Tell them identify doesn't work on these items, for reasons that hint at game lore

Despite the matter-of-fact wording in the identify spell, there is certainly precedent for items whose properties cannot be revealed by identify, or even items for which identify reveals false information. For example, in the DMG section on cursed items (emphasis mine):

A magic item’s description specifies whether the item is cursed. Most methods of identifying items, including the identify spell, fail to reveal such a curse, although lore might hint at it. A curse should be a surprise to the item’s user when the curse’s effects are revealed.

Obviously, your potions are not cursed items, but a similar logic applies: the whole point is that the potion's effects are meant to be a surprise. You can even do this in a way that makes the caster of identify feel like they got something useful out of their spell, even though they don't discover the actual function of the potion. Rather than simply saying "the spell fails", you can tell them that the identify spell reveals the potion's function, which is to produce a random effect when consumed. Make up some explanation, such as saying that the potions were created with fundamentally chaotic energies, so it's impossible to predict the effect without using the potion, because the effect isn't even determined until the potion is drunk. You can tell them that the matched pairs are quantum-entangled or some magic-sounding equivalent, which explains why the pairs both produce the same effect even though they're both "random". The point is, when the identify spell fails, you can have it "fail" in a way that rewards your players with hints at new and interesting lore about your game world.

Regarding the "really cool artifact with a lot of mysterious characteristics", I would say that it's entirely fair to just tell your players that the identify spell doesn't work. It's not at all unusual for identify to fail on such powerful and unique artifacts. Again, you can make up some reason for this that expands upon the lore of the game world. For example:

• Was the artifact created by a powerful, paranoid wizard? Then that wizard would certainly have shielded their creation from divination magic, lest it be discovered by other wizards. In fact, the identify spell tells the caster that this item is non-magical, which is clearly at odds with the magical properties they've already observed, hinting at the fact that this artifact is something special.


• Was the artifact created by long-forgotten magics from another plane? Perhaps it is simply outside the parameters of the standard identify spell, which was only designed to identify types of magic that was known at the time. When cast, the identify spell "works", but the description it returns is unintelligible gibberish. Perhaps your players will later find an old spellbook from a bygone era containing an archaic variant of identify does not have this limitation.


• Was the artifact created by a deity? Perhaps the deity didn't want it to be identified, and so they simply used their godly powers to make it so. Think about how that god's powers would manifest to prevent the identification. Perhaps the caster remembers receiving a full description of the item's properties, but for some unknown reason they are unable to recall a single word of that description.

Remember that you don't have to tell them exactly why the spell doesn't work (e.g. "a wizard shielded it from divination magic"); instead, just tell them what weird thing happens (e.g. "the item reads as non-magical even though you know this is demonstrably false"). This way, instead of identify ruining the mystery, the spell makes things even more mysterious, which hopefully makes your players even more invested in solving the mystery.

Change the rules

Between identify, and the rules for characters figuring out what items do by spending a short rest with them (DMG p. 136), 5e is signalling to us that by design, figuring out what magic items do should not be a challenge. I can see the argument: from a player's standpoint, they might just want to use their treasure and not jump through hoops to do so; they feel like they've already earned it by finding / acquiring it. But as someone who is primarily a DM, I see the other argument too.

Bottom line: decide whether or not you agree with that aspect of 5e. If you do, just let it go, if you don't, change the rules. As-is, it is almost trivial to identify a magic item. You, as the DM, are well within your right to change rules. Doing it mid-game with a precedent already set is another matter. If you wanted to go this route, you could say that those techniques only work on, e.g. uncommon magic items (thereby saving yourself from this issue when the party starts discovering more powerful items). You'll have to make a judgement call regarding what change, if any, is appropriate given the established precedent.

For the artifact, there you just house-rule that it doesn't work. Identify didn't work on artifacts in previous editions for this reason.

Now on your potion problem, this would seem to have a fairly simple in-game solution, which is that either:

a) the genasi doesn't let you cast spells on his potions. "Can I fondle your potion for a minute and use magic, which may or may not be identify, on it?" "No." Perhaps experimenting on people with the first potion of the pair is part of what he's getting out of this weird setup and doesn't want to let them cast identify - he's the one selling, it's his call.

b) the genasi lets you cast the spell, but then he sets the price to buy it based on the utility of the potion. I assume they're getting some sweet discounted price on a random potion (if not, it's a screwjob and they're right to be trying to get around it - "random" is fun for you but not so much for them, it's a danger to them). This makes the choice of "try a random potion and it's cheaper" a meaningful player decision.

