D&D 5E Problems with Illusions

[Coroc]

I want to think about illusions.

Illusion spells are tricksy, and are inevitably subject both to player creativity and DM judgement. My sense is that there is no agreement about how they work, and want to think about what are the legitimate uses of spells from this school. And this leads to problems.

I’m thinking specifically of Minor Illusion (cantrip), Silent Image (level 1), and Major Image (level 3), but there are implications for Hallucinatory Terrain (level 4), Programmed Illusion (level 6), Project Image (level 7). With each of these spells, the player defines some sort of image created. If the enemy interacts with it physically, it is revealed to be an illusion. If a creature discerns the illusion, it becomes possible to see through it (where it presumably no longer has an effect). Finally, an enemy can use an action to attempt to discern the illusion through an Intelligence (Investigation) check.

The first thing to note is how the three spells scale.
Minor: 5’ cube, affects either sight or sound. 1 minute, no concentration. Objects only.
Silent: 15’ cube, affects sight only. 10 minutes, concentration. Object or creature.
Major: 20’ cube, affects all senses but no damage. 10 minutes, concentration. Object or creature.
The variables therefore are size, senses affected, duration, and type of thing represented.

Specialist Illusionists gain the ability to amp up Minor Illusion (sight and sound, at level 2) and to change the nature of the illusion after it has been cast (for each of these three spells, at level 6). The upper level spells have specific uses, but are not for creative illusions in combat.

Tricksy elements. Let’s start with two basic uses in combat (I know there are lots of non-combat uses of illusions; I’m not thinking about them for now).

Cover. I am in a ranged combat and cast any of these spells to make a defensive barricade, no longer than 5’ wide and 5’ tall. Poof. Maybe I allow myself arrow slits, or a peekaboo window. Do I have 3/4 cover? I think I do, until someone interacts with it. That means they have -5 to hit me.

Problems:
* An enemy fires an arrow at me anyway. If they hit me, have they interacted with the illusion? No. If they miss me, have they interacted with it? Probably, but we don’t want any weirdness that interaction happens if you only interact if you come within 5 of the target number. Does any miss then count as interacting? Does the enemy have any chance to hit? Already things are subjective.
* Further, if the enemy does hit me (after -5), I make a concentration check if it’s a level 1 or 3 illusion, but not if it’s a cantrip. That also seems lame. Why would anyone use anything other than Minor Illusion to generate cover, except to make something 15’ or 20’ wide?
* What about choosing to disbelieve (make an Int (investigation) check). That uses an action, but (unlike attacking) has no ability to do me damage. It also can fail (and is more likely to do so, in most cases). I see no reason someone would choose to disbelieve.
* Maybe we need to bring in knowledge of spells. Does a successful Arcana check (not as an action) allow someone to know that there are no spells that could have brought this into existence except by being an illusion?
* Once one of the enemies has seen through the illusion, and they tell their friends, there is still no benefit for the friends, who must also somehow interact or disbelieve.
* What about my allies? They know I am an illusionist, and have seen my defensive barricade before. Can they see through it? Do they need to disbelieve? What if I have told them (maybe telepathically) that “I’m casting an illusion”?

None of this feels satisfactory to me. If it is fun (rule of awesome, etc.), it is only fun for the caster, and it would not be if an enemy caster did it to the party.

Distraction. So I’m in combat, and I see one enemy, a presumed spellcaster, hanging back. I cast an illusion of a brick wall in front of him, or a swarm of silent bees around his head. He knows or suspects it’s an illusion. Is my illusion (a 0- or 1-level spell) enough to shut him down for a turn, while he “interacts”? Is casting a fireball beyond where you can see (i.e. on me) interacting with an illusion at him? I don’t think so.

Problems.
* Is this a fair way to shut someone down for a full turn? Is it a more effective way to shut down a specific individual than color spray? Is there any way around this? Can a BBEG spellcaster consistently be occupied by a repeatedly cast cantrip? (which, given the players have multiple actions available to them, could be very powerful).
* Again, what about my allies? Can they get past the swarm of bees unaffected? Apparently. And do incoming arrows from my side create “interaction” to disempower the illusion for the enemy?

