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I'm playing a Bard in my current campaign and I like to use illusions to control the battlefield. Think of casting walls, boxes, and other goodies to hide or split the battlefield.

Now, the DM usually magnificently determines on the next turn that a monster investigates or touches the illusion to break it diminishing it's effect or usage entirely.

What are commonly accepted rules or gentlemens agreements on breaking illusions? If any. Do I have grounds for an argument?

On request, mainly for spells minor illusion and major illusion. Phantasmal force is quite clear and after a brief argument that is settled.

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    \$\begingroup\$ Can you specify which exact spells your character is using? All of the rules of illusions are spell-specific, so any answer would need to address the specifics of the particular spells being used. \$\endgroup\$
    – Marq
    Jul 16 '17 at 16:01
  • \$\begingroup\$ Updated my question. Question is mainly for minor and major illusion. Phantasmal force is quite clear and after a brief discussion we settled on that spell. \$\endgroup\$
    – bastijn
    Jul 16 '17 at 16:05
  • \$\begingroup\$ Most illusions are broken if you touch it, and you succeed on an investigation check. Even then, only you can see through it, not your allies. \$\endgroup\$
    – András
    Jul 16 '17 at 16:09
  • \$\begingroup\$ But the rules say "physical interaction with the image reveals it to be an illusion, because objects can pass through it". Separate from the lines about the investigation check. \$\endgroup\$
    – bastijn
    Jul 16 '17 at 16:10
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Minor Illusion and Major Image have the following paragraph (slight differences in the last sentence, to include the additional features of Major Image.

Physical interaction with the image reveals it to be an illusion, because things can pass through it. A creature that uses its action to examine the image can determine that it is an illusion with a successful Intelligence (Investigation) check against your spell save DC. If a creature discerns the illusion for what it is, the illusion becomes faint to the creature.

So, there are two ways to look through an illusion:

  1. If anything physically interacts with the image, it is revealed as an Illusion (automatically)
  2. If a creature uses its action and succeeds on a Intelligence (Investigation) check

What are fair ways for the GM to interact with those Illusions?

The core should be, that the GM does not use his meta knowledge.

That means if those illusions are cast out of the enemies sight the enemies would interact with it as if the illusion is real. You could use an illusory wall as full cover and peek around to attack your enemies, but as soon as you attack through the wall, the enemies will know that something is up. (physical interaction)

An exceptional case would be "what happens if you have three-quarters cover behind an illusion?" the core principle would be, that the enemy would try to avoid your cover and thus you could benefit from the cover bonus, but what happens if he misses? Does he hit your cover and reveal it as an illusion? That is up to your DM to decide.

(I personally would say, if the attack would hit if there were no cover bonus, the target hits the illusion and reveals it as such, but one can argue that the attack should have hit you then, too.)

If you happen to cast the spell while your enemies can see it there should be a few differentiations:

  1. an enemy who is stupid (beasts, low-int creatures)
  2. an enemy who is intelligent, but has little to no experience with magic (maybe bandits, barbarian tribes, more intelligent beasts)
  3. enemies who are intelligent and are either able to cast those spells themselves or at least know they exist

The first group should interact with an illusion as if it is true and avoid it.

The second group may use their action to determine if it is an illusion and tell their comrades of their results and then react accordingly.

The third group would know that this is probably an illusion and ignore it. Attack your spot based on position or just by listening (attack unseen target). The only reason for them to look closer to the illusion would be if they need to see their target for their spells. And as soon as the first enemy shoots through the illusion it would reveal itself.

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    \$\begingroup\$ This is only fair if the second group would use its action to examine a Wall of Stone too. \$\endgroup\$
    – András
    Jul 16 '17 at 17:31
  • \$\begingroup\$ "Cast the spell while your enemies can see it" meaning if I cast it to bring the illusion of a wall of stone that splits the room in two, dividing a group of attackers, the above will hold? They would question the sudden appearance of a wall. Groups one and two can be controlled by it, either entirely or forcing an action of at least one and potentially every member of the group (see comment above this one). Similar leaving a choke point would direct them to enter our section of the room via the choke point. \$\endgroup\$
    – bastijn
    Jul 16 '17 at 18:28
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    \$\begingroup\$ My main point is, that you should avoid metagaming, and as a DM differentiate between creatures which are stupid, intelligent and those who know enough about magic. András is right, that if you want to be 100% fair an enemy who examines a Major Image should also examine a Wall of Stone, but this part is completely decided by your DM. The only objective point one can make, is that you should avoid meta-gaming (knowledge which spell is cast, either illusion or actual wall, is included in this point) and different creatures should react differently. I would base that on intelligence. \$\endgroup\$
    – Thyzer
    Jul 16 '17 at 18:49
  • \$\begingroup\$ Another point one can make is, that an enemy "examines" a wall by just doing a simple shot/attack at it. This would take less time (1 attack instead of the whole action) and guarantees a valid result. It is really up to the DM. \$\endgroup\$
    – Thyzer
    Jul 16 '17 at 18:51
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    \$\begingroup\$ What's also relevant is how familiar the enemies are with the battlefield. Even less intelligent creatures would become suspicious when there is suddenly a wall right in their lair which wasn't there before. \$\endgroup\$
    – Philipp
    Jul 17 '17 at 10:35

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