D&D 5E Using & running illusory traps

[J.Quondam]

Looking for advice on how to run an illusory trap in D&D 5e. The particular situation* is an illusory rolling boulder trap that triggers when PCs enter a long hallway. The "boulder" chases the affected PCs into a real pit trap just before a room at the far end of the corridor.

(1) How would you run this? especially with respect to detection and disarming each part of the trap? (I've got my own idea, but would like to hear from more experienced 5e GMs.)

(2) Also just out of curiousity: What your experiences with illusory traps, in general? Any examples, published or otherwise? How-to's; what is/is not appropriate; successes & failures; do's & don'ts. It's all helpful for this discussion.


* This is a repurposed trap from an unused PF dungeon I made a long time ago. The whole (small) dungeon is a mix of real and illusory encounters, which is basically the fey bbeg's main schtick.

 

[Quickleaf]

My answer has two parts: general approach to traps (for the pit trap part) & my approach to illusions (for the rolling boulder part).

Running Traps in General (Pit Trap)
Here's how I do traps: Just like with monsters that have False Appearance, there is no single check you can make that will have me as DM say "there's a pit trap right here." I want more engagement, rather than more dicing for success – the roll gets the player more info, but it's up to the player's clever thinking to figure out what it means. This is a sharp divergence from how the 5e adventures handle traps. There are roughly 3-4 levels of information the players can discover about a trap, starting from no check at all, to passive skills, to checks, or even other abilities. Let's use a simple flagstone covered pit trap in a dungeon where the PCs know a rival group is also exploring as an example.

1. The obvious but subtle hint. This is just something I'll narrate as the PCs enter the area. Because my group is pretty savvy, I try to make this subtle. "Since descending the stairs, this stretch of hallway has more centipedes and cockroaches than the upper level you were just exploring." (This hints that there's some little passage – the pit – through which insects can enter the dungeon level)

2. Information I'll narrate with a sufficiently high passive Perception."Rell, you notice that the old flagstone floor is scuffed in several places – some are small streaks a few inches long, while others are scuffing along the edges of the flagstones." (This hints that someone may have been pushing with a 10-foot-pole and trying to lift up the flagstones...i.e. searching for a trap)

3. Information I'll narrate with a sufficiently high active check. Usually this is Perception, but sometimes it involves a player describing checking in the specific area where their PC would logically learn this information, or sometimes with clever spellcasting. For instance, say the player wants to run a finger along the mortar connecting the flagstones. "OK, Rell, as you rub around at the mortar between the flagstones you notice that a large seam is actually just dust and there's no mortar." And the player might ask for more information, so I might call for an active Perception check, and if it succeeds I'd say: "You notice the scuffing is most pronounced around the outer edges of this un-mortared seam." (This hints at the presence of the false flagstone tile which covers the pit trap)

4. Information I’ll divulge with a sufficiently high Investigation, Exploratory Action, or with a Special Ability/Spell. At this point, for this trap, the players probably have enough information to guess there's a trap on the flagstone tile. However, let's say the player wants to determine the nature of the trap and its mechanism/trigger. If their Investigation check is high enough, or perhaps if they cast a spell like thunderwave that causes vibrations, I'd narrate: "You're able to get the flagstone to vibrate slightly by stomping next to it, and knocking on this flagstone compared to others indicates that it is hollow. As you do so a centipede scurries down a seam at the flagstone's edge.”

Handling Illusions (Illusory Rolling Boulder)
Foreshadowing & Pattern Recognition. The best use of traps involves foreshadowing that makes a connection between the dungeon's story and the function/presence of the trap. You establish a pattern of how the trap works in that dungeon, then once the players have got the pattern, you change that up in small but interesting ways to keep the traps fresh. Very similar idea applies to illusions. You want to flag this dungeon in your players' minds as "hey, illusions are present!"

The Early Example. You can accomplish that with a minor (low damage/inconvenience) illusory trap right at the beginning of the dungeon – this serves as the teaching tool to (a) alert players to the existence of illusions, and (b) place the first clue nugget that there is a pattern to these illusions. The idea is that the more the players are paying attention to the dungeon's lore/story, the better prepared they will be to anticipate those places where they should be saying "Can I make a check to disbelieve that?"

