While there is more information in PHB 2024 about Illusions thanin 2014, there still isn’t enough, and problems persist in play. What follows is my trying to make sense of the rules so that illusions remain effective but are not game-breaking. Thoughts welcome. I am trying to get playable rules that work.
tl/dr: Illusion magic rules are poorly written but are only meaningful if they are not trivially overcome. Attacking an illusion to make it fade from sight is trivial and by itself should not count as "physical interaction". The final section presents the guidelines I think are implied by the PHB, and are (I feel) balanced for players and NPCs.
Preliminaries.
Illusion magic is cool, and it should be fun for both the player and the DM. The nature of illusion magic in combat (when things are most regulated; a consistent interpretation for combat will solve other applications), isn’t to cause damage (though some illusions do that), but to change the enemy’s behaviour; it is a form of battlefield control, with a lot of narrative freedom for the caster. Against a single opponent, illusions might affect a chosen target; in a crowd, many might be distracted or confused. However, Illusion rules have to be reciprocal, the same for a PC and an NPC, and that’s where the issues start. Illusions aren’t a “win button”, but they also can’t be trivial. (Everything that follows in PHB 2024 only – no legacy features or interpretations.)
Rolling to Disbelieve.
Central to an understanding of illusion magic is the way they are countered (what in AD&D we called rolling to disbelieve). The wording of Minor Illusion (cantrip) and Silent Image (level 1) are sufficiently similar to establish ground rules. For visual illusions with both spells:
1. “Physical interaction with the image reveals it to be an illusion, since things can pass through it.”
2. A creature can take a study action to examine an image. This can “determine that it is an illusion with a successful Intelligence (Investigation) check against your spell save DC.”
Additionally, “If a creature discerns the illusion for what it is, the creature can see through the image.” In both spells, this appears immediately after point 2. It is at least possible, then, that this is a corollary of case 2 – i.e. the discernment and the ability to see through the illusion is a result of making a successful Study action. In this case, to see through an image (a) requires an action, and (b) a successful Intelligence check. This is a high threshold: most monsters are not proficient in Investigation, and the DC of many casters will be quite high. An investigation check can (and often will) fail, and it will take an action.
How is this a high DC? One is a cantrip, the other a 1st-level spell. Not difficult compared to the majority of spells requiring a save.
If case 1, physical interaction, similarly dismisses the illusion, the threshold must also meet a high threshold. Otherwise, case 2 is meaningless: physical interaction would always be preferable.
Are you suggesting physical interaction is difficult? What do you mean by "high threshold"?
What does physical interaction do?
Physical interaction “reveals” the illusion to be an illusion. This is either the same result as a successful Investigation check, or a lesser result; and the most natural lesser result would be coming to the belief that something was an illusion, but not seeing through it. Natural language would suggest it’s the same effect. What do the rules say?
The PHB tells us (p. 24):
If you want to interact with a second object, you need to take the Utilize action. Some magic items and other special objects always require an action to use, as stated in their descriptions.
Interacting is tied to the Utilize action. This suggests to me that a character can have a “free” physical interaction with something within reach once per turn if they believe it is an illusion.
Do other activities count as physical interaction? There is no guidance, but I think the presumption is no. Many play that if you fire an arrow at a suspected illusion you reveal it at range. If so, the bar becomes trivially low, and this is always a better strategy.
What other activities?
If Interaction does create the same result as an Investigation check, then the offsets have to be vaguely commensurate. To my eye, they are.
Investigating:
- INT-dependent chance for failure.
- Can be done at range (though one can imagine a DM imposing penalties, such as disadvantage at more than 30’ or in dim light).
- No risk to self (of falling, burning yourself, whatever).
- The Keen Mind feat lets you Study as a Bonus action.
The time spent studying an object is risk, especially during combat when the character could be attacking instead.
Interacting:
- no chance of failure; auto-success at melee range
- one “free” attempt per round; otherwise costs an action.
- Risk of injury to self
- Possibly requires a free hand (DM call).
- A Rogue Thief can Utilize as a Bonus Action.
Interacting will be preferred most of the time, especially since it can be “free”, but at least there are natural circumstances in which either of these might be used. The separation of the description of the cause (physical interaction) and effect (seeing through the image) is suspicious but may just be bad writing.
How is that "suspicious"? It's simple cause and effect.
Attacking an illusion
Attacking an illusion, with the intent of doing damage, is mechanically different. Normally, if your eyes are closed but you know which square a target is in (when playing on a grid), you attack with disadvantage (Cover is different, and illusions do not provide cover). There is the possibility of doing damage, however to the target. If additionally this also let you see through an illusion, then this is always optimal, and the only reason to Investigate or Interact is to use the one free interaction: the Study Action and the Utilize Action are never preferred.
