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Some context: I'm a newbie DM running a campaign which requires the party to do a lot of scouting and infiltration missions. Especially in buildings. It's a city campaign happening in Waterdeep so I expected the party to do a lot of lock picking, disguise self and other similar standard "stealthy and rogueish" stuff.

Here's what's happened: the party came up with an idea: why don't we just cast See Invisibility on one of the casters, then Invisibility and then Gaseous Form and then Nondetection, all on the party's monk. So, instead of party's ranger doing some smart infiltration stuff in the Lord's mansion at midnight, we got a monk in a form of gas cloud, easily entering people's homes through the chimneys like Santa Claus 😆🎅, then going from a locked room to another locked room through the keyholes, looking at everything, listening to what everyone is talking about in every room, all in broad daylight...

And all that while not being detected at all. Because how could you reasonably detect such a thing?

  1. Is it visible? Definitely not. Even if there are some "buffed guards" with See Invisibility, it wouldn't work because of the Nondetection spell.
  2. Does the cloud smell? No, the spell doesn't say that.
  3. Can people hear the gas "moving"? I assume also no, because how in the world could you "hear" a slowly moving fog?
  4. Could the infiltrating person in a gas form throw badly on the Stealth check and accidentally tip that vase on the table? No, because the gas "can't manipulate objects".
  5. Maybe... A creature with blindsight could detect this sort of gas with a high enough perception check. But as I've said, this is a city campaign, so all guards in a mansion having blindsight may not sound reasonable...

Am I missing something here? Or should I just give up on trying to make all of the scouting / infiltration encounter challenging and admit that my party came with with an ultimate scouting / infiltration combo that just makes these encounters so easy? Based on your DM-ing experience, what you'd suggest to do to make the use of this combo more challenging?

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    \$\begingroup\$ Note that you need two casters to apply Invisibility and Gaseous form here at the same time, as both of them are concentration. (I think your party hase multiple casters, so it is possible). \$\endgroup\$ Jan 22, 2023 at 16:29
  • \$\begingroup\$ Related: How to Counteract Special Ops that use Gaseous Form \$\endgroup\$ Jan 22, 2023 at 16:49
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    \$\begingroup\$ One of the flaws in D&D's setup is that skill based infiltration - sold as a core competency of certain classes - is rapidly outclassed by magical means. \$\endgroup\$ Jan 23, 2023 at 12:03

8 Answers 8

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Yes, this is a good combo, but you don't need to nerf it

Invisible is not hidden

Just because you are invisible and cannot be detected by divination spells, does not mean you are hidden. You still need to succeed with a Dexterity (Stealth) check for that.

While I agree that narratively it is hard to see why that would be required when you are a cloud of gas, mechanically, by the game's rules there is no benefit to stealth from being in gaseous form. Gaseous Form does not make you hidden automatically, it does not make you noiseless. It does not even automatically confer advantage to Stealth checks. Spells do what they say they do, and Gaseous Form does not say it grants benefits to your Dexterity (Stealth) check, so unless you as the DM rule it does via circumstance, it does not.

Magical traps still work

For example, a glyph of warding does not state it requires a visible target. If you have a glyph triggering when a creature passes without giving a certain sign, wearing house colors or whatever other condition, then the gaseous monk will be blasted by it. Glyph is an abjuration, not a divination effect. If it is bad enough, it can leave the monk lying there alone and unconscious or dying, with nobody around to help them.

Successful scouting is fine

Now, it is quite possible that your Monk has a high Dexterity and is proficient in Stealth, and therefore will be able to successfully sneak.

That is not the end of the world though. There are many other ways the characters can listen in on conversations and see what is going on in various rooms. For example, they could just cast clairvoyance or clairaudience on the room of interest. They could make a small familiar like a spider or bat invisible, and let them in, looking through its senses. They can (with a range limitation of 60 feet) combine this with an unseen servant to open doors. These tactics have similar effects and are even cheaper, resource wise. We have done all of these things in our playthrough of the adventure. And learned a lot of secret stuff. At higher levels you can do the same thing with a single spell, Etherealness, as discussed here.

And that is fine. The PCs may learn some secret for all their efforts --great! Now they will have to figure out what they can do with it.

