Illusions and Passive Investigation

[iserith]

Throwing in my 2cp on this example...

The Barbarian stated his goal and is focused on looking for illusions. He should gain advantage on his passive score (+5). But that focus will cost him, he is alerting the goblins in the other room because of all that incessant tapping!

Given the stated goal and approach ("tap everything"), I'd probably just say he succeeds outright without reference to the passive Investigation score. There is no uncertainty as to finding the illusions if you're going to physically interact with everything. Your trade-off is a very reasonable one - you succeed outright, but of course it takes time and makes noise.

 

[strider13x]

Overall I would say let the dice decide when you as DM are not sure, apply Advantage or Disadvantage as appropriate. Keep in mind the motivations of the people that are being fooled, does the orc guard from the previous example really have motivation to be suspect of the additional crate? Or is he just making the rounds so he can get back to his card game? The is no one way to rule any situation which is the best (and sometimes worst) part of DMing! Reward players that are inventive, if they come up with something that you as DM have no contingency for then let them have it! Other times they will blunder all fall into exactly what you expected, then you really let them have it!!

 

[GMMichael]

Isn't an illusion sort of ruined if you ask players to make investigation checks when it shows up? So you use passive checks instead, and that one character with the high...whatever score always tells the party, "hey! It's an illusion!"

Since an illusion could be just about anything, the solution is very abstract: a character doesn't get to roll against an illusion unless he would have a reason to suspect an illusion. This shouldn't be a passive check, and the GM should do the rolling for the PC.

Why not a passive check:
[sblock]As stated earlier, a static number (spell DC) versus a static number (passive investigation) isn't a check. But more importantly, a character who is constantly checking for illusions, which could be anything, is basically doubting reality. Which would be horribly crippling.[/sblock]

That task must be described by the player like any other action, though sure, it's probably a common and fairly safe assumption at the table that unless the players describe some other action that is sufficiently distracting, the characters are keeping watch for hidden threats (and thus their passive Perception scores might apply to noticing hidden monsters and traps).

This reminds me of crawling through a Skyrim dungeon. I know that there will eventually be a floor panel that shoots poison darts at me or releases a spiked, swinging grate. So in theory, I'm always looking for them. In practice, though, I'm also watching for enemies, admiring the scenery, looking for the path forward, making sure my follower is behind me, swapping out gear, and even lining up the perfect bow shot. All of which reduce my ability to check for floor panels.

For the record, as soon as a player tells me he's tapping everything with his maul to make sure that it's not an illusion, as GM, I start sending in the monsters who have ears.

 

[Ganymede81]

Whether it is because you don't want to roll a bunch of dice (as in the example of a repeated task), or you don't want to roll any dice at all (as in the example of detecting a hidden trap or a cutpurse), the rulebook indicates that you use passive checks when you would rather not roll.

I think that's the key. Using passive perception is essentially the same as making any other perception check, with the only exception being your passive perception score functions as the ersatz result of your perception check. To me, this is roughly analogous to taking 10 on a spot/listen check, though used at the DM's discretion rather than the PC's discretion.

The big wrinkle is the Observant feat, which gives you a bonus to your passive perception score but not your perception check result. Why? It seems like the intention is to make you better at noticing things that are ordinarily within your ability to check, but not to make you better at detecting things you ordinarily couldn't detect (though there is also the fact that a +5 is, mechanically, the same as advantage; why give ersatz advantage on one but not the other?).


How would I play it? I'd probably do it like a take 10; your passive check score is a die roll minimum in low-pressure scenarios.

 

[iserith]

Isn't an illusion sort of ruined if you ask players to make investigation checks when it shows up? So you use passive checks instead, and that one character with the high...whatever score always tells the party, "hey! It's an illusion!"

The latter is a potential outcome of treating passive Investigation as an "always-on radar." Which is not to say you can't always been poking around for illusions, if you want, but there should be a trade-off, cost, or consequence in my view.

Since an illusion could be just about anything, the solution is very abstract: a character doesn't get to roll against an illusion unless he would have a reason to suspect an illusion. This shouldn't be a passive check, and the GM should do the rolling for the PC.

I would refine this to the character needing to state a goal and approach to discerning the illusion, if there is one. Simply suspecting isn't sufficient - what are you doing exactly to deduce that thing before you is an illusion? The goal and approach to discerning the illusion offered by the player may succeed, fail, or have an uncertain outcome at which point the DM can call for an ability check. Physical interaction often allows for automatic success in determining whether something is an illusion or not, so canny players will likely go this route. I know I would, if it was a viable option.

I'm not a fan of the DM rolling for the player, so I would suggest the failure condition be "You are unable to discern if the thing before you is an illusion or not..." rather than "You don't think it's an illusion." (Imagine a player hearing the latter from the DM and staring at a low result on the die.) Though really I'd prefer success at a cost or with a complication on a failed check.