There are other issues.
Repeat castings. An enemy disbelieves the illusion. So I cast it again. As a DM, I’d give them at a minimum a bonus to save, but is this even right? What if it’s the same illusion, in the same place? This strategy could work both for both of the basic uses above.

Sensory input. Problematically, it doesn’t seem to matter what senses the illusion affects. As long as sight is affected, it can occupy someone. Silent bees are just as distracting as buzzing ones. This means that the level 2 illusionist ability doesn’t gain anything as a combat use.

Triggers. What prompts someone to investigate an illusion, in or out of combat? For players, an omission in the DM description may be enough, but what about NPCs? Noyhing the playrs do make it easier or harder to determine what happens next. Creativity is not rewarded.

Sounds. Can NPCs get distracted by noise? Does the sound of a lion or call for retreat or the opening chord of A Hard Day’s Night or the sound of someone shouting “Sneak Attack” actually impact things in combat? I think it doesn’t. Or it requires interaction, in which case…?

All of this just feels so sloppy. I don’t like casting these spells, because they require negotiation with the DM, and I don’t like it when players cast them when I DM for the same reason.

Are these problems inevitable? Are there ways to make illusions work smoothly? Why would someone want to be an illusionist?

Help. What do you do in your games to make illusions fun for all?

Great post especially for pointing out that the dm needs to rule it in these cases else a cantrip is more powerful than a level spell.

With your cover example: Try to view it like an illusionary shield in the hand of the caster, the enemy sees it is there but for the three image spells it does not have no actual physical properties, the same an illusory box to cover a light source just will not work.

Therefore no -5 for cover for you, sorry.
Means: the enemy sees your cover and tactically decides to attack something else (eventually!), or he sees it and starts disbelieving because it appeared out of nowhere.

Now if someone interacts by shooting at you he should check, depending if you were hit anyway the arrow seemingly flew high or vanished into the "barricade"

I would set Int DCs depending on circumstances in this case.

The higher level illusions would create audible components so impact of a projectile eventually.

The example with repeated use to block sight of an enemy spellcaster or lets say another ranged class: he got movement too on his turn, as soon as he discovers your trick he will be quite immune to repetitions of it, therefore it works until he first disbelieves / interacts

Your allies: I would rule they know your tricks and automatically disbelieve they see your illusion as a diluted image so they know what you did and can act accordingly.

 

[Gadget]

As you say, illusion spells are tricksy. But I usually find a careful reading of the spell description, combined with comparison to spell effects of the same or similar level, helps quite a bit. But there is no getting around the fact that DM adjudication is required in many cases.

The first thing to note is how the three spells scale.
Minor: 5’ cube, affects either sight or sound. 1 minute, no concentration. Objects only.
Silent: 15’ cube, affects sight only. 10 minutes, concentration. Object or creature.
Major: 20’ cube, affects all senses but no damage. 10 minutes, concentration. Object or creature.
The variables therefore are size, senses affected, duration, and type of thing represented.

One of the key features not listed here is movement. Minor Illusion only creates stationary objects, with the others, the caster can spend an action to cause the illusion to move. Now, there is some interpretation here: can I use Minor Illusion to cause the Duke's Crest to appear on my cloak, or does it remaining hanging in the air where I cast it when I move? The concentration requirement on Silent & Major spells is onerous though.

Cover. I am in a ranged combat and cast any of these spells to make a defensive barricade, no longer than 5’ wide and 5’ tall. Poof. Maybe I allow myself arrow slits, or a peekaboo window. Do I have 3/4 cover? I think I do, until someone interacts with it. That means they have -5 to hit me.

Illusions do not provide cover, they give you concealment. Opponents may think you have cover though. Unless you make special effort to limit yourself to shooting through the 'arrow slits' (which would limit your field of fire, which may have a mechanical penalty), seeing arrows emerge from a wall may count as 'interacting' after the initial (surprise?) barrage.