Is the attacker targeting the illusion or the caster of the illusion? Your use of the word "target" here is confusing.
Attacking:
- takes an action
- Uses optimized attack stat and eventually magic bonuses
- No risk of failure to cause illusion to fade
- Possibility of damage to anyone inside
- Can be done at range
- No risk to self
- Costs a piece of ammunition.
Attacking an illusion that's concealing a trap or monster can be dangerous, thus there is risk.
If an attacker knows (or accurately suspects) where the target is, they can attack with disadvantage. By asking for a roll with Disadvantage, the DM signals something is amiss to the player, which may make them Study or Utilize, but it does not in itself cause the illusion to fade.
Again, what do you mean by "target"? The spellcaster? If so, the spell's caster may not be present when the character(s) encounter the illusion.
Consider, now, a gnome rogue casting Minor illusion of a 5’ crate with holes in it (or slats, whatever) as something to Hide within. Yes, you can Hide in it, and the enemy might not see you, but they have reason to know where you are, and can attack that location (at disadvantage, which more or less corresponds to ¾ cover anyways). Depending on the nature of the illusion, you might not need to Hide: an illusory door in a doorframe may arouse no suspicion at all (DM’s call).
I'm curious how the enemy has reason to know the gnome is under the crate? Is it out of place? How so? Also the illusion is providing concealment (hidden = invisible) not cover.
Anyone witnessing such an attack, though, may suspect something is an illusion, and either continue to fight through it (at Disadvantage), or spend an action (or a free interaction) to try to disable it. But, importantly, each person seeing the illusion must see through the illusion on their own (it’s like those Magic Eye autostereograms).
But the attack is "physical interaction" which would expose the target as an illusion to any one watching, correct?
Suspecting an illusion
You might believe something is an illusion even though you can see it. This could happen for a number of reasons:
- Someone tells you it is an illusion and you believe them (it is not enough for someone else to have successfully seen through the illusion);
- You suspect from the description from the DM that something is amiss (e.g. context doesn’t match setting);
- You have just seen another illusion being cast (the “fool me once” principle);
- You have taken an attack and the attack has passed through the illusion.
Suspecting something is an illusion, or witnessing someone interact with it, doesn’t remove the illusion for you, but may prompt you to Study it (a Perception check may reasonably be required if doing something else, like fighting).
The second one (GM description) is Meta-gaming, which is okay as long as the group has approved that kind of play.
Where in the rules does it state that if one character exposes something as an illusion via physical interaction, other characters watching that interaction still think the target/object is not an illusion? That looks like your opinion ...
Illusion rules for the PHB 2024.
The clarifications I want are pretty minimal, and with them none of the above would be needed. Of the following, 1 and 2 are in the PHB, but not stated as clearly as I would like; 3, 4, and 5 are, I believe, implicit, but should be spelled out.
1. Two things cause an illusion to dissipate/become faint for you:
- Study action investigation check (usually within 30’ in bright light).
- Move within 5’, take free interaction (or Utilize action).
What if 5' isn't close enough for physical interaction (can a gnome reach 5' up)?
2. You can’t attack an illusion, but if you know where an enemy is within an illusion, you can attack it with Disadvantage.
Why can't you attack an illusion that is within range of the attack? Do you mean "you can't attack a target concealed by an illusion"?
3. A creature who is aware of the illusion, but does not dissipate it, is still impacted by the illusion.
RAW, Physical interaction or a successful use of Investigation doesn't "dissipate" an illusion. They just reveal the illusion isn't real.
4. An illusion does not provide cover, but it does block line of sight, giving something you can Hide behind or within.
It "can" block LoS depending on its size in relation to the characters affected by it. "... but it can provide concealment ..." is a better use of words
5. Only those who have already seen through an illusion can offer Help to others.
"Those" meaning anyone who has experienced (via their senses) that the illusion isn't real?
An illusion will typically slow down an enemy, and make them change their behaviour. A spell is still constrained by concentration, area of effect, and perhaps range.
How does it slow down an enemy if it has no substance? Do you mean "the study of an illusion can slow down the enemy"?
Spells are also limited by duration (beyond concentration).
Finally, the Illusionist’s Improved Illusion ability has three effects: many illusion spells will have greater range; additionally, “You can create both a sound and an image with a single casting of Minor Illusion, and you can cast it as a Bonus Action.” The first of these makes an illusion more credible, and enemy viewers are less likely to want to Investigate. Its biggest impact will be out of combat, though. The second, though, means that Minor Illusion will be cast regularly in combat. And that’s why more robust Illusion rules are needed.
It looks like you might be overthinking illusions and how they impact their surroundings. Or maybe not. Overall I like that you brought the subject up since these discussions can lead to a stronger understanding of the rules for us