You can use a clock

These tactics cost a lot of resources (at least one level 2 and 2 level 3 spells in your example), and time. Invisibility and Gaseous Form only have a duration of 1 hour. You need to be lucky to observe a critical conversation between NPCs. You cannot keep this up 24/7, at least not at the levels the adventure is written for.

As I mentioned in my other answer about abusing the Divination spell, you create problems for yourself, if you do not put any clock on that adventure. When the PCs can take however much time they like to exploit their daily spellcasting over the course of many days and nothing happens in the meantime, then they eventually will be able to scout out everything without risk. But this is a dynamic city adventure with many factions. The opponents do not just sit around to wait for the PCs to scout them out. Things are moving, so keep them moving.

For example, in our playthrough, all the different factions were hunting after the Stone of Golorr, and passing it on and messaging one another, and, once they realized we are involved, also sending spies after us. If we did nothing, eventually one of them would have solved the puzzle and reached the vault of dragons before us, and ended the adventure.

So while we did invisibly spy on -- for example -- Manshoon's hideout, the Gralhund Villa and the Cassalanther Estate, and found some nasty secrets, we did not have the time to spend several days on this. We had to do it quick and with whatever resources were still at hand, because we had to follow through on any kind of leads we discovered, right then. By the next day, it might already have been too late to use what we learned, and the dynamic situation might already have evolved again. And if that led to a battle encounter, all the spells we spent on spying were not available any more to help in the fight.

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  • \$\begingroup\$ Thanks for the suggestions. Just to clarify, are you suggesting to explain to the party that while the fog IS invisible and it can't INTERACT with objects, someone can still "hear the air being moved around" or "feel some gas particles touching their body"? Would that trigger the Perception/Stealth checks? \$\endgroup\$
    – Lisa
    Jan 22, 2023 at 15:01
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    \$\begingroup\$ @Lisa, That is what the rules say. I am actually not suggesting to do that; I wouldn't as the DM, as I think using the rules in such a mechanical way is lame, but it would be legitimate (and our DM often does it to curb the power of our parties invisible ranger). I feel other methods like having a clock, or having traps that can trigger even on invisible creatures are more narratively accepteble to the players, and more fun. \$\endgroup\$ Jan 22, 2023 at 15:05
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    \$\begingroup\$ The clock idea is a great one - but remember this is a newbie DM. Explaining what that means and how to use one in 5e would be very helpful here. If you've got an example of one you've used in a similar situation that would help clarify (and bring this into good subjective.) \$\endgroup\$
    – NotArch
    Jan 22, 2023 at 15:08
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    \$\begingroup\$ Short of someone using See Invisibility, I would have a very hard time DMing even making a stealth check, let alone what failing one would look like. Normally, an invisible character can be revealed through sound, smell, footprints, etc. How would someone detect an invisible, intangible creature that can't make noises or interact with physical objects? Sure, there's an alarm going off. But even if the PC went right through them, all they'd be able to feel is a bit of wind. And if the PC is still? Waving your hand through mist just feels like waving your hand through air. \$\endgroup\$
    – Aos Sidhe
    Jan 23, 2023 at 16:56
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    \$\begingroup\$ @PeterCordes My math was off in the first place, anyway. Base (10'/rd): 100'/min. 100 x 60 = 6000 ft. x 2 for dash = 12,000 ft. Allowing UM (at 6th): add 15'/rd. = 30,000'. Allowing SotW (at 6th): add 6 x 25' = 30,150' of total maximum movement. So, maybe not as effective a mitigation factor as I first considered, unless you also factor in some time that max speed is not allowed, like trying to investigate to find a crack to slip through or stopping to listen in on a convo or things like that. \$\endgroup\$
    – Purplemur
    Jan 26, 2023 at 14:15
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In a world with no locks, anyone can get in

But your world does have locks. Specifically, Alarm.

This handy little ritual spell (doesn't even cost a slot!) isn't divination, so nondetection doesn't stop it. It doesn't matter that the creature is gaseous or invisible either, it'll still trigger the alarm because it remains a creature. Gaseous Form doesn't say you're no longer a creature, so you are simply creature of mist that can still be attacked, etc. Since Alarm covers creature sizes from tiny (the smallest) or larger, it'll trigger. And a non-hidden creature is immediately known is in there and anything hidden could be found by someone with a high enough perception (which they likely would have for responding to these alarms.)

