The minor illusion spell has been a major pain in the butt for me as a DM. My group is a bunch of math professors, a medieval history guru, and a biology PhD. Their ability to get creative with interpretations is... challenging. This is the problem with 5e and a bunch of math nerds - we love rules and without them, we assume there are no boundaries or we reach the edges of those grey boundary lines and jump up and down on them.
So... the uses that they are putting the spell minor illusion to are foiling even high-level encounters. As a DM, I do not want to discourage the use of creativity, but the ability to create any visual (stationary) illusion in a 5-foot-cube at will as a cantrip is incredibly over-powered. How do you deal with this?
I know that enemies can take an action to disbelieve, but to be honest, the way the players are using the spell (as walls, doors, images of themselves) wouldn't provoke the desire to disbelieve (otherwise, each combat would start with everyone taking an action to disbelieve.
Are you stuck in a pre-made adventure with a pre-made grid and your players won't let you adapt? If so it's hard to offer advice! I don't know an in depth example. But if you have room to get creative I'd start pre-planning enemies with built-in tools to foil fake walls, higher stats to make disbelieving easier. Maybe reinforcements show up and snap bad guys out of their disbelief. The big prize gets relocated by a reactive and increasingly paranoid ecosystem.
You never want them overdoing the same puzzle solution repeatedly! It's worth improvising some odd reactions from the universe.
Minor Illusion can potentially be a major pain. Just remember, it shouldn't be able to accomplish something that would take a higher level spell (like silent Image). It also says physical interaction reveals it to be an illusion, it does not say what has to physically interact or who it is revealed to be an illusion too. So if your PCs are using it as cover and poking their heads out to attack, anything that sees that can infer it is an illusion no action required.
You can also employ a few variant rules and/or house rules that can prevent it from being overly abused. With the Identify a spell reaction from XGtE (https://www.dndbeyond.com/sources/xgte/dungeon-masters-tools#Spellcasting), monsters can make a DC15 arcana check to recognise an illusion is being created. You can also house rule that a creature with a passive investigate higher than the check needed to perceive the illusion and sees the spell cast, succeeds automatically.
If they are using it to modify rooms, forest glades, or any other area that NPCs and monsters are familiar with there is a good chance the illusion will be noticed as out of place. If a new chair suddenly appeared in your living room you'd notice, right? Walls and doors will only work on people who don't know the area the wall or door is in, which is more likely to be the PCs.
An illusionary person is going to be revealed as such at the first hit in combat. Or the first time the person speaks (minor illusions can't do that,), this includes most spell casting. Also people created by Minor Illusion don't move. How long does it take to spot a waxwork or mannequin?
A duplicate person must be five feet tall or less, including headgear, or must be posed so as to fit in that five-foot cube. Remember that a standard door is taller than five feet, and a five foot wall is lower than most ceilings.
If you are the DM you can impose any limits you like. The rulebook is not the sum total of limits on the world. Remember the rulebook nowhere states that torches can be used to light anything on fire. There are no actual rules for lighting things on fire other than with a tinderbox, a Gnomish firestarter, and a few spells. Torches, and candles, are not on the list of "things which light other things on fire". I give this as an example of not everything true about the game world being given a rule.
The existence of illusions is a known thing in D&D. People are well aware that they exist. So they are much more likely to try to disbelieve things they see and hear than you or I would be.
I was going to say the same thing. Who wouldn't notice a new half height wall, or miniature door that doesn't match the rest? Or a person who doesn't move, doesn't react to the wind, has leaves passing through them....?
First, thank you to all respondents. The examples I can provide include creating a 5-foot-cube representation of the surrounding area and hiding behind it, making a representation of the caster in a static position to fool opponents, etc. My statement a few times at the table has been that some of these are a bit too complex to authentically and accurately recreate; however, I really like DxJxC's suggestions below (with Identify and passive investigation).
You can also employ a few variant rules and/or house rules that can prevent it from being overly abused. With the Identify a spell reaction from XGtE (https://www.dndbeyond.com/sources/xgte/dungeon-masters-tools#Spellcasting), monsters can make a DC15 arcana check to recognise an illusion is being created. You can also house rule that a creature with a passive investigate higher than the check needed to perceive the illusion and sees the spell cast, succeeds automatically.
You could decide that "it must be no larger than a 5-foot cube" means it must be no longer than 5’ in any dimension, rather than it must no be larger than the volume of a 5’ cube. That rules out most doors and walls, as well as any character over 5’ tall. Additionally a creature trying to decide between attacking a PC and the minor illusion of that PC doesn’t have to break the illusion to notice the differences between the two (motion, sound, etc.) Or you can have them fight a score of illusionists....
