If I cast phantasmal force on an NPC to make him think a creature has jumped onto his back and is trying to strangle him, assuming he fails his initial roll, why would he use an action to make an Investigation check to examine the illusion?

Would it be that he suspects he is the target of an illusion? Would he know that investigating it would get rid of it?

Phantasmal force also takes place during the player's turn, so he still has his turn, but it seems like he would probably use his actual turn attempting to get rid of what's attacking him, or if it's say, a monster standing in front of him, he might attack it with his weapon. Would you count attacking or attempting to defeat it as an Investigation roll, or does it specifically have to be that he is investigating it?

I can't see many reasons why your average person would actually stop and study a monster. If it were an actual monster that jumped on his back, or an actual skeleton that was summoned to fight him, he wouldn't investigate, he'd just try to attack. So I'm a bit confused about the Investigation part, unless one of his friends shouts 'the monster isn't real, look closely', but then they'd have to know he was fighting a monster and not just confused.

Is there a reason to investigate illusions like phantasmal force other than some kind of metagaming?

For the sake of roleplay, is it considered typical/expected for anyone affected by such an illusion to simply make an Investigation check each turn to attempt to end the illusion? Or is it considered normal for them to suffer during the player's turn a monster on their back trying to strangle them, and on their own turn, perform some unrelated action like attacking a player with their sword while ignoring the effect?

  • \$\begingroup\$ I see, the question is more that I couldn't think of a reason to investigate other than metagaming. As for normal, I meant in that the game takes certain liberties that are pretty de facto, for example it is 'normal' for players to act like heroes, and not plunder a temple once then buy a nice house and retire, to always save the damsel in distress, fight the dragon, etc. and I wondered if it was also 'normal' for characters to act with a little meta information. Much like how most monsters eg goblin raiding mob will attack the party despite the fact they will without a doubt die horribly. \$\endgroup\$
    – NibblyPig
    May 4, 2020 at 12:09
  • 1
    \$\begingroup\$ I've edited the post to clarify the question a bit. Please check to make sure it still matches your intent. \$\endgroup\$
    – V2Blast
    May 4, 2020 at 19:24
  • \$\begingroup\$ @V2Blast Looks good, thank you. \$\endgroup\$
    – NibblyPig
    May 5, 2020 at 14:26

5 Answers 5


Think of the investigation check as 'fighting off the illusory whatever' and working out what is it by doing so

I have always thought the spell was really difficult for a DM, because the natural reaction to a monster appearing is probably to take a swing at it, or run away, rather than ignore it and keep attacking the party. That means that in effect this level 2 spell adds a status effect as well as damage, and becomes pretty powerful quite easily.

I actually ran into this situation the other day, in a strange situation where I was running a DMPC (for reasons) so was both caster and target, and like I normally do I made an attack roll in the first round against the illusion. I have spent the last few days rethinking my ruling.

You are correct that 'I want to investigate this whatever' is probably metagaming, but every action taken can be looked at through a different lens.

I now think of the investigation check as more than just having a thorough look at it, you can RP it as they are fighting off or attacking the illusion, but rather than making an attack roll, you make the investigation roll.

If they fail the save maybe the illusory whatever avoids the blow and no information is gained.

If they succeed then maybe they hit and the illusion didn't react as they would expect, so they realised what it was.

  • 3
    \$\begingroup\$ I like the direction this answer is headed: the target of the illusion probably will try to fight off the illusion, which causes them to make an Investigation check (thus "using their action"). They don't have to choose to investigate the monster, just to interact with it in any way. \$\endgroup\$
    – Mark Wells
    May 3, 2020 at 23:33
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    \$\begingroup\$ I like this because it makes sense with the illusion not being a valid target for most attack, being neither a creature nor object. Attempting to attack triggers the investigation check rather than the character choosing to investigate. +1 \$\endgroup\$
    – linksassin
    May 4, 2020 at 7:29
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    \$\begingroup\$ I think your answer applies to a lot of 'normal' illusions, but phantasmal force is different in that it is purely in their mind and that they will rationalize what is happening per the wording of the spell. How does your approach here work with that (and this is why I made my answer the way I did.) \$\endgroup\$
    – NotArch
    May 4, 2020 at 13:19
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    \$\begingroup\$ I do like this answer, but as NautArch points out, it would be improved by addressing the part of the spell that says "The target rationalizes any illogical outcomes from interacting with the phantasm." As it stands, this part of the spell seems to foil your suggestion... \$\endgroup\$
    – NathanS
    May 6, 2020 at 7:40

There is no one rule

Not only do different tables respond differently to illusions, but the specific illusion and the specific monster and/or their intelligence comes into play with response.

