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I was wondering what would prompt an investigation check into an illusion. The description of illusion spells (such as minor illusion and silent image) say that a creature can take an action to make an investigation check against my spell save DC to identify an illusion.

This "take an action" means that the NPCs have to make a conscious decision to make this check, otherwise the mechanic would be completely broken, with the caster forcing everyone to constantly make checks while an ally with truesight tears them down with no resistance.

So my question is what would prompt an NPC to make such a check? Would it be that the illusion is out of place, seems particularly odd, or something along those lines? This is never really specified.

In spells such as Phantasmal Force, the description says that:

The target can use its action to examine the phantasm with an Intelligence (Investigation) check against your spell save DC [...] While a target is affected by the spell, the target treats the phantasm as if it were real. The target rationalizes any illogical outcomes from interacting with the phantasm.

This means that any event that might cause the target to question the illusion and lead to an investigation check are now rationalized and no longer seem "odd" by the target. So what sorts of events would cause this check in this setting?

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    \$\begingroup\$ Chepelink makes a good argument that Phantasmal Force is a special case. If that's your real question - and your comment on aramis's answer suggests it is - I suggest you edit to make that clearer. If you only bring it up as one example of an illusion spell, aramis's answer is correct. \$\endgroup\$ – SirTechSpec Aug 24 '16 at 16:32
  • \$\begingroup\$ (If you want to know about both, you could certainly split the question into two by asking another one and then editing as appropriate.) \$\endgroup\$ – SirTechSpec Aug 24 '16 at 16:34
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(Disclaimer: the power of the illusions are controlled by the DM unlike most spells, the reason is because illusions might be too powerful and game changing for so little. Therefore, the best course of action is to ask the DM about your question, if you are the DM here are some guidelines but at the end is up to you)

Narrative wise

From the DM to the players, the way they present the illusion is the way it works (mostly, rules of the spell still apply). This is very important, specially in how the magic works in the world, for example, shield: does it make sound when you cast it or while it is active? What about light or shimmers? Some illusions like silent image and minor illusion+ don't make sound. A beast probably won't question why the wall that suddenly appear didn't make sound, but an intelligent creature familiar with a tiny bit of magic, yes, it would.

An interesting spell, narrative wise, is major image. Since major image can create illusion with smell, temperature, and sound, etc, the narrative of the spell should reflect the illusion presented. A wall emerging from the deeps of the earth making crumble sounds and the characteristic smell of earth, dust and cut grass will be way harder to disbelieve than a wall that just appear.

It just odd

This is the most basic way of disbelieving an illusion. Something might be misplaced and should not be there. For example, a cardboard box in the middle of a room undoubtedly will attract attention but the same box in a corner won't be noticed as much. Another example would be that the caster was to greedy and create an illusion so big that there wasn't room for a fake shadow. A giant guppy floating in the middle of the battlefield is a tell too. An example list would be:

  • The illusion is in a place where it attract attention
  • The action used by the illusory creature is out of its common behavior
  • The spell does not make sense (the wall just pop in from thin air with any "common" warning, like displace air current and so on)
  • The creature do nothing at all relevant, e.g. it does not attack.

Failed roll

Some DM will ask for roll checks when you want to create an illusion of a creature that you have not seen or that you usually do not use. These checks vary from illusion to illusion, the most common are arcane, nature and religion. A failed roll just increase the oddity of the illusion, it does not make an automatic failure, the "enemy" still have to pass the corresponding check if the illusion seems odd for him. For example, you know of a giant floating fish, you have seen one once, but you fail (nature roll) to recall the details, but the Orc that you are facing only heard of tales of such creature, he have to pass an history check to remember details of such creature. On a failed, the big fish is safe. This goes for hallucinatory terrain and other spells that require some degree of knowledge.

Phantasmal Force

This is a special case, mainly because of two particular things:

  1. While a target is affected by the spell, the target treats the phantasm as if it were real.
  2. The target rationalizes any illogical outcomes from interacting with the phantasm.

If we consider the initial save roll as failed, there is very little that the affected target can do for itself. Number 1 take care of things like thinking it is to odd for it to exist or to be real. Number 2 is particular problematic, since it make any interaction feel real and in order no mater how illogical the reaction or action was. But, there is a solution to this problem, third parties. Since any third party won't see what the affected creature sees, it is very easy for them to connect the dots and establish that something is very wrong with him. They can yell it's in your head or it's an illusion, they can ask what is wrong and latter say there is nothing there or the like. In that case, the affected target have a reason to suspect might be no real and act accordingly.