Which Senses Are Being Tricked? Also, most illusions have a flaw of some kind. For example, maybe the rolling boulder is visual and auditory only – you can see it and you can feel the hall rumble from it – but let's say it blocks a passage that leads to a sulfur hot springs cave, so the PCs should feel a warm breeze and smell the sulfur once the boulder starts rolling. However, for this illusion that isn't the case. To make this work, it requires a bit of dungeon design / narrative adeptness so that you ensure the players KNOW there is a sulfur springs behind that wall which the boulder has just "revealed." Again, this is rewarding player cleverness – if they're connecting the dots, then the question might arise "does it smell like sulfur?" To which you slyly answer: "Strangely, it doesn't."

Preserving the Illusion. Another element is the preservation of the illusion. This gets into "game within the game" territory – which may be controversial – but often when one player disbelieves or sees through an illusion, the temptation for other players to attempt to do the same is strong – whether that's because (1) the first player is shouting to them, (2) because of metagaming, or (3) something their PC would have seen that didn't add up. In your case, this could be a PC getting "squashed" by the illusory boulder. I like to interrupt at least one of those temptation routes to preserve the illusionism a bit longer... For example, I might say the rumbling sound of the rolling boulder is so loud that the "squashed" PC's shouting can't be heard by their companions. Or I might use false attack/damage rolls and describe the "squashed" PC dying horribly, putting on my best "I'm sorry, but the dice said so" DM face. Or I might include a blood smear that becomes part of the illusion.

Unique Weakness. Finally, just as vulnerabilities/weaknesses are too infrequently applied in the Monster Manual (in my humble view), illusions are often left to be solved one of two ways: dispel magic or an Investigation check. With my illusions, I always like to include at least a third option which is uniquely tied to the story of the dungeon or the NPC that created the illusion. For example, maybe your fey's illusions vanish "by the light of the moon" – which a PC might accomplish by entering dungeon at night & tearing away moss/plants at certain locations in the caves to allow moonlight to pour in, by casting moonbeam, or perhaps use some class feature involving moonlight.

Hope something in there is helpful to you!

 

[iserith]

Looking for advice on how to run an illusory trap in D&D 5e. The particular situation* is an illusory rolling boulder trap that triggers when PCs enter a long hallway. The "boulder" chases the affected PCs into a real pit trap just before a room at the far end of the corridor.

(1) How would you run this? especially with respect to detection and disarming each part of the trap? (I've got my own idea, but would like to hear from more experienced 5e GMs.)

(2) Also just out of curiousity: What your experiences with illusory traps, in general? Any examples, published or otherwise? How-to's; what is/is not appropriate; successes & failures; do's & don'ts. It's all helpful for this discussion.


* This is a repurposed trap from an unused PF dungeon I made a long time ago. The whole (small) dungeon is a mix of real and illusory encounters, which is basically the fey bbeg's main schtick.

1. I would not telegraph the boulder. They enter the hallway, boom, the boulder starts rolling toward them and it looks big enough to flatten anyone in its path - "What do you do?" This might already seem "off" to my players who would immediately start thinking about some clue they must have missed - after all, I don't do gotchas and always telegraph traps. Now I would just have to telegraph the pit trap ahead in some way as they run toward it. Assuming they are running away from the bolder and toward the pit trap, they are moving at a fast pace, and have a -5 on their passive Perception checks to detect hidden threats, assuming they are engaged in that task at all. The PCs in the first rank of the marching order hit the pit, triggering it as normal if they don't detect it. The remaining PCs can stop, but then have to figure out what to do in the face of the oncoming boulder.

2. Here's an example I ran recently:

The PCs come across this hallway with a spike pit in the middle and two paths on either side. (The players can't see the pressure plates marked in red.) Some plaster has broken away from the ceiling on either side of the pit revealing a row of holes. If you step on the pressure plate, a spring mechanism shoots the PC up to the ceiling, where spikes descend out of the holes, impaling them, at which point they fall prone onto the floor for more damage.

As it turns out, the spiked pit is an illusion - nothing but bare, easily traversable floor underneath. The creators of the adventure location wanted something to deter and kill intruders moving to the southern areas but needed to get through unscathed themselves. The PCs in this case fell for it and sent their fighter along the west side where he got chucked, pierced, and proned. They suspected something was up with the row of holes in the ceiling, but did not figure out what it was. Later on they figured out the pit was an illusion.

