How to counter special-op attacks that use Gaseous Form?

Question

As DM I have always some problems to apply the magic world of D&D onto the real world, which is the base of our common understanding of how things works. Especially, since I have experienced players who just wanted to kill kings to take over their kingdom, purely acting on impulse. I stumbled upon a new situation and I want to learn from your experience:

In my opinion, the spell gaseous form is very powerful for special-ops (by this I refer to rogues and assassins in particular). It allows to fly and, used at nightfall, it should generally provide a good amount of stealthiness as long as not used in very confined and lit spaces. Also, it provides the beneficiary of this spell the ability

to pass small holes, narrow openings, and even mere cracks,

Of course, it only allows slow movement (10 ft) and provides only resistance against common damage types, but the beneficiary can take all his potentially deadly equipment on its hour-long way. There might be a drawback; if the beneficiary is not the caster, the spell cannot be ended by the beneficiary, but that should be just a thing to be considered not a real problem here.

Consider a castle (or the home of a very wealthy merchant) which is well defended by regular means and maybe even some magical backup. An assassin who wants to follow his profession will certainly find a "mere crack" which can be used to enter the castle's building and even rooms (I don't think that any (land-based) door or window is air-tight).

This spell can be used from characters against anyone, as well as from assassins sent against the hero group (see Tyranny of Dragons, 2nd part for example). It could be used between hostile rulers - in fact, this could be nothing out of the ordinary: As gaseous form is a 3rd-level spell it even should be not a problem to find spell caster hirelings.

I don't see a realistic chance to protect a whole castle from such an intruder, do you? Certainly, spells like Alarm can 'protect' sleeping rooms, but they cannot prevent an attack (but, maybe a deadly success, depending in the fanaticism of the killer).
Do you consider a guard standing next to a bed all night as realistic?
Sure, while in gaseous form the intruder cannot interact with any object, but if that would be an issue I would send a special-ops team with potions of gaseous forms: whenever needed, a team member would end this effect, the remaining team proceeds.

Answer 1

Glyphs of Warding and Symbols on doors and windows, possibly paired with Alarms would probably be the go-to defense against such intruders.

Such spells could also be placed on bed frames of a castle's most important occupants or throughout their chambers to cover the case that an assailant managed to slip through cracks in a wall or floor rather than a door's key hole or drafty window.

Glyphs of Warding used to store a spell like Dispel Magic would be perfect for ending the Gaseous intruder's spell. For higher level casters, a Forcecage stored in a glyph would be perfect, as even a gaseous creature cannot escape from an enclosed Forcecage. Because Gaseous Form requires concentration, a Sleet Storm could be another useful spell to store, since it forces a concentration save against the caster's spell DC (rather than being based on the damage), and with only 10 feet of movement and the Sleet Storm creating difficult terrain, the caster would need to make multiple saves before leaving its area. Any spell that incapacitates, such as Hold Person or Hypnotic Pattern, would also break the gaseous interloper's concentration, as well as leave them temporarily unable to move.

Symbol offers several options that incapacitate, such as Pain, Sleep, and Stunning.

Otherwise, you could just try to do enough damage to knock the intruder out by placing multiple Glyphs of Warding (with the Explosive Runes option) or Symbols (with the Death option) in various places of import throughout the building.

If you placed Alarm spells in the same location as each Glyph or Symbol, you would also be aware of where your intruders were, and if the Glyph or Symbol worked as you hoped, you could quickly get to the location to find your intruder incapacitated or at least debilitated when you arrive.

Answer 2

Gaseous form is not invisible

The spell explicitly states that the person is transformed into "a misty cloud". It does not become invisible and observant guards who know what they're looking for might very well notice a cloud moving about in the fortress, I imagine it would look not unlike a wisp of smoke or steam. So if castle is on high alert and the personnel knows to look out for this trick, it might not go as smoothly as your assassins might think.

A couple of Alarms in strategic location will make spotting the gaseous form even easier as once it's tripped the guards will all know the location to concentrate on.

