The fourth edition book "Sanctuary of Ice" introduced Amaranth flowers. These are a type of faerie-tainted Creo vis which can be consumed for the following effects for the next year and a day:
No increase in Apparent Age
1 Confidence Point
10 points of Magic Resistance
It's also possible for an Alchemist to refine the Amaranth to produce a powder, which gives Gifted the following effects:
For half an hour after taking "see the underlying mystical forces that are expressed through the material world"
1 extra confidence point for the next month
+3 to all Perception rolls for the next month
+3 to effective Parma Magica score for the next month
The ever so slight minor downside to this is that Amaranth is addictive - every year that you take it, you need to make a Stamina roll against an Ease Factor of 9+ to avoid developing the personality trait "Amaranth Addict", which opposes any action that doesn't directly involve acquiring more Amaranth. This roll is made when the first dose is taken each year, and the personality trait builds with 1xp per year taking it.
For bonus fun, the Amaranth Addict personality trait hurts your rolls to avoid and comprehend Twilight, and Amaranth addiction can't be detected or controlled by Hermetic Magic.
That's all fourth edition. There is one passing reference to Amaranth in 5e, in Against the Dark, where it is described as causing pleasant hallucinations in magi, but also addiction and warping.
In a fifth edition game, some of the characters (including my own as a Beta Story Guide due to the covenant vis distribution mechanism) have ended up in possession of some Amaranth vis without really knowing anything about it other than that it's Creo vis (and that the people who gave it to them are rather dodgy). They're also planning to create longevity rituals in the next year or so, and I'm trying to work out what the effects of that should be in the fairly likely event that the vis gets used for them.
Having the Longevity ritual completely eliminate Apparent Aging whilst in effect seems like one option - how good do people reckon this is?
I might give a confidence point when they take it.
Obviously the magic resistance goes out the window.
I'd probably not give the bonuses from the powder - that's usually an extra step, which the characters will have no idea is even possible.
I'd say make an addiction roll when first taken. Do you then have to make a new roll every year, if you're not reapplying the Longevity ritual?
If you do end up addicted, does the Personality trait build every year you're under the ritual, or only when you take a new dose?
Is being under the ritual sufficient to satisfy the cravings? (I'm inclined to say not here)
Should I keep the quasi-immunity to Hermetic Magic? It's a bit weird, although I can see that removing it makes it a bit trivial to avoid. On the other hand, I don't actually want to create a covenant full of drug addicts (I realise I should possibly have considered this earlier, although I do have plans for an explanatory tractatus to get circulated by someone who wants Amaranth banned. So the PCs may find out about it in time, if they actually read it.)
Longevity rituals do not have to be a consumable. So it is quite possible that some of the Magi who use the Amaranth will not actually consume it.
Even if they used it in a Longevity Ritual in which they did consume it, wither it caused any special effects would be a judgement call since depending on the ritual only the actual vis rather than the Amaranth might have been consumed. Though anyone adding any Amaranth for additional vis beyond the base required to increase the effect (and the ritual is in a form that is consumed) would for sure be effected by it.
So all that given, if they meet the requirements for consuming it then they have only consumed one dose. It does not matter if the ritual list for decades, since the ritual and the drug duration are separate things. So any special effects from the Amaranth would only last one year.
Addiction rolls if you feel required, though the Magi will have no way of knowing that they are addicted to it and no way of finding out due to the "quasi-immunity". Might come up in the future if they ever start messing with Amaranth again, especially if they find out more about it. In other words if there is nothing planned with Amaranth in the future, don't worry about addiction rolls.
Now if your group is ok to play potential drug addicts, you could insert a story-line about them discovering a potential Longevity Ritual booster.
I'm not sure I want to quibble with exactly how the longevity ritual is applied - it feels like if the longevity ritual is able to affect you, its ingredients should be able to affect you. Arguing that you might have extracted the vis without actually using the flowers is probably a better argument, although it's not clear that Amaranth's properties aren't inherently linked to the vis rather than just their surrounding flowers. Refining it changing the effects is possibly an argument against this, but that does require a specific magical process to do.
I'm not following the logic for there being a difference in Amaranth used to bump the potency up vs Amaranth inherently required by the ritual?
They are likely to come into contact with Amaranth smugglers again (they're mercenaries who've done one job for them, albeit without starting out with any knowledge of what they were transporting).
Maybe tell the players using it that they feel that there are some properties in the flowers that they could use to enhance the longevity rituals, but they're not sure of all of the side effects.
That is something that came from my current game and I didn't catch it when I was writing an answer. We decided that to gain the "special effects" of vis that provides such, they need to be added as additional vis rather than the base (required by age) vis. Sorry, I normally clearly define interpretations from play (which might be of use) vs actual raw in my post.
Both the smugglers and the "enhance longevity ritual with possible side effects" can become interesting story seeds. You know your players so have a much better chance of knowing how they will react to a potential character changing drug addiction. Some players would enjoy it, some would hate it, and some would depend on the character.