 

[J.Quondam]

This is all very helpful info, thanks for the input.
I've been looking at this very systematically, in terms of detect, disarm, save DCs, etc; or else as a special magic trap, modeled on glyph of warding or something.
But maybe that level of detail isn't necessary, since NPCs can do things in ways PCs can't-- something I keep forgetting, coming as I do from 3.5e/PF. Clearly I need to recalibrate my thinking on this a bit!

 

[Nefermandias]

1. I would not telegraph the boulder. They enter the hallway, boom, the boulder starts rolling toward them and it looks big enough to flatten anyone in its path - "What do you do?" This might already seem "off" to my players who would immediately start thinking about some clue they must have missed - after all, I don't do gotchas and always telegraph traps. Now I would just have to telegraph the pit trap ahead in some way as they run toward it. Assuming they are running away from the bolder and toward the pit trap, they are moving at a fast pace, and have a -5 on their passive Perception checks to detect hidden threats, assuming they are engaged in that task at all. The PCs in the first rank of the marching order hit the pit, triggering it as normal if they don't detect it. The remaining PCs can stop, but then have to figure out what to do in the face of the oncoming boulder.

2. Here's an example I ran recently:

View attachment 143799
The PCs come across this hallway with a spike pit in the middle and two paths on either side. (The players can't see the pressure plates marked in red.) Some plaster has broken away from the ceiling on either side of the pit revealing a row of holes. If you step on the pressure plate, a spring mechanism shoots the PC up to the ceiling, where spikes descend out of the holes, impaling them, at which point they fall prone onto the floor for more damage.

As it turns out, the spiked pit is an illusion - nothing but bare, easily traversable floor underneath. The creators of the adventure location wanted something to deter and kill intruders moving to the southern areas but needed to get through unscathed themselves. The PCs in this case fell for it and sent their fighter along the west side where he got chucked, pierced, and proned. They suspected something was up with the row of holes in the ceiling, but did not figure out what it was. Later on they figured out the pit was an illusion.

This is Grimtooth's.

 

[iserith]

This is Grimtooth's.

Yep. I think I modified it some so it was telegraphed, but the idea comes from there.

 

[jgsugden]

I'd use Programmed Illusion.

Unless they interact with the boulder, or they use an action that involves with the beholder without physical contact (which is enough to trigger the investigation role in my book), they perceive it to be real. To that end, I'd run it just like it was a real boulder until such time as they reveal it to be false.

 

[iserith]

This is all very helpful info, thanks for the input.
I've been looking at this very systematically, in terms of detect, disarm, save DCs, etc; or else as a special magic trap, modeled on glyph of warding or something.
But maybe that level of detail isn't necessary, since NPCs can do things in ways PCs can't-- something I keep forgetting, coming as I do from 3.5e/PF. Clearly I need to recalibrate my thinking on this a bit!

While the mechanics can help with inspiration and adjudication, ultimately we can just envision what we want, sketch out its triggers, effects, and limitations in a fictional sense, and then just describe it to the players. They'll figure out a solution on their own. So one doesn't even really need to come up with that part. The only reason I even write some of it out is as part of my thinking process. I rarely need to refer back to it in play, nor do I need to justify anything about how it works. The trap builders are geniuses and also a wizard did it.

 

[Shiroiken]

Passive Investigation exists (there's a feat that gives a bonus to it). I'd use that when the "boulder" appears, and ask if they have any plans other than running (ala Indian Jones), as they may have a spell or ability to try and avoid/destroy it instead. As someone pointed out, they now have -5 to their Passive Perception to notice the pit trap. Those behind the ones who fall now have to immediately decide if they want to jump into the pit, or try to jump over it.

 

[iserith]

Passive Investigation exists (there's a feat that gives a bonus to it). I'd use that when the "boulder" appears, and ask if they have any plans other than running (ala Indian Jones), as they may have a spell or ability to try and avoid/destroy it instead. As someone pointed out, they now have -5 to their Passive Perception to notice the pit trap. Those behind the ones who fall now have to immediately decide if they want to jump into the pit, or try to jump over it.

Only the corridor past the pit is blocked with a wall of force that anyone trying to jump over the pit slams into!