PCs buff up, enter building, alarm notifies...someone(s)...continue on telling your shared your story :)

People who need Alarm probably also have more than going on to protect them. Whether it's guards, traps, magic mouth, or really anything else a clever mind can dream of can make things interesting for someone trying to infilrate.

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    \$\begingroup\$ OK, good point. But would the Alarm going off really stop the infiltration / scouting? Most likely the guards and maybe even the mansion owner would check the surroundings, but they would find no one... Then they'd probably decide that it was just some random spider or a flee... And then go away to mind their own business (although this would probably stop them from discussing their secret evil plans this day) \$\endgroup\$
    – Lisa
    Jan 22, 2023 at 14:37
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    \$\begingroup\$ that's actually the part I don't understand. How exactly a monk in a Gaseous Form + Invisibility + Nondetection could be caught? You've said that "your world does have locks. Specifically, Alarm." But the Alarm spell is not a LOCK. It's an alert. Raise suspicion? Yes. But it doesn't help to "catch" the monk in any way. He just triggers the alarm casually and then casually gets to another room... How would you "catch" it? Throw "dispell magic" at a random 5ft squares of the room? \$\endgroup\$
    – Lisa
    Jan 22, 2023 at 14:44
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    \$\begingroup\$ The PC is visible for anyone that can see invisible creatures. Invisible thiefs are a thing, so whomever responds to the alarm is likely prepared for what might have triggered it. \$\endgroup\$
    – NotArch
    Jan 22, 2023 at 14:46
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    \$\begingroup\$ They aren’t hidden, though. They can still try to hide, but a sufficiently high perception will know someone is in there and then they can do other things. Once this type of infiltrator is known, then people start being ready for it. You have to assume that someone of sufficient wealth or paranoia is ready. \$\endgroup\$
    – NotArch
    Jan 22, 2023 at 14:51
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    \$\begingroup\$ @Eddymage it was a play on words, not a literal statement \$\endgroup\$
    – NotArch
    Jan 23, 2023 at 13:39
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Nondetection does not thwart See Invisibility

You need to read your spell descriptions carefully:

The target can't be targeted by any divination magic or perceived through magical scrying sensors.

See Invisibility's target is the spellcaster, not the invisible thing being seen. It also does not create a magical scrying sensor; it augments the spellcaster's normal visual sensors.

Alarm and other precautions

Other answers have addressed that particular spell, but there are loads of others. For example, particularly sensitive conversations may take place using Telepathy.

Invisible is not Hidden

See What advantages does hiding have? and Can invisibility be countered by "ramming" the whole room with lots of conjured animals?

Don't nerf your players

You gave them a problem and expected they would solve it using method A. Instead they solved it using method B.

Why is this even a problem?

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    \$\begingroup\$ While I'm not sure you are wrong on Nondetection vs See Invisibility, there are mulitple in-depth Q&As on this site that come to the conclusion it does trump See Invisibility, like this one \$\endgroup\$ Jan 23, 2023 at 7:33
  • \$\begingroup\$ @GroodytheHobgoblin I'm going to guess that that related post didn't help much, considering the poster of that question is the same as this one! \$\endgroup\$
    – Michael W.
    Jan 23, 2023 at 23:29
  • \$\begingroup\$ @MichaelW.: Or that it did clarify that RAW Nondetection beats divination spells like True Seeing and See Invisible (and Crawford has agreed with that interpretation of the rules wording). And that's why they see this combo as fairly powerful and resistant to being caught, hence this question. (For the record, Alarm is an abjuration spell, not divination.) \$\endgroup\$ Jan 25, 2023 at 19:25
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Certain important buildings or rooms may be deliberately constructed to block 'gaseous form' spying.

The easiest option I can think of would be a curtain of water falling from the ceiling and running into a drain on the floor.

Alternatively, for something more simple but more annoying for the occupants, have a corridor that slopes down for a bit and then slopes back up, and keep a 10 ft wide section at the bottom completely flooded up to the ceiling.

Just don't overuse this or your players will get frustrated. Not every NPC will go to such lengths to hide their base from magical infiltration.

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Is it visible? Definitely not. Even if there are some "buffed guards" with See Invisibility, it wouldn't work because of the Nondetection spell.

Accurate - the See Invisibility vs Nondetection is something that can be debated, but it does seem to follow the spirit of the spell for Nondetection to win.

