My players think they found a loophole that gives them infinite poison and XP. How can I add the proper challenges to slow them down?

Question

(I'm the game master.) The party (lvl. 4) reached a Kobold village. It has several dozen Kobolds, many of them pretty powerful. The players aren't supposed to kill everyone in it, but to solve the issue through diplomacy. In total the village is worth about 20,000 XP.

But you know how sociopathic players can get. The wizard suggested she could continuously cast Poison Spray into the water supply and have them all take damage, at least killing many simple Kobolds. She argued that because Poison Spray is a cantrip, she basically has an endless supply of poison. Reading the description of Poison Spray, I highlighted that the spell creates a gas, not a liquid, and because chemistry it wouldn't mix with the water directly. My players started throwing ideas:

• An Alchemist Artificer could distill the gas into a liquid.
• Filling up a large container with poison spray, sealing it, then having an Alchemist put it inside of balloons.
• If they do have a vast supply of liquid poison, they can contaminate the water supply of a large city and reap the XP of whoever lives in it.

I don't wanna just say no, but I also don't want them to have an unlimited supply of poison and XP without the proper difficulty... Any ideas on how to tackle this?

Answer 1

No. Just, No.

• Mechanically, the duration of the spell is instantaneous. Nothing in the description of the spell even hints that the poison lingers after the instantaneous duration. So it doesn't.

• Mechanically, the spell affects one creature. One. Nothing in the description of the spell even hints that it could affect more than one creature, nor an object like a water supply. So it doesn't.

• Descriptively, the spell creates a puff of gas, not a liquid, as you note. Trying to apply real world intuitions to spells is always fraught, and the principle of "Spells Do Only What They Say" tells us generally not to do that. But if you were, there is no real reason to expect a magic toxic gas to be water soluble.

• Descriptively (and again, we're not really supposed to do this) it is a lot harder than most people think to chemically poison a municipal water supply. In this case, trying to effectively poison dozens of kobolds would imply at a minimum casting that cantrip dozens of times. But more realistically ("realistically") the water dilutes the poison to some unknown degree and they might need to cast it thousands of times to get any meaningful health effects.

• Game balance wise, this is just nuts. A cantrip doesn't let you casually mass murder an entire city. Cloudkill can't do that. I wouldn't even let a full Wish spell accomplish that.

I would just say no, and move on.

But every table has different dynamics; the first two mechanical objections are more than sufficient if you need to provide rules-as-written reasons.

Answer 2

This "loophole" does not work

There is a principle that spells only do what they say they do. Poison Spray targets a creature:

You extend your hand toward a creature you can see within range and project a puff of noxious gas from your palm. The creature must succeed on a Constitution saving throw or take 1d12 poison damage.

That is, you cannot just cast poison spray into water or a container. You need a creature to cast it at. Moreover, nothing in the spell says that the poison stays around after the creature made, or did not make its save, so it won't. It just deals damage if the save is not made, there is nothing to harvest.

How you justify this narratively is up to you as the DM. Maybe the poison is transient and vanishes again after it was created. It also is your right and duty as the DM to shut down exploits (when they actually work according to the rules) and curb abuses that would turn the game into a non-challenging cakewalk, by overruling the rules if need be. DMG p. 4:

The D&D rules help you and the other players have a good time, but the rules aren't in charge. You're the DM, and you are in charge of the game.

But in this case, there is no need for that, as the games rules and back you up here: this exploit does not work.

Answer 3

“No, this is not how it works”

Just shut it down. You say that you don’t want to say “no” to them, I as a DM generally agree with the sentiment and only very, VERY rarely shut down players’ ideas outright. Having said that, this is a situation to do just that. Your players seem to be treating your campaign like a video game and trying to apply video game rules like XP farming to it.

The very idea of

contaminate the water supply of a large city and reap the XP of whoever lives in it

is a video game logic and goes against the spirit of what tabletop roleplaying is about.