Ultimately, this comes down to a DM decision, and it's not necessarily metagaming. It's just the DM playing/controlling the NPC.


There can still be reasons to investigate the illusion created by phantasmal force.

The description of the phantasmal force spell says (PHB, p. 264; emphasis mine):

You craft an illusion that takes root in the mind of a creature that you can see within range. The target must make an Intelligence saving throw. On a failed save, you create a phantasmal object, creature, or other visible phenomenon of your choice that is no larger than a 10-foot cube and that is perceivable only to the target for the duration. This spell has no effect on undead or constructs.

The phantasm includes sound, temperature, and other stimuli, also evident only to the creature.

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. For example, a target attempting to walk across a phantasmal bridge that spans a chasm falls once it steps onto the bridge. If the target survives the fall, it still believes that the bridge exists and comes up with some other explanation for its fall - it was pushed, it slipped, or a strong wind might have knocked it off.

The key to understanding why a creature might investigate the illusion is the parts I've bolded.

The illusion is only perceptible by that one creature, so if the target has any allies, they might wonder what their ally is doing or talking about (e.g. if the target thinks something's on its back trying to strangle it). The target might say, "Get it off my back!" and its allies might just think "Uh... What are you talking about? There's nothing on your back..." Or if the target sees an illusory pool of lava all around it, it might mention this to its allies, who can clearly see that there's no lava around the target and mention this. Obviously, this gives the target a reason to examine the illusory "strangler" or "lava" more closely.

In addition, the target will come up with rationalizations for any illogical outcomes from interacting with the illusion. For instance, if the target thinks there's something on its back trying to strangle it, it might fall onto its back to try to crush whatever's there - but it will, of course, just land on its back. It might rationalize this by thinking the (illusory) creature swung over onto its chest instead. The target might then roll onto its chest instead (to crush the strangler underneath), or try to grab the illusory strangler - but, of course, this too will fail (maybe the strangler will swing onto the target's back again, or dodge its grasp). At a certain point, the repeated rationalizations are likely to strain credibility, at which point the target has reason to examine the illusion to figure out what's really going on.

Other illusion spells are often a lot simpler to identify as an illusion. For many illusion spells, physical interaction with the illusion usually reveals that it is one - or the illusion can't move or otherwise change as needed to fit the situation. Phantasmal force is certainly trickier to identify in this regard, but there can still be reasons that the target might question the nature of the illusion and thus have a reason to examine it.


Illusions exist - it’s not metagaming to be aware of this

Assuming that you are thinking of this definition of metagaming:

In particular, metagaming often refers to having an in-game character act on knowledge that the player has access to but the character should not.

This isn’t that.

Because illusion magic is an inherent and known part of the world the character lives and grew up in, they know that it exists. Therefore investigating for it isn’t metagaming.

When an antagonist casts a spell and a creature suddenly appears trying to rip your head off there are only 3 possibilities:

  1. It was summoned.
  2. It’s an illusion
  3. Something really esoteric is happening like it was there all the time but hidden or a gate to another plane opened or something similarly unique.

Now, 3. can probably be discounted but 1. and 2. are viable (possibly not equally) so it’s perfectly reasonable to check.

Alternatively, if the DM announces the character has been targeted by XYZ spell then, barring table rules to the contrary, it’s perfectly reasonable that this real-world knowledge reflects in-game knowledge in some way and that the player is free to - and, indeed, should - use what the DM tells them. Remember, the RAW way to play is that DM does not tell the player’s what spells their characters have been targeted with but many (most?) don’t play RAW.

So what if it is?

There’s nothing inherently wrong with playing games intelligently and according to the rules. Metagaming is inherent in every game people play, not just RPG.

It’s only a problem where it exceeds the norms of any given group of players but that’s a problem with establishing and agreeing those norms, not with metagaming per see.