The smugglers have all sorts of possibilities, even if they are just a small scale operation. Who else they are doing work for, where they are getting their goods from, and groups wanting to stop them all can bring about interesting stories. You could also use them to pull the players into some cross Tribunal events.
I've used 4th ed material before in my game, but I'm cautious in how I use it, and typically, I don't translate it directly into 5th ed without some adaptation. There's a lot that changed. Including the realms. As far as I know, that includes the border between what was faerie and what was magical. I say this because when I look at vis in 5th edition, at least the vis types described, those of faerie origin... don't strike me as fitting Amaranth. To put it mildly, unless you're interested in waking up a faerie, most faerie vis comes with little side effects, and is basically just boring vis to most magi.
On the other hand, the other realms offer more interesting options, and I think they could give you plausible answers for how to handle this. Magic realm has several types of extraordinary vis which can lead to interesting answers to your dilemma:
You could put Amaranth as a sort of spell-like vis that doesn't have a fully Hermetic effect. With spell-like vis, you have an either or type of situation, where if they use the power of the vis, they don't get the vis, and if they use the vis, they don't get the power of the vis. That's a simple answer to your situation - they either consume the vis, or they use it to fuel a longevity ritual, but if it's the later, there is no complication.
An herb (e.g. flower) of virtue that needs to be enriched would fit very well with the lore you're describing. Normally, such things don't benefit the magi unless they are worked into a Talisman because the effect would need to penetrate the parma magica. Arguably, I'd say if it was worked into a longevity ritual, it would make total sense for a flower of virtue to impact the magi. Herbs of virtue typically come with drawbacks when enriched, and have a temporary duration, so this fits very well. However, bear in mind that enriching a flower of virtue again is normally a deliberate action from a knowledgeable individual, it's not something that merely happens by accident. So if you're going down this route, just like spell-like vis, the players shouldn't accidentally become addicts. They should be allowed to roll a magic lore, and depending on how well they roll, determine that this is a flower of virtue rather than normal vis, know about the enrichment process, know the benefits, know the drawbacks as well. The DC for each info is something you should set in advance, but even a failed check could come with tidbits that might let the PCs investigate for more rather than simply use up the special vis.
Now if you really dig the idea of turning your players into accidental drug addicts, and to give them a confidence boost while doing it, neither Faerie vis nor Magic vis will help you. Look up to your infernal vis sordida. It happens to generate Personality traits (e.g. Amaranth-addict) when you work with it in the lab for a season, it will give you confidence when you act on your sinful trait (e.g. consume more of your drug), and it will lead you to want to use it because it's oh so awesome (e.g. each pawn counts as double, which you can substitute for the "no increase in apparent age" for longevity ritual by giving them a higher bonus on the longevity ritual). Incidentally, this choice leads to the rest of your plot well: those dodgy drug dealers really are infernalist who relish on corrupting magi of the order by feeding them vis that will make them fall in the end - a rather good reason for some magi in the order to want to see it banned, because using it isn't a crime, but they know very well sooner or later, those drug dealers will make you break the code and deal with demons to get some more.
So that's my advice for you. Pick one of those three vis, and all your questions about how this incompatible stuff should work in the new edition if your PCs do XYZ will go away, because you will have rules that are already mostly written.
Focussing on the smugglers side was (and mostly still is) my original intention - there are people (probably increasingly greater in number and power, although that will depend a bit on the PCs' actions) trying to stop them, but the smugglers are willing to pay well. They're also likely to want things that increasingly "push" Hermetic law - transporting Amaranth in a tribunal that hasn't outlawed it yet is technically fine, even if it might not make you popular. There might be some jobs that come from the anti-smuggler people - why not work for both sides? Although the smugglers do pay better, if push comes to shove...
There are some interesting ideas in there, thank-you.
I agree that the idea (speculation) in 4e that Amaranth is a faerie plot against the Order doesn't gel that much with 5e Faerie - at most they'd be playing out a role that involved getting people addicted, but I'm not sure to what extent concerns about drug addiction were a thing in the thirteenth century (beyond "drunks are bad").
You could probably have faerie versions of the various types of unusual magic vis by having a faerie encasing it?
I wonder if there's any link available to faeries' addiction to vitality.
Enrichment is an interesting idea, although no-one's Magic Lore is all that great at the moment.
Infernal vis does feel like it could work...although a pawn of vis for a confidence point doesn't feel like it's going to be that tempting to magi under most circumstances, so there needs to be something else to make it feel like a good idea to start with.
The big lure is the pawn is worth two pawns. If used to boost a spell with several pawns, the magi should feel that his spell is much more powerful. Mind you, an extra pawn on a longevity ritual, would that be noticed unless it pushed the longevity total up into the next category? Not sure. Maybe if they used several pawns of it on the same potions.