You can also set Glyphs of Warding in places like corridors, near windows, doors or other probable entry points, loaded with a spell of your choice (Dispel Magic to end the transformation early, Hold Person to incapacitate them, some damage dealing spell or Explosive Runes if you just want to hurt them or anything else you can come up with).

You would also probably do your best to reinforce any cracks, openings and such within the walls, the fewer points of entry, the better.

Answer 3

And that is why there is still Guards and Wards

This spell has existed since 1st Edition

You create a ward that protects up to 2,500 square feet of floor space (an area 50 feet square, or one hundred 5-foot squares or twenty-five 10-foot squares). The warded area can be up to 20 feet tall, and shaped as you desire. You can ward several stories of a stronghold by dividing the area among them, as long as you can walk into each contiguous area while you are casting the spell.

Without copying the whole description, you can:

• magically lock down doors
• cast obscuring fog in some areas
• place a couple magic mouths
• place a permanent gust of wind
• place a suggestion spell
• and a few more tricks

The spell can be made permanent if cast every day for a year. There is a password that must be spoken aloud (so no using it while gaseous) to not be affected by any of the wards. And dispel magic can only take out one effect at a time so it would take multiple castings to get through all the defenses.

I could see a situation where the final corridor to the king's chamber is guarded by a gust of wind that pushes someone out a window, and then have the suggestion triggered by the window sill to say, "You should stop using gaseous form."

There is a similar spell called druid grove that works outside.

Toss in things like forbiddence to stop teleporters and you've removed a lot of the possibilities.

Now add in the actual humanoid guards, the alarm spells, glyphs, and symbols, and you've got a fortress.

Add if you're really powerful...

Sleep inside a magnificent mansion

You conjure an extradimensional dwelling in range that lasts for the duration. You choose where its one entrance is located. The entrance shimmers faintly and is 5 feet wide and 10 feet tall. You and any creature you designate when you cast the spell can enter the extradimensional dwelling as long as the portal remains open. You can open or close the portal if you are within 30 feet of it. While closed, the portal is invisible.

Imagine the look on the intruders face when they make it all the way to the king's bed chambers only to find out that they are on a different plane with the only entrance invisible and warded against everyone but his family and personal guards... Talk about the princess being in another castle

Answer 4

To add to all the answers and to stay in a "not poor but not super rich too" scenario, a basic alarm spell that wakes up the merchant/king/noble and alerts guards plus a glyph of warding with the invisibility spell reduce drastically the amount of gold required to protect properly the person you want to protect.

The guy to protect/assassinate is invisible for a duration equal to the gaseous form (and the gaseous form will end before as it would have been cast before the invisibility from the glyph), and now the assassin is screwed as they can't find their target, the alarm is up, the guard will come, forcing them to run.

Instead of putting a glyph everywhere, you put it near the target's bed, and when the alarm rings, the target runs to the glyph, dash action runs again, and waits for guards to come by running every turn from the assassin, and/or hiding. Also work for stealers. When they enter the room, a silent image illusion spell is cast over things that should be hidden, making it look like the room is empty, or full of thing that aren't worth trying to steal.

Answer 5

On top of all the good points made so far...

These Special Operators can't get far

Movement is reduced to 10 Ft per Turn (6 seconds). That is 100 Ft per minute, or 6,000 Ft in the Hour duration. Moreover, that is Vertical and Horizontal. If you are both 100 Ft below and 500 Ft away from an arrow slit you seek to infiltrate, you'll need at least 600 Ft and 6 minutes! And then you have to consider how fortified corridors wind, twist, and will consume movement by design; it's one of the principle purposes of fortifications.

This might be alleviated if your DM let's you Dash in Gaseous Form, but not by much (because you can't Dash every Turn without gaining Exhaustion Levels). Realistically, such a tool for clandestine operations is primarily useful for suicide missions (where they can Gaseous Form inside only), emergency escape (where they use it to escape), or as part of their egress (navigating through a lava trap).