Problems:
* An enemy fires an arrow at me anyway. If they hit me, have they interacted with the illusion? No. If they miss me, have they interacted with it? Probably, but we don’t want any weirdness that interaction happens if you only interact if you come within 5 of the target number. Does any miss then count as interacting? Does the enemy have any chance to hit? Already things are subjective.

The enemy has to deal with your partial or full concealment, if their attack roll overcomes that, they hit you. On a miss, DM adjudication comes into play, depending on how far away they are and what the illusion is depicting, it could be they do not notice their arrows going through the wall or can't tell. Personally, after several 'misses' (without other distractions) I would rule that having arrows go 'through' a solid wall counts as interacting and they no longer have the partial/full concealment penalty.

* Further, if the enemy does hit me (after -5), I make a concentration check if it’s a level 1 or 3 illusion, but not if it’s a cantrip. That also seems lame. Why would anyone use anything other than Minor Illusion to generate cover, except to make something 15’ or 20’ wide?

Agreed.

* What about choosing to disbelieve (make an Int (investigation) check). That uses an action, but (unlike attacking) has no ability to do me damage. It also can fail (and is more likely to do so, in most cases). I see no reason someone would choose to disbelieve.

Being attacked by a silent swarm of bees may be a reason in and of itself.

* Maybe we need to bring in knowledge of spells. Does a successful Arcana check (not as an action) allow someone to know that there are no spells that could have brought this into existence except by being an illusion?

I would allow a successful Arcana check while the illusionist is casting to determine that the spell being cast is an illusion spell, thereby prompting the observer to spend and action to 'investigate' any phenomena that appeared suddenly. This is as far as I would go unless I felt there was an repetitive abuse of illusions going on.

* Once one of the enemies has seen through the illusion, and they tell their friends, there is still no benefit for the friends, who must also somehow interact or disbelieve.

Correct. But their friends may not have known to spend the action to investigate or interact without the hint.

* What about my allies? They know I am an illusionist, and have seen my defensive barricade before. Can they see through it? Do they need to disbelieve? What if I have told them (maybe telepathically) that “I’m casting an illusion”?

Your allies must interact or investigate. A simple way around this is stepping through the illusory object after you cast it to show them (assuming you have the time/opportunity to do so), otherwise they can just assume that 'wall' that appeared is one of your tricks and run right through it without pause (followed by a nasty surprise when you get your Illusory reality ability and want to mess with them).

None of this feels satisfactory to me. If it is fun (rule of awesome, etc.), it is only fun for the caster, and it would not be if an enemy caster did it to the party.

Much of this can be alleviated if the DM is consistent with their rulings and the players become familiar with the advantages and disadvantages with using these spells. But yes, it is difficult.

Distraction

So I’m in combat, and I see one enemy, a presumed spellcaster, hanging back. I cast an illusion of a brick wall in front of him, or a swarm of silent bees around his head. He knows or suspects it’s an illusion. Is my illusion (a 0- or 1-level spell) enough to shut him down for a turn, while he “interacts”? Is casting a fireball beyond where you can see (i.e. on me) interacting with an illusion at him? I don’t think so.

A swarm of 'silent' bees around him may count as 'interacting'. If you made it so the 'swarm' was not quite around him, I might require a concentration check for him to get his spell off, but then again, the bees are silent, not as much of a distraction. Also, you cannot create a swarm of silent bees with Minor Illusion, as you cannot create creatures or movement with it, so it would have to be Silent Image. A wall would block his line of sight (assuming it is positioned properly) until he investigates or interacts. I might allow one of his allies pushing through or shooting through the wall to count as interacting, depending on the situation.

Problems.
* Is this a fair way to shut someone down for a full turn? Is it a more effective way to shut down a specific individual than color spray? Is there any way around this? Can a BBEG spellcaster consistently be occupied by a repeatedly cast cantrip? (which, given the players have multiple actions available to them, could be very powerful).