Does the cloud smell? No, the spell doesn't say that.

You know, there's a hidden danger in all the claims that spells only do what they say they do and nothing else.

Because not only does the spell not say that the cloud smells, but it also doesn't say that the cloud can see.
We simply assume it can because it's a creature, and creatures can generally see....
But creatures also generally smell and make noise when moving....

This DM would say that yes, the cloud smells as much as the person did, and the cloud also isn't automatically perfectly silent. Meaning a stealth check should be rolled and we're back into the realm of a powerful-but-not-unbeatable tactic - which seems to be the appropriate power level for something that involves multiple synergistic spell expenditures.

Can people hear the gas "moving"? I assume also no, because how in the world could you "hear" a slowly moving fog?

How in the world could a person become a slowly moving fog? Oh right, magic does things that defy regular explanation.
Let's remember that if it was intended to make a person exceedingly stealthy, it'd probably say something about that.

Could the infiltrating person in a gas form throw badly on the Stealth check and accidentally tip that vase on the table? No, because the gas "can't manipulate objects".

Good thing there's a bunch of other ways to fail a stealth check besides "physically manipulate an object".
A spiderweb unnaturally bows, indicating the presence of a not-very-solid creature to a keen observer. Or some of your gas reaches a person's ear, causing it to pop without an elevation change, again noticeable to a keen observer. Or a wind chime rustles to a breeze that shouldn't be there. Or or or....

Maybe... A creature with blindsight could detect this sort of gas with a high enough perception check. But as I've said, this is a city campaign, so all guards in a mansion having blindsight may not sound reasonable...

Blindsight wouldn't even need a perception check. If there's line of sight from blindsight to your gasmonk, they'll be seen immediately. But all guards in a mansion having blindsight is an obvious overstep by the DM unless there are serious extenuating circumstances.

Like don't get me wrong, this is a powerful tactic you've got here, and it should be powerful. They're spending 8 spell levels and 25 gold to do this. It neatly sidesteps the whole door problem that ordinary invisibility has, and it's a fantastic mechanism for reconnaissance. This DM would probably have it confer advantage on stealth and disadvantage on perception to locate you, and until you're either caught or publicly use information you shouldn't have, I wouldn't even have anyone prepare countermeasures like expertised perception guards, scent hounds, or just a blindsight fighter.

But you're actively making it far more powerful than you should be - it's pretty clear that it shouldn't make stealth checks irrelevant, just make them harder.

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Make the recon interesting

  • You can and should give them loads of interesting and relevant stuff about what's going on in the area. They will overhear many conversations and see some places they shouldn't. They've spent a fair amount of time and spell slots for this - but that doesn't mean you have to give them everything.
  • Time limits are one way to do this, as others have mentioned. Spend some time exploring a museum or other unfamiliar public building with a stopwatch some time... An hour can go by awfully quick.
  • To stretch the time & resources spent on the powerful recon, the PCs will want to spend multiple shifts searching, to get as close to the target as possible before spellcasting, and to supplement the undetectable-cloud monk's recon with some cheaper & faster alternatives: Can they get a map of the Baron's estate before going in? Do they know what the important NPCs look and sound like? Do they know the schedule of the warlord they're targetting?
  • Important documents can and should be kept in locked cabinets. Even if the gaseous monk can get in, that doesn't mean he has a light source to read the things or can get the docs out.
  • Important conversations can be done behind secret doors, although this shouldn't be overused. Various warding spells are available that could keep out unwanted trespass, or alert the people there. Guards & Wards, Alarm, Glyphs, Antimagic Shell, and so on. If the invisigas monk wants to find things that have been deliberately hidden or disguised, that should require extra time and search checks.
  • What are the rest of the PCs doing while this is going on and might they be spotted? If they're coming right up to the outer wall of the castle once a day, casting a series of spells, and then leaving just over an hour later, someone's going to eventually notice them. Separate the monk from the other PCs and play through a scene for each of them, perhaps swapping between scenes every now and then - you can give each group clues that something is happening with the other, but what happens when the monk returns and finds his friends aren't at the rendezvous? What happens when the rendezvous spot is being very obviously watched by the town guards with the monk's time limit running out?
  • Definitely have the monk make stealth checks occasionally - knocking papers off a desk while trying to read them, or brushing up against people will generate some suspicion... and if the player doesn't know whether the check succeeded or failed, you can describe ambiguous outcomes to make the player sweat. You should have a lower target number for those checks than normal, but even a 1 comes up occasionally.... Similarly, have the occasional guard make a perception check - or seem to be staring directly at the monk, or otherwise take an ambiguous action. Maybe he's just in a bad mood and happens to be scowling in the cloud's general direction... and maybe he's suspicious of the dust swirling around the bookcase. The more frequently the PCs use this trick, the more likely it is the NPCs will catch on to it.
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The players have come up with a powerful and effective plan for scouting, and that should be rewarded, but you don't want that to steal the tension and excitement from the game either. The 1 hour time limit on the two spells gives you a fun way to add back the tension to the game as the GM while still rewarding the players for their creativity. You can describe how they spend the first 10 minutes drifting slowly past guards, through keyholes, scouting out the house and then finding the Lord's study, where they can float up to the ceiling and listen in.