You don’t even necessarily need a mechanical RAW explanation for why it doesn’t work (although other answers do a great job of providing it). There is no “XP farming” in D&D, they can play Skyrim or World of Warcraft for that and don’t need you to DM for them.

Poisoning a city water supply doesn’t get them a bunch of XP for all the commoners they killed, it gets their characters declared as wanted criminals and eventually caught, thrown in jail and suitably punished for genocide.

Make sure they are aware that if they do manage to find a way to poison the well (and Poison Spray does not work for this but assume they get hold of a suitably strong poison that could) in the kobold village, you will advance the plot accordingly, but you will NOT award them any XP for kobolds that died of the poison, that’s just not how D&D works.

Answer 4

I disagree with the other answers. "Yes, and..." is the way things work in improvised storytelling. Don't stop them from trying, just enact consequences for their choices.

The dose makes the poison. What might be fatal to one person in a concentrated dose, may be beneficial to a large population when diluted.

Let them try to poison the water supply, and have them fail at it.

The rules can help guide you as to the specifics:

You attempt to dissolve the poison into a glass of water and...

• ...accidentally poison an ally because they're the only creature you can see

• ...accidentally clean the water supply because you've introduced a dilute chlorine solution.

• ...created a new magic cocktail due to how the poison reacts with the water.

• ...get caught when attempting to dump the glass of water into the water supply due to an overzealous Kobold Karen in the Neighborhood Watch program.

Failing doesn't even have to be entirely negative. If you want to railroad push them toward a diplomatic solution, you can have them get caught doing a "good" thing, by having a Kobold discover that they cleaned the water supply and make them local heroes so that the whole village knows who they are and won't leave them alone. This can have additional consequences of making enemies of the local political leader, who then can't outright attack them.

Answer 5

Your players being creative is not a bad thing

How D&D works is, that you present your players with an adventure (location), in this case that kobold community, and your players then have the agency to come up with a solution on how to tackle that adventure and you as DM then adjudicate if and how those solutions work. In your case your players have come up with the solution of poisoning the water supply. To which you say:

I don't wanna just say no, but I also don't want them to have an unlimited supply of poison and XP without the proper difficulty

I get you. You don't wanna just block your players, as they seem to have fun coming up with this creative solution, but of course you also don't want them to just skip all the work your did with your adventure and especially don't want them to get XP for effectively doing nothing, as that would simply break the game. As others have pointed out (and even myself in my first comment under your question) you could very well just say: 'No, you can't poison the water supply'. And you actually did that, when you said:

I highlighted that the spell creates a gas, not a liquid, and because chemistry it wouldn't mix with the water directly.

To which your players responded with coming up with more intricate solutions on how to make their plan work, like further processing the poison by an alchemist. I want to tell you, that this really isn't a problem. Your players are creatively engaging with your world and having fun, and that by itself is great. The problem here is just that your players think that their ideas they will get them free XP, which again just isn't how D&D Works. XP and other rewards are awarded for completing encounters, adventures and quests.

So what do to? There is a simple solution, that will satisfy both you and your players. They will be able to test out their ideas and you will be able to give them rewards for doing them, while they overcome challenges:

Turn poisoning the water into an adventure

As stated earlier your job as DM is to adjudicate the players ideas and this is exactly what you are going to do here. For this you need to determine a few basic things about the adventure. First off, how can they poison the water supply, here are some examples:

1. They have to spend a full 8 hour day at the river upstream of the kobolds continuously using the cantrip on the water and perhaps succeed on an ability check, maybe medicine, every 1-2 hours of casting.
2. They can prepare the poison in town with one of their ideas, e.g. having an alchemist distill it. They then just only have to throw the result into the river and perhaps wait 1 hour until it arrives at the kobold lair.