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    \$\begingroup\$ The question is how deep the inception of phantasmal force penetrates. Maybe you rationalize that your poor comrade cannot see the dragon, because it is invisible. You could rationalize he is under some kind of spell and you shall not listen to him. "rationalize ANY illogical outcomes" is too mindboggingly strong. I would also set a limit by illogical outcomes which do not result directly from interacting with the illusion (such as a third party) but this is just DM nerfing. \$\endgroup\$ – Falco Aug 24 '16 at 13:07
  • \$\begingroup\$ @Falco Good point, but the spell specify interaction with the illusion, not interaction with others. If the companion interact indirectly without knowing with the illusion I would rule that the affected target (AT) would rationalize that interaction. But, if they told the AT that its in its mind it has a reason to suspect because of its companions, not because of the spell. The AT might curse the companions on a second failed role and stop listening, though. \$\endgroup\$ – Chepelink Aug 24 '16 at 13:20
  • \$\begingroup\$ @Falco I'm not entirely convinced that someone saying "You are suffering under the effects of an illusion spell" is an "illogical outcome" that needs to be reconciled. Your best friend not knowing who you are (because it is an illusion) is something that needs reconciliation. \$\endgroup\$ – Shane Aug 24 '16 at 19:38
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The most obvious reasons to doubt an illusion:

  • It doesn't react to something you did that it should have.
  • you can't smell it.
  • it casts no shadows
  • it's not affecting the environment.
  • it's a critter not native to this region and/or plane.
  • It's the wrong shape/color because the caster failed a nature check.
  • It's too dangerous for the current party level.†
  • It's unlikely to have survived here.

† Sure, that's metagaming, but still, it's a reason used.

Now, "unlikely" can be interesting. A wall of ice in a dungeon... is it ice? Or is it an illusion of ice. If it's not dripping, or there's no puddle, and it's been there a bit, odds are it's an illusion. Or the fire that casts no shadows and provides no light.

Things of that nature.

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    \$\begingroup\$ Thank you for your response. What about phantasmal force though, wherein the reasons that cast doubt on the illusion that you listed are automatically rationalized? \$\endgroup\$ – Yapoz Aug 24 '16 at 7:27
  • \$\begingroup\$ So what you are saying is that a successful Perception(Wisdom) to notice any of that could prompt an Investigation(Intelligence) check? \$\endgroup\$ – Slagmoth Aug 24 '16 at 17:05
  • \$\begingroup\$ @slagmoth - no. A successful nature roll, perhaps, to know it either can't survive or isn't native. Asking the right questions, rather than reliance upon mechanical rolling. \$\endgroup\$ – aramis Aug 28 '16 at 1:24
  • \$\begingroup\$ Was just indicating that a great many things on your list are perception based. Personally, I allow rolls a lot but if they ask specific questions I may just give it to them. Like if they specifically ask if there is a shadow being cast and it was broad daylight (amateur illusionist if ever there were one in that case) I would simply give them the information without asking for a roll. \$\endgroup\$ – Slagmoth Aug 28 '16 at 2:12
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The player (DM if applicable) saying "I use my action to investigate this X to see if it is an illusion."

This method of determining if a thing is an illusion is specifically called out as something a creature can do in response to it. As an inherently metagame construct in the spell there is no issue with a metagame solution. If the player suspects then they can just burn an action to see (or not if they fail the check).

If this makes you uncomfortable, you can simply decide that there is always something about an illusion that isn't quite right and that is what triggers the character's suspicion, irrespective of what triggered the player's.

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I would say in this question the frame of mind about rolling checks is incorrect. You want the PC to find a reason to doubt the illusion first then make the check, which would lead to situations where the PC calls out something strange and then shrugs it off.

The order of actions should be as follows:

  1. It is up to the DM to describe it in a way that makes the Player (not PC) suspicious.
  2. The Player calls out the illusion.
  3. The Player Rolls.
  4. The PC acts: either calling out the illusion or is simply distracted without realizing why.

Note the difference between Player and PC above. The Illusion is a GM tool that is used against the Player which says: "You may know it is fake, but your PC can only know if you succeed the next roll." (informal quote)

PS. I believe it prudent to have rules such as cannot make this investigation check more than once every 10 rounds, or when the pace of story dictates.

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Don't worry about the "metagaming". Let the dice take care of that.

When presented with an illusion, assume that the NPCs or monsters are always aware that an illusion is possible. Whether they consume an Action to investigate should depend on whether they consider that the consequences of successfully penetrating the illusion are worth the loss of the Action, weighing in the probability of success or failure. Outside of combat, they almost always should take that Action. Within combat, it should depend on how much the illusion affects the situation.

Then apply Advantage or Disadvantage according to all the factors listed in the other answers provided. Particularly "natural-seeming" illusions should provide Disadvantage, poorly-considered illusions should be easy to see through and give Advantage on the roll.

There is no particular difference with Phantasmal Force. Although the character under the spell will rationalize what he perceives, that does not mean that he suddenly forgot that the spell exists and wizards enjoy casting it. Unless knowledge that such spells exist is rare in your campaign, PCs and NPCs alike should be granted the opportunity to investigate any illusion under any circumstances and doing so should not be rare. This is actually the opposite of meta-gaming - this is letting the dice and character sheet determine the outcome.

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Passive Perception may be your key. In actuality, you can calculate the passive vales for each of the abilities, just exactly like the players did for Passive Perception. As the DM, have the Evil McNasty roll against that for the party's belief. If everybody is effected, they'll know it. Otherwise, part of the party might not believe the illusion and might noe, but that's OK.

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