing a poison is as simple as making a Physik roll against its quality. Ta-da. Let's not draw this out. For story purposes, the GM might decide that it takes days of intensive care to do this, but that's not something that necessarily needs to be quantified. If it makes your game better for it to take a long time, have it take a long time; otherwise, make it something reasonable (e.g., a few hours spent brewing an antidote or foraging for the right counteractive herb) and move on.