It depends on the situation. I have already pointed out some of applications of the spell that would not work properly. Repeated attempts of the same thing might incur a bonus action Investigation Check with advantage or something, but it all depends on what specifically the spell(s) are being used to do.

* Again, what about my allies? Can they get past the swarm of bees unaffected? Apparently. And do incoming arrows from my side create “interaction” to disempower the illusion for the enemy?

Anyone can get past a swarm of Illusory silent bees unaffected. Would your allies have an advantage with ignoring them if they knew (because you told them or this is a favorite tactic)? Probably. Remember, you are spending a 1st level spell to create this 'swarm', and your action to have them move like a swarm would.

There are other issues.
Repeat castings. An enemy disbelieves the illusion. So I cast it again. As a DM, I’d give them at a minimum a bonus to save, but is this even right? What if it’s the same illusion, in the same place? This strategy could work both for both of the basic uses above.

The enemy could just choose to ignore said illusion cast repeatedly if that is at all beneficial, knowing it for what it is. If repeated castings become a problem, I could see myself ruling that 'seeing through' the first effect counts against further manifestations for the next while.

Sensory input

Problematically, it doesn’t seem to matter what senses the illusion affects. As long as sight is affected, it can occupy someone. Silent bees are just as distracting as buzzing ones. This means that the level 2 illusionist ability doesn’t gain anything as a combat use.

You seem caught up in the Silent Bee plan. I would argue that Silent Bees are not as distracting as buzzing ones, and either are easy to overcome through object interaction. Since 'Silent Bees' requires Silent Image and your action to make it at all plausible, yes the 2nd level Illusionist ability does not come into play here. Truth be told, it is difficult to think of a situation where that ability is all that beneficial, since immobile objects rarely make sounds.

Triggers.

What prompts someone to investigate an illusion, in or out of combat? For players, an omission in the DM description may be enough, but what about NPCs? Noyhing the playrs do make it easier or harder to determine what happens next. Creativity is not rewarded.

Well a swarm of silent bees would enough for me. If something seems suspicious, then an investigation check will likely follow. The creativity is in not making your opponent suspicious.

Sounds

. Can NPCs get distracted by noise? Does the sound of a lion or call for retreat or the opening chord of A Hard Day’s Night or the sound of someone shouting “Sneak Attack” actually impact things in combat? I think it doesn’t. Or it requires interaction, in which case…?

I would think this falls squarely under DM Adjudication. Also, don't be a jerk DM if the effect is plausible and believable for the situation. Opinions will obviously vary on what is plausible and believable. Did I mention the DM should not be a jerk?

All of this just feels so sloppy. I don’t like casting these spells, because they require negotiation with the DM, and I don’t like it when players cast them when I DM for the same reason.

Are these problems inevitable? Are there ways to make illusions work smoothly? Why would someone want to be an illusionist?

Yes, it is more wishy-washy, but this has been a problem with illusions since the beginning. Like I said, a careful reading of the spell and comparison of the desired outcome to other similar level effects helps tremendously. Things like you can't turn Silent/Minor Image into an in place invisibility spell that allows you to attack from 'cover' with impunity for the combat, etc. Consistency helps as well.

 

[Caliban]

Help. What do you do in your games to make illusions fun for all?

Nothing. I hate them and you deserve whatever happens if you cast one.

 

[neogod22]

The funny thing is, there way better illusion spells that can be cast in combat, and are actually for combat. These spells the OP are fixated on are definitely not the best for combat are are more useful for out of combat situations. Magic in general, and illusions especially. The easiest way to stop people from disbelieving your illusions is to not use them exclusively. Try changing out that illusionath wall of stone for a real one after they run through the fake, they will probably think it's a fake too and run into it. Lol

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[Kobold Stew]

Thanks, all for your responses so far.

Really the trick to illusions is to create things that won't make the creatures want to question it. Like for example creating a cloud of fog.

Sage advice, for what it is worth, is clear that atmospheric effects are precluded.