Why not a passive check:
As stated earlier, a static number (spell DC) versus a static number (passive investigation) isn't a check. But more importantly, a character who is constantly checking for illusions, which could be anything, is basically doubting reality. Which would be horribly crippling.

The passive check may come into play if the player has described the character as performing the task repeatedly and, despite this, the DM still finding the outcome uncertain. I imagine this is going to be a thing that a character chooses to do while traveling about through the adventure location, not unlike being constantly searching for secret doors, keeping watch for hidden threats, tracking, foraging, navigating, map-making, etc. Point is, you can do it, but you might not be able to do other activities at the same time while traveling.

Another thing to consider in discussions like these is whether players at the table ask to make checks or whether they simply describe their goal and approach and wait for the DM to decide on success, failure, or uncertainty. I advocate the latter while many people play in the former fashion, perhaps because of the influence of previous editions.

 

[Awesome Adam]

The characters walk up to a bridge, which happens to be an illusion.

* If they check the bridge at all, before crossing, they should get a check.
* If they simply walk across the bridge, the first guy is going to make a Dex save or fall in.

Always on Passive Investigation turns this into:

The characters walk up to a bridge and the Wizard go's "IT'S A TRAP!!"

No Drama, No risk, No XP should be awarded. It's a wasted opportunity.

 

[Psikerlord#]

Whether it is because you don't want to roll a bunch of dice (as in the example of a repeated task), or you don't want to roll any dice at all (as in the example of detecting a hidden trap or a cutpurse), the rulebook indicates that you use passive checks when you would rather not roll.

I think that's the key. Using passive perception is essentially the same as making any other perception check, with the only exception being your passive perception score functions as the ersatz result of your perception check. To me, this is roughly analogous to taking 10 on a spot/listen check, though used at the DM's discretion rather than the PC's discretion.

The big wrinkle is the Observant feat, which gives you a bonus to your passive perception score but not your perception check result. Why? It seems like the intention is to make you better at noticing things that are ordinarily within your ability to check, but not to make you better at detecting things you ordinarily couldn't detect (though there is also the fact that a +5 is, mechanically, the same as advantage; why give ersatz advantage on one but not the other?).


How would I play it? I'd probably do it like a take 10; your passive check score is a die roll minimum in low-pressure scenarios.

The tip off "problem" can be avoided by making rolls every now and then behind the screen for no reason, but raising eyebrows, muttering darkly, etc. Or use a phone rolling app. Or play online. Or get players to roll up 10 perception checks ahead of time and go up, down or some other way through the list.

 

[CapnZapp]

The characters walk up to a bridge, which happens to be an illusion.

* If they check the bridge at all, before crossing, they should get a check.
* If they simply walk across the bridge, the first guy is going to make a Dex save or fall in.

Always on Passive Investigation turns this into:

The characters walk up to a bridge and the Wizard go's "IT'S A TRAP!!"

No Drama, No risk, No XP should be awarded. It's a wasted opportunity.

Don't use "always on" passive checks.

And don't follow some rulebook to tell you when to use passive checks.

Use passive checks if and only if you the DM want to utilize one of the two traits of passive checks:
1) the way a passive check often equates to auto-success if everyone in the party gets to "check".
2) the way a passive check is faster and less intrusive than active rolls

If the party decides to mistrust the bridge and check it, you the DM might decide they should find out it's an illusion with no risk of failure since they did take the trouble of checking it. Perhaps you feel it would be highly anticlimactic to ask for rolls and see them all roll a 1.

In this case, use the passive check mechanic because of reason #1. Then simply tell them "the bridge is illusory, what now". You might not even bother checking their individual passive Investigation scores against the illusion DC, partly because you really don't want to have them fail here, partly because experience tells you most 5E parties will contain at least one character with a good passive value that is better than the relatively modest save DCs of the game.

If the party encounters a lot of illusions and this bridge isn't special, you might instead decide to speed up the illusion-checking procedure to keep down the number of rolls that really doesn't lead anywhere "okay so this statue is also an illusion, nothing to be gained here, moving on".

In this case, use the passive check mechanic because of reason #2. Then tell them "you pass by a lot of towers and bridges and windmills and statues, but most of them turn out to be illusions. After an hour, however, ..." and by this time, they've come across something crucial to the adventure.

Now, and this is the lesson boys and girls, this final illusion might very well be one the passive mechanism would also reveal... but this time you decide NOT to use the passive mechanism, and instead ask for rolls.

This way, the party Wizard must roll at least average and the rest must roll high, or the illusion stays unrevealed and the party has its challenge!