You also need to determine what successfully poisoning the water will achieve, like:

• The Kobolds are dead (this is what your players want, but not want you want, that's okay, there are other options)
• The Kobolds lose half of their hit points to poison damage
• The Kobolds have the poisoned condition
• One of the above, but the kobolds get to make a Con Save to avoid the effect
• One of the above but it only effects a limited group of kobolds, maybe half of them or you determine it randomly e.g. by rolling 2D12+3

Now you have the basis for the adventure and can go ahead and fill it with encounters in the form of possible complications, or how you describe it in your question "the proper challenges to slow them down".

If the players spend 8 hours at the river, these things might happen:

• Local wildlife, such as a Griffon is disturbed by the players presence and attacks
• A merchant or other friendly NPC passes by and observes the players casting poison into the river. If they don't successfully deter him, he will tell the local militia or nature loving forest rangers about what they are doing, and they will come to intervene
• A flood pours down the river and the caster and all players near them must succeed on STR saves or be swept away
• Kobold scouts patrol the area and see what the players are doing. If the players don't spot and stop them, they will tell the kobold community who will then come and attack them.
• The hands of the wizard start to hurt from casting all day. The other players have to find a way to ease their pain, so they can continue casting, e.g. by collecting an herb to make a salve, which they can find with Survival rolls and they need to decide who goes herb collecting and who stays with the wizard.

If the players develop their poison in the city, these things might happen:

• The alchemist needs a certain ingredient such as the eye of a Basilisk or vines from a Shambling Mound to make what they want, which the players must then seek out and defeat
• The alchemist needs special equipment, that was stolen from a rival or can be bought from a wealthy merchant in a nearby town, which he tasks the players with doing
• The townsfolk find out that the players are distilling large amounts of poison, making them very suspicious of them. If they can't talk their way out of it they might be banned from town or people will refuse business with them.
• Kobold spies are in the town and might get a chance to find out the players plan beforehand and either try to sabotage their efforts in town, warn their people to not drink the water and/or await them with a small army, when they try to pour their finished poison into the river.
• Once they have their poison they now need a way to transport it from town all the way to the river. While transporting it, they might have to defend their payload from bandits or other dangers.

Be more flexible in your planning

As you can see, it is absolutely possible that both you and your players get what they want in this situaton. They get to successfully carry out their idea, while you can ensure that they also play the game and earn their XP reward. For this to work you have to be open to your player's ideas and use your DM tools to work them into the game in a way that is fun for both of you. One lesson that will help you achieve this is to not determine the solution for the adventure upfront, which I think you did, when you said:

The players aren't supposed to kill everyone in it, but to solve the issue through diplomacy.

I don't know if this is something you as the DM just intended for them to do or if this is something their quest giver explicitly asked of them. If the players are just passing through the area and the kobold community is a road block for them, they should be able to freely choose what to do: diplomacy, attacking directly, just going a different route or poison the water to weaken the kobolds, so they can proceed.

If the players were asked to deal with the kobolds by an NPC, they would have a specific goal, such as stopping the kobolds from attacking a village or getting back an item that the kobolds stole. They get a reward if they achieve that goal, but their choice of how to achieve it is ultimately their own. Though maybe the NPC would tell them some stipulations like 'please don't hurt any kobolds, dissuade them with diplomany'. They then still would be free to choose their method of achieving the goal, including breaking the rules the NPC set, but the NPC might not be happen if they report back to him, that they achieved their quest by poisoning all the kobolds. The quest giver might then decide to not give them their reward or only give them part of it, because they didn't do it the way that they had agreed to.

What kind of game do you want to play?

At the end of the day, the most important thing is that you all have fun. If creating adventures from you player's ideas sounds fun to you, go ahead and do it, it sounds like they would enjoy it. If you would rather have a less freeform experience, so you don't have to prep or improvise as much as a DM that is fine too. You just need to talk with your players about this out of game and agree how you want to handle your game moving forward.