* The DM's job is to make the heroes feel like heroes. Interpreting the illusions in ways that allow the PCs to be creative and effective is not a problem - it is giving them the great story they want to tell!

Agreed, but part of the challenge is establishing standards that allow this and are still fun when applied to NPCs. That is a big part of where I get stuck.

* There are a lot of threads on illusions that go through all of the standard arguments. I suggest digging up some of them if you want to get the discussion you seek a bit faster than waiting for it to repeat here.

I have not found many that discuss illusions for 5e, and those that do are not looking at the school generally. If there are some you have found helpful, please point them out, as I may have missed them.

I’m not sure there’s any further quantification that can be done short of redoing the spells entirely with an intense amount of granularity (i.e., Create Small Illusory Monster, Create Large Illusory Object, etc.) that I don’t think most people would want in their game. I think there will always be a degree of subjectivity to illusions, and it’s probably a good idea to have a conversation with your DM if you plan on making use of them heavily.

Agreed. I am certainly not suggesting a mass re-write. But I do not see clarity in what we have.

The easiest way to stop people from disbelieving your illusions is to not use them exclusively. Try changing out that illusionath wall of stone for a real one after they run through the fake, they will probably think it's a fake too and run into it. Lol

Though I did have a fun occasion where an illusionist made a fake fire, and then cast bonfire, which I had a half-orc stand just to prove it was fake.

Thanks for these. I get that, even if it is not a solution a dedicated illusionist might want.

 

[Gadget]

One thing more, as linked above in Sage Advice, I would not allow the Image spells (or at least the lower level ones) to create or alter lighting conditions, thereby turning them into poor man's light or darkness spells. This is not readily apparent from the spell descriptions. Minor Illusion should not be allowed to be a poor man's Fog Cloud either, I'm unsure about Silent Image as the spell description says: "You create the image of an object, a creature, or some other visible phenomenon that is no larger than a 15-foot cube." A fog cloud could be considered to fall into the "some other visible phenomenon" aspect.

In fact, I'm not sure my above statements about movement on Silent Image are quite RAW. It could be interpreted that the 'movement' portion of the spell description for Silent Image refers to Movement outside the 15' cube of the effect, just like the effects on the victim of the Booming Blade spell only trigger when the subject uses some or all of its allotted movement speed, not scratching his nose, defending, attacking etc. As such, a swarm of silent bees that remained in the 15' cube could be allowed without the caster spending additional actions. Or a banner flapping in the wind. Or a sentry pacing back and forth withing the cube? I'd be inclined to allow all of these, though the creature pacing back and forth sounds a bit too much for some reason. If you want to have the Image move beyond the 15' cube or appear to react or interact with the environment, then I would require an action from the caster.

 

[Shadowdweller00]

Sage advice, for what it is worth, is clear that atmospheric effects are precluded.

That is explicitly for Minor Illusion - which specifies object. The higher-level image spells allow for "object, creature, or some other visual phenomenon" which would tend to include fog.

 

[neogod22]

You know what minor illusion is excellent for? Giving arcane tricksters something to hide behind.

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[Li Shenron]

Illusion spells are tricksy, and are inevitably subject both to player creativity and DM judgement. My sense is that there is no agreement about how they work, and want to think about what are the legitimate uses of spells from this school. And this leads to problems.

Yes there's always a good deal of subjectivity when it comes to illusions, so be prepared to read opinions rather than factual solutions

Cover. I am in a ranged combat and cast any of these spells to make a defensive barricade, no longer than 5’ wide and 5’ tall. Poof. Maybe I allow myself arrow slits, or a peekaboo window. Do I have 3/4 cover? I think I do, until someone interacts with it. That means they have -5 to hit me.

I wouldn't grant cover from an illusion, only concealment. So use it for hiding purposes. It is not obvious if the caster can automatically see through her own illusions. If you allow that, then obviously you always create the illusion of total cover.

* An enemy fires an arrow at me anyway. If they hit me, have they interacted with the illusion? No. If they miss me, have they interacted with it? Probably, but we don’t want any weirdness that interaction happens if you only interact if you come within 5 of the target number. Does any miss then count as interacting? Does the enemy have any chance to hit? Already things are subjective.