  • \$\begingroup\$ I don't think it's metagaming to be aware that illusions exist in general, but the 'player' (which may be the DM) knowing there is an illusion (which his or her character does not) and thus choosing to make an investigation roll feels exactly like metagaming. \$\endgroup\$
    – NibblyPig
    May 3, 2020 at 20:47
  • \$\begingroup\$ I'll rephrase, it appears to fit your definition of metagaming if the DM/player knows something is illusory and thus chooses to investigate it, when their character does not know and believes it genuinely to be real. \$\endgroup\$
    – NibblyPig
    May 3, 2020 at 20:52
  • \$\begingroup\$ @NibblyPig does the edit help? \$\endgroup\$
    – Dale M
    May 3, 2020 at 21:15

For an illusion like Phantasmal Force, RAW says (PHB, p. 264)

"The target can use its action to examine the phantasm with an Intelligence (Investigation) check against your spell save DC."

It can. Technically it doesn't need a reason. For that matter, if the spell is used against your character, neither do you. The trick is, if you're going to play it as written, how do you rationalize the target deciding to "investigate" something it has no reason to believe is fake?

It helps to think of the mechanics of a check like Investigation a little more broadly, and couple that with a little bit more thought about how magical illusions work. If you read the description of the Investigation skill (PHB, p. 178) it says (emphasis mine)

"When you look around for clues and make deductions based on those clues, you make an Intelligence (Investigation) check."

It doesn't say "You only make an Intelligence (Investigation) check when you look for clues." As an Intelligence check, Investigation is as much about how good you are at putting the pieces of a puzzle together to reach a conclusion as it is about knowing how to examine things.It's the die roll you use to decide whether a character figures something out that is not obvious, but is creating clues.

So it's not necessarily that the NPC decided for some reason to examine the creature more closely. It's more likely that there are clues, and the check is a way to see whether the target catches on to the clues, puts 2 and 2 together, and breaks through illusion's grasp on their mind.

That said, I work in computer simulation during my day job. When you're trying to create simulations of reality, there are infinitely many little factors that reality contains that the illusionist has to try to simulate. Generally you cover most of the big ones, and hope the little ones get overlooked.

But there are going to be discrepancies (the object's shadow doesn't move quite right, a small stone or blade of grass pokes through the illusion's foot, the weight on your back shifts oddly when you try to shake them off, etc.). My rationalization for the RAW in this case is that those discrepancies allow for the target to get clued in that something isn't as it seems, if they have the INT for it, and especially if they are skilled at making deductions from a collection of small clues.

  • \$\begingroup\$ I think that's the way to play it that makes the most sense. "I'm on fire AHH am I really on fire something feels off... checks..." Since it consumes your action though, it seems like you'd focus on not being on fire rather than checking but I guess it would just be broken otherwise. \$\endgroup\$
    – NibblyPig
    May 3, 2020 at 20:50
  • \$\begingroup\$ The problem here is still that making the Investigation check appears to be voluntary. It uses your action, so you have to choose to investigate rather than Attack / Disengage / Hide / whatever else you do when being attacked by a monster. \$\endgroup\$
    – Mark Wells
    May 3, 2020 at 23:43
  • \$\begingroup\$ I don't really see it as a problem. Narratively something seems off, do I divert my attention to that while trying not to get killed, or ignore it and focus on my attacks? Mechanically, the cost keeps illusions from being nerfed by free disbelieve attempts, and keeps you from spamming disbeliefs all the time just in case. \$\endgroup\$
    – Zimul8r
    May 4, 2020 at 0:39
  • \$\begingroup\$ I think you are going to run into gameplay like "it feels a bit off, so I ignore it" since most of the time that is more effective than trying to dispel it. \$\endgroup\$ Jul 7, 2020 at 6:43
  • \$\begingroup\$ This is powerful magic, as powerful as Suggestion and Hold Person. The check is to determine whether you can successfully ignore it. Unless you end it or become no longer affected, it can do damage, in the case of Phantasmal Force up to 1d6 psychic damage per round. Ignoring it without breaking the spell would be no more effective than ignoring an actual creature on your back choking you out. \$\endgroup\$
    – Zimul8r
    Jul 7, 2020 at 13:54

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