Illusions and Passive Investigation

[Elredion]

I've seen much debate on how different DMs treatment of illusions can cause their power levels to vary wildly from game to game. It just occurred to me that there is already a precedent for automatically determining whether a creature sees through an illusion, in addition to the "use an action to check" clause.

Note: I haven't read through the DMG but I haven't seen this mentioned before so correct me if this is redundant.

Passive Investigation is mentioned in the PHB under the Observant feat (pg 168). The intention seems to be that any creature with a passive investigation score above an illusion's spell DC should automatically see through the illusion. If no one sees through it passively, THEN the DM discretion comes in as to whether the creature should make an active investigation roll. Now here there is some room for leeway, but it makes sense for the DM to use some sort of familiarity level to determine whether the illusion should be checked, as well as considering if the creature saw the illusion come into existence

I.E. Very Familiar or Saw Illusion Materialize - Automatically check (maybe advantage if it saw it come into form)
Somewhat Familiar - 75% chance to check
Seen Once or Twice - 50% to check
Never seen - 25% chance to check

For example - a gnome illusionist is hiding in an illusion crate in an Orc storeroom. An orc patrolman walks through on his daily rounds. The orc's passive perception and passive investigation are low enough so that he doesn't hear the gnome and doesn't detect strange features with the crate, however he is very familiar with this storeroom and hasn't seen this crate here before. He makes an investigation check (brief stare studying the crate) and rolls high enough to determine that something is wrong. THEN he goes and tries to interact with the crate and discovers that it is an illusion and the gnome is busted.

DMs - what do you think? Do you already use Passive Investigation? Would Passive Investigation create a more solid baseline for the power of illusions? Is this a stupid statement that everybody already knows about? Let me know

 

[iserith]

Basic Rules, page 59 states that, "A passive check is a special kind of ability check that doesn’t involve any die rolls. Such a check can represent the average result for a task done repeatedly, such as searching for secret doors over and over again..."

"Passive" refers to there being no dice rolls in the check; it does not mean that the character isn't actively engaged in a task. Therefore, if you are not actively investigating in an ongoing fashion - perhaps to the exclusion of other activities such as keeping watch for hidden threats, navigating, foraging, tracking, or map-making - then your passive Investigation does not apply should you happen across an illusion. You simply fail to discern it. It is not a form of "always-on radar."

If, on the other hand, you have stated that you are actively investigating for illusions or similar effects while exploring the dungeon - perhaps because you expect gnomish trickery or the like (due to DM telegraphing) - then the DM can use your passive Investigation score to determine an outcome, should he or she believe that your chance of noticing any illusions is uncertain.

 

[Awesome Adam]

Passive investigation like that would pretty much destroy the usefulness of illusions.

It would be an arms race and illusionists would be at a disadvantage.

Base DC for Illusion = 8 + Ability MOD + Prof Bonus
Passive Investigation = 10 + Ability MOD + Prof Bonus

Any Wizard worth his salt who took Investigation would generally see through every illusion, cast by anyone appropriate for his level , automatically, even when he had to no reason to suspect an illusion.

Which would encourage a DM to use higher DCs on the invisibility, putting everyone else at a sever disadvantage.

A level 1 wizard with a 20 INT rocking a Passive Investigation of 17, could only get his own illusions up to DC 15. In fact, he could NEVER create an illusion capable of fooling his equal.

 

[CapnZapp]

Use a passive check when you don't want to roll dice.

That is all.

Really, that's all there's to it. You certainly don't auto-detect everything below your passive DC.

What the passive check allows is: for your DM to tell you stuff without asking you for a roll.

It also allows the DM to not apply (ignore) your passive DC. Either because she asks you to roll, or because she decides you don't deserve either (perhaps because you aren't playing your character in a way that would give you either the roll or the passive check)

 

[Psikerlord#]

I dont believe in passive scores at all and dont use them. If I need a roll, I roll, and I think the game works best that way. I like your % chances to investigate though. Illusions is the kind of area I think best left to GM discretion.

 

[CapnZapp]

I use passive checks to justify telling the players what I would like to tell them anyway.

For instance, if they pass by a bandit ambush, the maths tell me it is very likely SOMEBODY in the party will roll high enough to spot the clumsy bandits. This means, rolling dice essentially only decides WHO spots the bandits.

With passive checks, I can simply say "anyone meets DC 14 spots bandits in the woods" and get on with it. You could say this unduly rewards the high-Perception character, but really, what it does, is tell the group what the group would like find out anyway.

In cases where there is a significant advantage from being the one succeeding on a roll, then roll. Don't use passive Perception to decide who finds and gets to keep the King's Crown.

---

I would not feel obliged to apply their passive checks on everything. I could apply it on everything if I want to focus on the action and/or speed things up, but I don't have to.

That would include illusions.

All the passive check does is, when the player has his character suspect foul play ("I disbelieve!") you can immediately tell him whether he sees through the illusion or not, without having to ask for a die roll. You could also ask for the roll. Or decide the player hasn't earned such a check yet.

This speeds up the game, since players only spend actions to percept, investigate or sense motive if their passive checks isn't enough to satisfy the players curiosities and suspicions.

Without the passive check, there would be many more actions taken on these skills. And many of them would be trivial or boring, and above-all time-consuming.

 

[Elredion]

Passive investigation like that would pretty much destroy the usefulness of illusions.

It would be an arms race and illusionists would be at a disadvantage.