As an example: a fortified castle commonly had a 300 Ft square footprint, and to be generous we can say it only winds mildly, requiring 600 Ft to get from one end to another, and again that distance to reach the top levels. They have an outer wall 300 Ft away, and then an outer trench line another 500 Ft from the walls. If you have a perfect route requiring no avoiding of any patrols whatsoever, you still will take 2,000 Ft and 20 minutes if you started your journey right at the outer line. Considering how slow Gaseous Form is, and how easily spotted it can be, this aggressively average castle would be nearly impossible to sneak into with even Commoner guard; your AC would be laughable, and Resistance only goes so far when you're swarmed.

Answer 6

If it is common, there will be countermeasures

An important thing to remember is that magic isn't something people aren't aware of, especially when thinking about protecting an important person like the ones you mention, and even more so when said person is rich enough to afford specialized protection.

So to answer to the question "Is it realistic to have countermeasures against this kind of magic?", the answer is yes, as long as the VIP has enough money, influence or network to afford it.

NPCs aren't bound by the same rules as PCs

It is always interesting to figure out what player tools could be used to counter such a problem. However, in the case of NPCs, as the DM, you are not bound by the same limitations.

Be it through the modification of existing magic items and spells, or simply the creation of a new one altogether, the only limit for creating anti-assassination devices is up to your own imagination.

You could have magic traps that trigger when someone in range is under the effects of any transformation magic, such as polymorph spells or Gaseous form. The guards could have special amulets that let them detect this kind of magic effect. You could even create creatures made especially to counter this kind of assault.

Careful if the assassins are the players

If the players are the ones doing this kind of stealth trick, this kind of specialized countermeasure can be risky to use, at least if the players get surprised by it. As having seen that mistake both as a DM and as the receiving player, I can affirm it isn't fun to devise a plan based on the clever use of an ability, then have it shut down by a perfect counter out of nowhere. Situations like a druid in mouse form scouting a place, only for a cat to appear out of nowhere and make the druid into lunch.

However, if your players have at least some knowledge of this kind of countermeasure, is where it becomes interesting. If the players are aware of those difficulties, and they still want to go through with this plan, they'll have to outthink those countermeasures. If they manage to do so, great! They've made use of their creativity on a problem you've given them, and their reward is being able to go through with their original plan.

You can even make this kind of countermeasure into a major plot point. Make your players go to a remote location to find an artifact or a mage capable of dealing with a unique magic guarding the place. Negotiate the help of a specialist that knows the layout of the place and the locations of the traps.

To sum it up

In a world of magic, it seems logical that VIPs would have countermeasures against magic-based assassination methods, and in the case of an NPC you can create just about anything to prevent such methods, without being bound to the magic items and spells listed in the books.

If the players are the ones doing the assassination, you can use such countermeasures as fun challenges or plot points to spice up your adventure. Just be careful to not jump it on them last second, or it'll feel like an improvised counter to their plan and not be fun for anyone involved.

Answer 7

The spell list of 5e is not every magical effect in the world.

Protecting houses with various kinds of hedge magic -- iron horseshoes, blood in the foundation, iron powder on window sills -- is something that people do and did in the real world. In a world where magic works, it seems plausible that these kind of hedge magic rituals would either work, or variations would exist.

Treat the spells of 5e as ones practical for adventurers. Minor magics will be all over the place: you'll leave milk out for the fey, and the fey will bless your house, stonemasons will put marks on the cornerstones of buildings, the hair of a first born will be woven into garments, wards will be woven into the walls of castles. Waterfalls will spin prayer wheels, prayer scrolls will be embedded into door frames, lanterns will be lit and float away with wishes. The walls will be carved with deals with the spirits, the first drink of every meal will be given to a god, and sheep will be sacrificed to prophesy the best course of action.

And the most effective hedge magic will be part of castle walls and noble houses.

As a DM, what this means is when the players do clever things like this, you are justified in coming up with clever responses. World-building wise, these sort of tactics should only work when unexpected, or require more than just casting the spell; before someone uses gaseous form, they also arranged to disrupt the protective rituals: maybe they substituted a fake priest for the real one a week before, or adulterated the iron powder used with lead.