Answer 6

The poison described in the spell is fast-acting and unsubtle. It will be obvious to the kobolds that the water has been poisoned when they drink it. They will send a party upstream to investigate--perhaps a laborer, in case it's a dead animal, but most likely some warriors as well. Or if it's a well, they'll condemn the well and start searching for outsiders.

If your players release the poison slowly and subtly, maybe all the kobolds drink the water before realizing where the damage comes from and are down 1d6 hp for the combat encounter. If your players build a large supply of poison and release it all at once, maybe two random kobolds from the village cark it before the rest catch on.

Maybe put a gold price on this--alchemical reagents for the alchemist to transmute the poison cloud into liquid, and special barrels to store the poison while they build up a supply. I don't know how much would be appropriate for your party.

Lastly, if kobolds die outside of combat, you are not obligated to give the party combat experience for their deaths.

Answer 7

Poisoning a community is not so easy

Even if you could create durable poison for free with this spell, which you cannot, and even if you could harvest sufficent amounts to kill everyone in a village or city, and even if you could distill it or precipitate it from gas into liquid form, and even if that liquid was water-soluble, and even if the poison was tasteless and had no bad smell (that are already five big ifs, each of which would have stopped even trying), poisoning a community with this by spiking the water supply is not going to work.

Not everybody is going to drink a large amount of this water at the same time. Someone is going to drink from it first, and at that point will experience pain or die, and with that everyone witnessing this will know something is not right. It will not take long to figure out it is the water. At best, you can kill a few individuals before the alarm is raised and the water avoided or subjected to some purifying magic.

I still would strongly advise against allowing the players to collect unlimited amounts of poison this way. The poison is pretty harmful -- one dose can deal 1d12 damage upon exposure. The poison in the spell makes no requirement for the target to need to be able to breathe to damage it, so mere exposure is suffient to trigger the save. What do you do if the characters collect bottles with hundreds of doses worth of the stuff, and use them as thrown granades? This will be an autokill against any opponent that is not immune to poison. Your campaign would have much bigger problems than just the poison a city ploy.

Answer 8

You try that? Really?

Ask the players if they are sure they want to do that. Ask them twice. Yes, I make it deliberate, because ohhh boy, they want me to unpack the hard bandages...

First, the rules don't work that way.

Spells do exactly what is on the tin. Poison cloud creates, for the duration of an instant, a poisonous gas that harms a single creature and nothing more. The moment the spell is over, nothing lingers on.

Tell them, and if they argue against it... they're violating the Rule that the GM is always right. Undermining your authority after a final ruling is not ok, people get kicked from the table for that shit.

Second, they want to spend hours at the well even knowing their plan is not going to work?

The Kobolds aren't dumb, blind, and stupid. They move through town, and wells or other water supplies are natural gathering spots. Standing hours at the well doing stuff with it is highly suspicious. They will notice.

Third: Player/PC choices have consequences

Their shenanigans will have consequences - such as being caught red handed and curb-stomped because every Kobold in the village will be there, and they won't pull punches.

Answer 9

You don't get XP from mass murder

The whole point of xp is learning, not how many things you can kill. Killing a dragon is a hard fight where you're balanced on the razor edge between success and failure which hones your skills.

Poisoning a bunch of kobolds requires no real skill thus you don't improve your skills tipping poison into the water supply.

Their logic leads to things like an XP vending machine where a person drops a gold coin into a slot and a skeleton is animated and then killed giving the XP to the user.

XP comes from challenges, not deaths which is why you can get xp from avoiding fights

Answer 10

You can just say no

You are the DM.

You can say "Spells do what they Say"

And no more. Poison spray lets you do exactly its effect, and nothing else. I find this a boring argument.

Cantrips are work

While you don't have a limit on how many you can cast per day, you also don't have a limit on how many attacks you can make per day.

If you take a sledgehammer and start swinging it at a rock, you will get tired. If you swing it every 2 seconds for an hour, you'll probably fall over dead from exhaustion, and if you don't collapse dead, you'll be crippled for a week.