Assuming we're talking about partial cover, I might give them a check to disbelieve if they hit when they should have hit the cover. Say e.g. your normal AC is 14 and behind cover it would be 19. If they roll >19 then they hit you and don't notice anything weird; if they roll <14 they miss both you and the cover and don't notice anything; if they roll 14-18 they hit you while they should have hit the cover and thus get a check vs Illusion to understand why.

(But I might also not. I don't think it's absolutely necessary to grant such check. It's just as possible that in the midst of battle and at a certain range, an archer might not immediately notice that his arrows should hit the wall and are instead going through it. I might also decide to grant the check after a few rounds, but not the first time.)

If you created total cover, they might see your attack spells coming from that direction, and decide to shoot towards that, in which case again they'll see their arrows disappear into the wall, and this can count as an interaction enough for the purpose of revealing the illusion to them.

* Further, if the enemy does hit me (after -5), I make a concentration check if it’s a level 1 or 3 illusion, but not if it’s a cantrip. That also seems lame. Why would anyone use anything other than Minor Illusion to generate cover, except to make something 15’ or 20’ wide?

If the cantrip is better, then don't use anything higher level... it's a perk

* What about choosing to disbelieve (make an Int (investigation) check). That uses an action, but (unlike attacking) has no ability to do me damage. It also can fail (and is more likely to do so, in most cases). I see no reason someone would choose to disbelieve.

Uhm... where are you getting this "choose to disbelieve" option? There is usually a possible voluntary investigation activity described in illusion spells, although I'd say a more proper way to talk about it is a PC "choosing to investigate". Maybe it's just semantics, but "I choose to disbelieve" to me sounds like what you do when your PC decides to act regardless of the illusion. A famous example is Indiana Jones (in the Last Crusade) stepping towards the chasm without seeing the bridge.

Investigating is supposed to the a safer choice, and more "expensive" in terms of actions because it's an activity on its own. Indiana Jones doing this would have meant him prodding forward with hands or a stick, or throwing the rocks before trying to walk.

In combat with some illusionary cover it's assumed that the attacker has no reason to suspect, and so investigating shouldn't even cross his mind. Interacting casually doesn't always have to grant a check, but (as I said previously) I might decide to grant it anyway. In a way, the illusion has probably already served some purpose, as maybe others have not tried to attack the target because of it.

* Maybe we need to bring in knowledge of spells. Does a successful Arcana check (not as an action) allow someone to know that there are no spells that could have brought this into existence except by being an illusion?

I am not too fond of this idea.

* Once one of the enemies has seen through the illusion, and they tell their friends, there is still no benefit for the friends, who must also somehow interact or disbelieve.

But they can choose to believe their ally and start shooting at the target even tho they still see the cover. As the illusion doesn't grant the AC bonus because it doesn't stop the arrows, it makes sense to do so.

* What about my allies? They know I am an illusionist, and have seen my defensive barricade before. Can they see through it? Do they need to disbelieve? What if I have told them (maybe telepathically) that “I’m casting an illusion”?

They don't see through it, but they can know it's an illusion if you tell them so.

Distraction. So I’m in combat, and I see one enemy, a presumed spellcaster, hanging back. I cast an illusion of a brick wall in front of him, or a swarm of silent bees around his head. He knows or suspects it’s an illusion. Is my illusion (a 0- or 1-level spell) enough to shut him down for a turn, while he “interacts”? Is casting a fireball beyond where you can see (i.e. on me) interacting with an illusion at him? I don’t think so.

He can choose to cast through the wall, but it's risky (what if you instead cast a real wall? he cannot know for sure), and as long as the wall is visible he might not be able to target his spells beyond it.

BTW, I don't think you can use Minor Illusion to create illusionary creatures, only objects.