So... the uses that they are putting the spell minor illusion to are foiling even high-level encounters. As a DM, I do not want to discourage the use of creativity, but the ability to create any visual (stationary) illusion in a 5-foot-cube at will as a cantrip is incredibly over-powered. How do you deal with this?
I know that enemies can take an action to disbelieve, but to be honest, the way the players are using the spell (as walls, doors, images of themselves) wouldn't provoke the desire to disbelieve (otherwise, each combat would start with everyone taking an action to disbelieve.
Minor Illusion doesn't let you create "any visual illusion in a 5-foot-cube", for starters. It's very picky: it creates an image of an object, and objects have a very specific definition in D&D: "a discrete, inanimate item like a window, door, sword, book, table, chair, or stone, not a building or a vehicle that is composed of many other objects." That means no atmospheric effects, no lighting or shadow effects, no creatures (including images of themselves), arguably not even loose liquids. It also means no walls - at least not a wall that's seamlessly connected to the rest of the structure - because that wouldn't be a discrete item of reasonable size, it'd be an extension of the building.
Once you realize how incredibly mundane the spell is, it shouldn't pose a significant problem.
Good call on the object definition. I was also thinking about how your viewing angle would only hide someone if you view a 5 foot cube sized object from straight on and only if you're short enough to not see over the top of the cube. You're not going to hide from a giant behind a 5' tall anything if they're close enough, and if someone is walking by once they shift their viewing angle enough they'll see you crouching behind the cube.
First, thank you to all respondents. The examples I can provide include creating a 5-foot-cube representation of the surrounding area and hiding behind it, making a representation of the caster in a static position to fool opponents, etc. My statement a few times at the table has been that some of these are a bit too complex to authentically and accurately recreate; however, I really like DxJxC's suggestions below (with Identify and passive investigation).
You can also employ a few variant rules and/or house rules that can prevent it from being overly abused. With the Identify a spell reaction from XGtE (https://www.dndbeyond.com/sources/xgte/dungeon-masters-tools#Spellcasting), monsters can make a DC15 arcana check to recognise an illusion is being created. You can also house rule that a creature with a passive investigate higher than the check needed to perceive the illusion and sees the spell cast, succeeds automatically.
As InquisitiveCoder pointed out, those are things that Minor Illusion can't actually do. Re-creating the surrounding area is more than a single small object. And it can't create illusions of creatures. I think the worst is you could make a Metal Gear Solid style box to hide under as long as no one saw you enter into the illusion (which would reveal it as fake)
First, thank you to all respondents. The examples I can provide include creating a 5-foot-cube representation of the surrounding area and hiding behind it, making a representation of the caster in a static position to fool opponents, etc. My statement a few times at the table has been that some of these are a bit too complex to authentically and accurately recreate; however, I really like DxJxC's suggestions below (with Identify and passive investigation).
You can also employ a few variant rules and/or house rules that can prevent it from being overly abused. With the Identify a spell reaction from XGtE (https://www.dndbeyond.com/sources/xgte/dungeon-masters-tools#Spellcasting), monsters can make a DC15 arcana check to recognise an illusion is being created. You can also house rule that a creature with a passive investigate higher than the check needed to perceive the illusion and sees the spell cast, succeeds automatically.
I was not referring to the identify spell, to be clear. I was referring to the "identifying a spell" optional rule that I linked.
My answers were to deal with possible cheaty uses during combat.
A Minor Illusion of the surrounding area is likely to be of less use to hide behind than the players expect in any case. in order to be convincing the parts of illusion that are thin air are going to be transparent any way. So I would disallow the hiding behind an illusion of "empty space" trick. Also known as I cast an illusion of empty space on myself. The spell you are looking for to do that is called "Invisibility".
I once had a player who was also a brilliant marine biologist playing a halfling warlock/rogue(arcane trickster). His deviousness still haunts me to this day.
He thoroughly explored the use of Minor Illusion and constantly had me scratching my head and wondering things like, "How would someone react to suddenly finding themselves standing in the lower 2/3 of an iron maiden?"
Truthfully his use of the sound effect was more bothersome than the visual; he'd have enemies fighting and doubting each other more often than I can recall. If your crew of intellectual giants hasn't yet explored that, stall it as long as you can.
Anyway, one of the ways I made him adapt was first to have enemies learn his tactics. The long term villains adjusted and acclimated to his illusions after surviving encounters; it turned into a chess match of sorts. They would start using spells like True Seeing, Detect Magic, Detect Thoughts, and Locate Creature, so he switched to Nondetection, Dispel Magic, and Nystul's Aura and so forth.