Base DC for Illusion = 8 + Ability MOD + Prof Bonus
Passive Investigation = 10 + Ability MOD + Prof Bonus

Any Wizard worth his salt who took Investigation would generally see through every illusion, cast by anyone appropriate for his level , automatically, even when he had to no reason to suspect an illusion.

Which would encourage a DM to use higher DCs on the invisibility, putting everyone else at a sever disadvantage.

A level 1 wizard with a 20 INT rocking a Passive Investigation of 17, could only get his own illusions up to DC 15. In fact, he could NEVER create an illusion capable of fooling his equal.

Which makes sense, because he/she has studied magic and knows what to look for as tell tale signs of illusions: how they interact with light and such. Unless the enemy illusionist is significantly more proficient

This also creates a tool for DMs to deal with players who abuse illusions - make them deal with enemy wizards. Force them to find another solution. If they are in a campaign with no wizards, tough luck, the illusionist has a leg up

 

[Elredion]

Basic Rules, page 59 states that, "A passive check is a special kind of ability check that doesn’t involve any die rolls. Such a check can represent the average result for a task done repeatedly, such as searching for secret doors over and over again..."

"Passive" refers to there being no dice rolls in the check; it does not mean that the character isn't actively engaged in a task. Therefore, if you are not actively investigating in an ongoing fashion - perhaps to the exclusion of other activities such as keeping watch for hidden threats, navigating, foraging, tracking, or map-making - then your passive Investigation does not apply should you happen across an illusion. You simply fail to discern it. It is not a form of "always-on radar."

If, on the other hand, you have stated that you are actively investigating for illusions or similar effects while exploring the dungeon - perhaps because you expect gnomish trickery or the like (due to DM telegraphing) - then the DM can use your passive Investigation score to determine an outcome, should he or she believe that your chance of noticing any illusions is uncertain.

I see that different DMs interpret and use passive scores differently from this thread (or not use them at all). However it seems to me that 9 times out of 10 when players run into hostile NPCs they are going to have that "always on" radar: whether it be patrolling, guarding, or moving in to attack. The only situations I can see is if they are face first in a spell book, eating, sleeping, or drinking in which case the player should be rewarded for catching them surprise. If the player creates a distraction that should also be rewarded, however if the distraction itself is an illusion then that should have the attention of the creature and then the DM can decide whether to make a check or not based on familiarity.

I suppose another way to word this thread would be: the same mechanics that creatures use to detect stealth can (or should) apply to illusions, except with investigation instead of perception. At least that is what I inferred from the Observant feat.

The beautiful thing about this game is that no two DMs interpret the rules the same way. At the end of the day illusions will still be powerful at some tables and weak at others. I have seen many debates about how to treat illusions, I was hoping that this could provide some insight.

 

[iserith]

I see that different DMs interpret and use passive scores differently from this thread (or not use them at all). However it seems to me that 9 times out of 10 when players run into hostile NPCs they are going to have that "always on" radar: whether it be patrolling, guarding, or moving in to attack. The only situations I can see is if they are face first in a spell book, eating, sleeping, or drinking in which case the player should be rewarded for catching them surprise. If the player creates a distraction that should also be rewarded, however if the distraction itself is an illusion then that should have the attention of the creature and then the DM can decide whether to make a check or not based on familiarity.

The activities the rules call out as obviating a passive Perception check for keeping watch are navigating, tracking, foraging, and map-making. Any similarly distracting activity could do the same. They key thing here is that there is a potential trade-off. You can continually keep watch and gain the benefit of your passive Perception or you can do some other activity and potentially not gain that benefit - choose.

I suppose another way to word this thread would be: the same mechanics that creatures use to detect stealth can (or should) apply to illusions, except with investigation instead of perception. At least that is what I inferred from the Observant feat.

I read it again and I don't infer that from the feat. Sure, if a monster is taking steps to remain vigilant for the telltale signs of illusions, then the DM could reasonably rule that passive Investigation applies, if the outcome the monster's effort is uncertain. But this level of scrutiny (which probably includes more than just gawking) should come with a trade-off in my view, as above.

The beautiful thing about this game is that no two DMs interpret the rules the same way. At the end of the day illusions will still be powerful at some tables and weak at others. I have seen many debates about how to treat illusions, I was hoping that this could provide some insight.

Treating passive Investigation as a form of "always-on radar" strikes me as an attempt to justify making illusions less effective.

 

[BoldItalic]

If the DM wants some, all or none of the PCs to see through an illusion that he himself has created, that's fine. That's his job, deciding things like that. He can decide arbitrarily what makes the best narrative to entertain the players. He can reveal the illusion immediately and leave the players to admire it and wonder what it signifies and who the NPC was who supposedly created it; or he can decide to play 'gotcha' and not reveal it unless and until it occurs to a canny player to make an active check. But either way, he doesn't have to justify that decision to himself by appealing to some sort of formula.

If an NPC casts an illusion, the DC is determined by the NPC's stats which the DM decides arbitrarily. There's no element of chance. The PCs skill adjustments are known as soon as they enter play. Again, there is no element of chance. Adding 10 and calling it a passive score doesn't change that. The result of comparing two numbers that are known in advance is a foregone conclusion.

If you use a foregone conclusion to try to justify an arbitrary decision, you are not being honest with yourself. I might decide to wear red socks tomorrow, on a whim. But if I tell you that I had to wear red socks because pi is more than three, you would be entitled to cast doubt on my thought processes.

Passive scores make sense when they are compared to a dice roll (such as a Stealth roll). They don't make sense when they are compared to a predetermined stat (such as a spell save DC) because the comparison is redundant.