Do note that in each and every case the decision to use or not use passive checks is solely up to the Dungeon Master: the needs of the story and the pacing of the play session. At no time does any rule from the PHB whatsoever come into this at all.

This is how I recommend you put the passive check mechanism to good use. Anything else will only lead to confusion and frustration. Like this very thread is a perfect example of, I might add.

 

[iserith]

Here are some examples showing different goals and approaches to the situation and adjudication for each based on what I have been saying thus far in this thread.

Scenario: The characters are exploring the secret underground laboratory-dungeon of Auric Gnomefinger, a villainous artificer known to make use of illusions. Having just defeated his tuxedo-wearing owlbear butler, Hootsworth, they have acquired a partial map of the dungeon which shows a wide chasm cutting through it, but no bridge or other means to get across it.

Player 1: Khegg Steadfast leads the way through the dungeon toward that chasm, feeling like a million gold after getting the deathblow on that ridiculous fedora-sporting owlbear. I'm keeping an eye out for any hidden threats.
Player 2: I've got the map. Wanda Curelight will navigate so we don't get lost. "It's this way, Khegg."
Player 3: Percy Sly will also keep an eye for hidden threats. "Let us make haste. If we don't catch him soon, Gnomefinger will activate the Doomsday Device."
DM: What about you, The Ritual?
Player 4: As a fellow fey, I expect gnomish trickery in the form of illusions. Using my keen elven eyes and my knowledge of all things arcane, I'm going to scrutinize everything important for the telltale signs of illusions as we move through the dungeon.
DM: Okay, so two of you on watch for danger, one navigating, one investigating for illusions. Wanda and The Ritual, you run the risk of being surprised outright if there are sneaky monsters about. What's your passive Investigation score, The Ritual?
Player 4: Let's see, I'm proficient in Investigation and my Int is 16, so 15.
DM: After a while of traversing winding corridors and homogeneous chambers, you find the chasm marked on your map. The far side, some 30 feet distant, is reachable by way of an ornate and sturdy-looking bridge that spans the gap. The bottom is somewhere in the darkness below, beyond the range of your light and vision.

Adjudication: The Ritual is performing a task of searching for illusions repeatedly. The DM has decided that being on watch for illusions, but not physically interacting with them makes the outcome uncertain. Therefore, this is a good place for a passive check to decide the outcome. The DM sets the DC at 14, just short of medium difficulty, because that is Auric Gnomefinger's spell save DC and this seems reasonable as it was his Illuso-Matic invention that created the illusory bridge. The Ritual's passive Investigation score beats the DC.

DM: Something about the way the bridge casts a shadow in Wanda's light catches your eye, The Ritual.
Player 4: "Hold, Khegg - go no further. I sense gnomish trickery about this bridge..."

---

Player 1: Khegg Steadfast leads the way through the dungeon toward that chasm, feeling like a million gold after getting the deathblow on that ridiculous fedora-sporting owlbear. I'm keeping an eye out for any hidden threats.
Player 2: I've got the map. Wanda Curelight will navigate so we don't get lost. "It's this way, Khegg."
Player 3: Percy Sly will also keep an eye for hidden threats. "Let us make haste. If we don't catch him soon, Gnomefinger will activate the Doomsday Device."
DM: What about you, The Ritual?
Player 4: "Yes, look lively." I'll keep an eye out for danger as well.
DM: Okay, so three of you on watch for danger, one navigating. Wanda, you run the risk of being surprised if some sneaky monsters come calling.
Player 2: The light will see me through.
DM: After a while of traversing winding corridors and homogeneous chambers, you find the chasm marked on your map. The far side, some 30 feet distant, is reachable by way of an ornate and sturdy-looking bridge that spans the gap. The bottom is somewhere in the darkness below, beyond the range of your light and vision.
Player 2: "This bridge is not marked on the map."
Player 4: "Hmm, perhaps it is more gnomish trickery afoot." From a safe distance, I examine the bridge for the telltale sign of illusions, using my keen elven eyes and knowledge of things arcane.

Adjudication: The DM has decided that investigating illusions, but not physically interacting with them makes the outcome uncertain. The DM sets the DC at 14, just short of medium difficulty, because that is Auric Gnomefinger's spell save DC and this seems reasonable as it was his Illuso-Matic invention that created the illusory bridge.

DM: Let's see an Investigation check, The Ritual.
Player 4: *rolls* 12.
DM: Other than Wanda's mention of it not being on the map, you discern nothing unusual about the bridge. The clank of metal and the sound of heavy footsteps alerts you to the approach of a squad of Gnomefinger's inept guards coming from the corridors behind you. Their echoing voices indicate they are arguing with each other about which way you went, but they'll no doubt be here soon. What do you do?