Talk to your players about this

For any situation where you want to change the rules (either mid-campaign or pre-campaign) you should talk to your players about the rule change and why you want to make it. Ultimately any change to the rules should be making the game more fun for you and your players.

If your players like the proposed rule changes then great, you've gotten everyone on the same page.

You should be prepared, however, for your players to say that your suggested rules changes are not something they will enjoy. If that's the case then listen to them. It may mean you need to change some detail of your plot, or how you award the magic items.

Let's say your players want to keep the status quo with identity: Using your potion shop as an example, you could change Ocean's Potions to be instead:

• each potion comes with a companion potion, which is required for the magic potion to work on someone


• the price of buying a potion is that the character's drink a "wild magic" potion (which rolls on the sorcerer's Wild Magic table for its effects)


• it just sells one copy of a potion

How can you make identification more difficult?

My house rule for this sort of situation (which I established with my players in a Session 0) is the following:

There are three tiers of items:

• Artifacts


• Magic Items


• Regular Items

Identify (and other related methods of identification) then has the following properties:

• For all items identify will reveal whether or not it's magical


• For non magical items identify will find out what the item is, its classification (martial/non-martial weapon/other item) and its purpose


• For non-cursed magic items
  
  
  
  
  • a ritual identify will get the current beneficial properties
  
  
  • a level 1 identify (using the spell slot not the ritual) will get all of the (current and future) properties of the item
  
  
  • attunement and experimentation will get you some of the properties
  
  
  
  


• For cursed magic items a level 2 identify will reveal the curse's effects


• For Artifacts
  
  
  
  
  • A level 3 identify is required to get the main beneficial properties
  
  
  • A level 4 identify is required to get all the properties
  
  
  • A level 5 identify is required to find out the method of destruction (of which there is only one)
  
  
  
  

In game this tiering is justified as the magic imbued into the item/artifact resisting the magic from the identify spell and the spellcaster has to overcome that magical potential in order to extract the information from it using identify.

This has worked well in the last campaign I ran. The players were happy with the changes, and how they affected their character's decisions (for example, choosing to attune to an item they could not identify despite the risk that it might be cursed was worth the trade off for one player).

What if I don't want to houserule this?

The DMG has a variant rule for making identification of magic items more difficult in your campaign:

Variant: More Difficult Identification

If you prefer magic items to have a greater mystique, consider removing the ability to identify the properties of a magic item during a short rest, and require the identify spell, experimentation, or both to reveal what a magic item does.

One additional method you can utilize is to have the object in question, specifically any artifacts you want to remain mysterious, have Nystul's Magic Aura cast on them, which would allow you to make a magical object seem nonmagical, or make it appear to be from a different school of magic.

Although it wouldn't prevent the Identify spell from determining the secrets of the item RAW, the text of the spell does state that "You place an illusion on a creature or an object you touch so that divination spells reveal false information about it." You can always houserule this to imply that the information revealed is incorrect, potentially confusing players when they try to use it. And, the spell can be made permanent, so bonus there.

For the potions: they're quantum-magically-entangled wild potions. At the time of creation, the potion is in a superposition of all possible effects. Once you drink one potion, you collapse the possibilities into one for both potions. Using the identify spell will give you confusing results - the magic equivalent of looking at an optical illusion, where everything seems to be constantly changing.

Use the 'Awakening Items' concept to turn Identify into a Mystery creator rather than a Mystery solver.

Identify will only tell you what an object does 'right now'. If it has the capacity to become empowered later, then its new properties will not be revealed until that point. Identify might, at your discretion' reveal that it has dormant properties, buried too deeply to be identified. This uses Identify as a way to generate mystery rather than ruin it.

This is used to great effect on Critical Role Campaign one with the Vestiges of Divergence. In it, the DM, Matt Mercer, uses three tiers for these powerful artifacts. There's a Standard level, an Awakened level, and an Exalted level, with additional and enhanced properties that are revealed only when the artifact is empowered, either by some milestone being reached, some greater power charging it up, or in the case of items found later in the campaign, already active, due to the high level of the characters. (I'm avoiding specifics so as not to give spoilers to those who may be working through the show)

Enforce the spell component requirement of a pearl worth at least 100 gp and an owl feather

The players will then have to choose carefully which items they identify.