* Is this a fair way to shut someone down for a full turn? Is it a more effective way to shut down a specific individual than color spray? Is there any way around this? Can a BBEG spellcaster consistently be occupied by a repeatedly cast cantrip? (which, given the players have multiple actions available to them, could be very powerful).

It could be a valid tactic, but not fool proof. After the first illusion, he might choose to ignore the other illusions by assuming they are in fact illusionary, and so not need to spend an action to interact with each one. But they might still block vision, which in turn can block targetting with certain spells. And who knows if you decide to conjure something real after the first couple of illusions instead...

* Again, what about my allies? Can they get past the swarm of bees unaffected? Apparently. And do incoming arrows from my side create “interaction” to disempower the illusion for the enemy?

Indirect observation might give them the suspect, but I think only direct investigation (if successful) gives them proof.

Repeat castings. An enemy disbelieves the illusion. So I cast it again. As a DM, I’d give them at a minimum a bonus to save, but is this even right? What if it’s the same illusion, in the same place? This strategy could work both for both of the basic uses above.

As before... the next illusion remains effective unless they interact with it, but they may choose to ignore it.

Imagine someone casting a brick wall illusion to run away. A pursuing enemy stops at it, and spends an action trying to prod the wall, and figures out it's an illusion so decides to walk though it. Then the fugitive casts another wall illusion then another... The pursuer might decide to skip prodding and run straight into any walls beyond, faithful that they're all illusionary. If they are, then good for him, but if they aren't...

Sensory input. Problematically, it doesn’t seem to matter what senses the illusion affects. As long as sight is affected, it can occupy someone. Silent bees are just as distracting as buzzing ones. This means that the level 2 illusionist ability doesn’t gain anything as a combat use.

Then it means the ability is not meant to provide a combat benefit.

Triggers. What prompts someone to investigate an illusion, in or out of combat? For players, an omission in the DM description may be enough, but what about NPCs? Noyhing the playrs do make it easier or harder to determine what happens next. Creativity is not rewarded.

The DM roleplays the NPCs as best as she can. I wouldn't metagame and have the NPCs do the right thing just because as a DM I know what it is. I'd just try to make them behave reasonably.

Creativity can definitely be rewarded. A player who goes into the specifics of what the illusion should look like (instead of just staying on the generic) will certainly affect my decisions on what the NPCs would do.

Sounds. Can NPCs get distracted by noise? Does the sound of a lion or call for retreat or the opening chord of A Hard Day’s Night or the sound of someone shouting “Sneak Attack” actually impact things in combat? I think it doesn’t. Or it requires interaction, in which case…?

Yes they do. Distraction is the main point of an auditory illusion, in or out of combat. Whether it works, it depends on whether the caster has a good idea and on what kind of enemies. The result doesn't have to be major, such as the enemies actually retreating, it could be something minor such as not choosing the best possible action.

Interaction with sounds is much less obvious to define, because you can't verify by touch. It might be that just not seeing any lion around after a round or two could be a reason to be granted a check to recognize it's an illusion.

 

[Kobold Stew]

Thanks for the detailed response!

Uhm... where are you getting this "choose to disbelieve" option? There is usually a possible voluntary investigation activity described in illusion spells, although I'd say a more proper way to talk about it is a PC "choosing to investigate". Maybe it's just semantics, but "I choose to disbelieve" to me sounds like what you do when your PC decides to act regardless of the illusion.

As I hoped was clear in the passage you quoted, I used the legacy language of disbelieving to map onto the 5e language of "make in Intelligence (Investigation) check". Because it's not a real investigation taking place -- it's an action that can take place at range, just by looking. (How do I know this? because it's something separate from interacting with the illusion -- so it's an investigation, but without an interaction. I think it's a bad design that was overly influenced by the legacy of disbelieving illusions.

And how does a PC investigate? By squinting?

The Intelligence check is fishy in other ways. How many monsters have proficiency in Investigation? To my knowledge, only the Deep Gnome and the NPC (appendix B) Spy have proficiency in Investigation. So it's also an unfair expectation, that gives an advantage to PCs (without DM fiddling).