Swinging a hammer is also at-will. It doesn't mean it isn't a pile of work.

5e "at-will" combat abilities are all things that require similar levels of effort - aka, sprinting-levels of work. Something that will tire you out if you keep doing it at a pace. This is supported in the rules (look at dashing in chase rules) if you don't want to take my word for it.

The Poison Spray Cantrip doesn't make much Poison

Distilling it into a different form will take a fair amount of work. The poison is likely to have a short half-life, as it is being created and maintained by magic, and is designed to be inhaled not mixed with water and drunk.

Converting that into a water-soluable stable poison is a major project, and it might not even succeed. The alchemist will need a lab and materials and experiments.

In the end, what will probably come out of the experiment is a way to use magical energy to form a projectile that sticks around for a bit longer. The water-dissolved poison actually dissolves flesh, becoming more of an acid. Congratulations, you discovered the "Acid Arrow" spell.

Ok, time to iterate again. That only took a year!

Dilution is the Solution to Pollution

Very few poisons are strong enough that you can pour them into a central water repository for a large community and have much effect. Your best bet here is disease, honestly. Poison Spray isn't "save or die", so it isn't a super-strong poison either - even when administered directly to the lungs of the target (a soft spot in biological defenses). The stomach is treated as "outside" the body - creatures are donuts with a digestive hole down the middle - and we break stuff down in the stomach with strong acids before absorbing it selectively through our intestinal walls.

A poison that crosses the blood-air boundary in the lungs has a much easier problem to solve than one crossing the acid of the stomach followed by the intestinal membrane. So even if it can cross, you'll need a lot more of it.

The water supply is enough for all 10,000 Kobolds and their crops and animals, call it 100,000 Kobold-equivalent-uses. If poisoning via the stomach is 10x harder than through the lungs, then you need enough poison to kill 1,000,000 Kobolds. Lets assume the water source has some mobility, and that it refreshes over a period of a week.

Killing a Kobold with poison cloud would take 1-3 castings; so call it 2.

The loss from the artificer turning the poison gas into a liquid soluable form might be 5 fold.

So you need 10 million casts of poison spray within a week, or 100 wizards chain-casting poison spray into the alchemical factory run by the alchemist, manned by a horde of apprentices (I suppose Kobolds?), in turn pouring the resulting poison into the water supply.

If you force someone to exercise at peak rates for 24 hours a day, they'll probably die within the day. So you'll have to dispose of 700 wizard corpses as well. You can maybe used the Kobolds to ensure the wizards keep up their work.

I hope the Kobolds don't notice.

Answer 11

If you want to go down this road, then go down this road.

So many great answers here give you great advice on how help the PCs execute their plan.

There's opportunity for a conversation with your players here.

I think there's something to the idea that spells can be used in off-book ways. The challenge is then you don't really know how magic works any more. If poison spray can be used to create liquid poison, can ray of frost be used to freeze water? It wouldn't necessarily be bad for it to work that way, it's just that allowing imaginative off-book uses of spells means you'll have to adjudicate that.

So, one thing to discuss with your players is your overall approach to adjudicating spells. When might off-book uses might work, and how you'll adjudicate them.

What's good for the PCs is good for the monsters

A second thing to discuss with your players is that what works for them can work for the monsters, too.

In other words, where are your PCs getting their water from?

If poison spray creates a liquid poison capable of poisoning a water supply, kobolds might not think of that, but an intelligent, organized opponent absolutely would.

Or maybe the kobolds would think of that.

There's precedent for kobolds to be challenging adversaries.

Also, Mordenkainen Presents: Monsters of the Multiverse presents some kobold variants with some unusual attacks which might be of some interest; for instance, the kobold inventor. Or, if you don't want to go that route, you can think of something yourself.

After all, if the PCs get to think up clever things that bend the rules a bit, then the kobolds ought to get to too.