Describe how the Lord comes in shortly after with an interesting guest (whatever fits the scenario) and then how over the next 20 minutes they enjoy a good snifter of brandy, talk about the politics of the day and how their families are doing, before finally getting to the juicy part with the information they're looking for just as time on the spell is running out. Does the Monk want to slip through the keyhole into the closet and try and hide there and sneak out later? Do they wait until the last possible minute before trying to float up the chimney before they rematerialize or reappear and hope they aren't spotted by guards getting off/away from the building?

Does the monk even know when the spell will end? Without some kind of timekeeping device they probably don't, and while maybe the caster would know how much time is left they don't have a way to get a message to the monk.

Similarly, once the monk is inside and not able to be contacted by the other characters you could have something interesting happen outside where the visible party members can see it - without a way to let the monk know what's happening they'll need to decide if they risk sneaking over to eavesdrop themselves, or miss out. If you do this I recommend having something the monk will find so their role isn't entirely useless - it also helps avoid someone metagaming a justification to go back outside if they're busy with what they came for.

These reward your players for coming up with a creative solution to the problems their characters are facing, while still adding tension and interest to the encounter where the players will have to take risks to succeed.

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Some additional points

In addition to the other fine answers here, I would like to add a few additional points:

Lean into it

As others have suggested, that your PCs have come up with this is fantastic. Lean into it. But that doesn't mean there aren't complications.

This uses up resources

Specifically spell slots. also, non-detection uses up 25gp of diamond dust, each casting. Even for lower-level parties, the resource drain is not significant, but that doesn't mean it's not there.

Time

As others have mentioned, there's a clock on this. The trick only lasts an hour, and how does the monk know how much time has elapsed? Even the Keen Mind feat only allows the beneficiary to "know the number of hours left before the next sunrise or sunset".

Interpretation of the spell

It's not unreasonable for the DM to say, "I think what you're doing is great. But the spell is woefully inadequate in describing life as a blob of vapor. For instance, it doesn't say you can see or hear or smell or feel. Here's my ruling. You can see and hear, but your perception is normal. You can't smell or feel or taste. We may well need to make some additional rulings as we go along."

Object interaction

Just because the spell says you can't manipulate objects, that doesn't mean that you are incorporeal. Our reasonable DM might also say, "While you can't manipulate objects, that does not mean that your passage doesn't possibly cause dust to swirl, or plant fronds to move."

Being perceived

Yes, the gaseous form is invisible. That does not mean it is indetectible. In addition to possibly causing the dust to swirl, the gaseous form is "a misty cloud". It's not unreasonable that the misty cloud feels different than normal air. Cold and clammy maybe. Maybe a servant claims they felt a ghost. Or maybe the family dog starts barking at a potted plant.

Countermeasures

Several answers mentioned the alarm spell. It is not unreasonable that a high-level mage, confronted with an alarm spell that went off, or dogs barking at nothing, or servants talking about ghosts, might not become suspicious, and start using things like detect magic, see invisibility, or true sight.

What's going on with the support team

Where's the party at? They have to be close, otherwise the monk uses up precious time in traveling. Maybe they get roused by guards, or who knows what. And wouldn't the plot get deliciously thick if they got involved in a confrontation and lost concentration on either invisibility or gaseous form, or both.

Conclusion

I think this is a great opportunity for some fantastic play. The players have come up with a great idea, but it is not at all an automatic win, and it has consequences and complications. Have fun!

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