Another way I kept things interesting was to include enemies that were automated to some extent or had blindsight. Oozes and grimlocks were particularly effective during a stint in the Underdark. Illusions don't hinder things that can't see them, so those creatures just keep on trudging.
I also included defensive mechanics in encounters that launched spells and such at living targets within range (e.g. Glyph of Warding on steroids). That forced the party to do more than just fool the monsters, they had to react to fortifications as well.
Regardless, that campaign had some very memorable moments that left me in awe of what a creative player can do. I completely understand the struggle!
* Adding more environmental hazards like hidden pits, trap walls, and auto-teleportation tiles to split the casters from the Rogues/Rangers/Bards. Have some archers or monsters with reach attacks (ropers? displacer beasts? hydra?) attack them while they are separated.
* Give your supervillain a homunculus. Or plant a spy on them as an NPC who is there as a "guide" or fake prisoner. Intelligence works both ways.
* Anti-magic zones.
* Feature more monsters that not only see through illusions, but have some tricks of their own: naga, gynosphinx, umber hulks, doppelganger sorcerers, warlocks running their own alter self or witch sight powers. Give them a taste of their own medicine.
*Blindsight - "A monster with blindsight can perceive it's surroundings without relying on sight". Characters using a sight based illusion? Too bad, monsters " see" through it. (Bad pun unintended)
*Tremorsense - "A monster with tremorsense can detect and pinpoint the origin of vibrations within a specific radius". Using illusions to create hidey-holes? Monster sees through it. Creating duplicates? Monster feels no vibrations and doesn't care. (Warning - use of the spell " prestidigitation " to create harmless vibrations could bypass tremorsense unless you also get creative.
*Truesight - The monster sees everything. No saves or discernment needed. And when I say everything, it includes the ethereal plane and high level spells.
*Touchy feely NPCs. No longer does a covert nod constitute a hello, this NPC needs a nice long hug, slap on the back, some physical.
*Superstitious NPC - have a group walk down the hall / tunnel, talking about something (plot point or other) and one needs to knock on wood. Conveniently, the characters are hiding behind their minor illusion door; and oh dear, the NPC's hand just went right through. Let's have an initiative roll everybody.
*Environmental effects - This was touched upon earlier. Wind, dust, leaves, floor falling (fun way to turn the tables), whatever. The illusion is not affected? Free discern rolls with bonuses for all! And to all, a good fight!
I usually roll for something. When my gnome wizard (who is 3 ft tall) creates a 4 to 5 ft large tree stump, or rock, the creatures that walk past roll wisdom saves. If they succeed, they may roll a perception check. Yes. It is two rolls. But remember, a dire wolf can smell real good. And maybe they assume the creature is behind the stone or the illusion. So suddenly the wolf jumps at them! The minions may remember that there wasn't a door there. When I quickly make a wizard NPC for the town (Assuming the town would have one), I usually take into account that they would probably have a nondetection (or mind blank if high enough level), and a true sight. Wizards in my world(s), experiment with stuff. Creating potions, salves, and other objects or effects that would assist them in everyday life. Maybe a wizard/sorcerer/bard, and arcane (maybe even divine) spellcasters would be able to better see through such a low level illusion. You can rule that the DC of a spell, such as minor illusion is 10 + spell level. If the creature has a CR or class (or arcane spellcaster) levels equal to the DC. They can make a perception or wisdom save against their spell save DC. Seeing through the illusion on a success. Also, like invisiblity. Sounds are not masked.
Or, make a (long) arc that evolves around magic. I have a faint idea of an arc or at least multiple sessioned plot for my wizard. An area that has wild magic all around it. Giving weird buffs at odd times. Such as max damage, no expended spell slot. But everytime he casts a spell of 1st level or higher (maybe even cantrips), he has to roll a 20. Depending on how close to the center he is, the number changes. But say he just entered it, if he rolls a 1. A wild magic effect occurs. The closer he gets, he may have to roll conc checks to even cast the spell, and not lose it. And at the very center. No magic. Blab. Nope. Nothin'.
Challenge them in ways that endanger them. Perhaps they are starting to get famous for their illusions. And suddenly, bugbears are paranoid of all rocks. So they see them touch EVERYTHING. You let them get creative. But I think it's you who needs to be creative here. And I don't mean that in a bad way. Not as in you suck a DMing. No no. But, it's totally okay to challenge them against their usual tactic. If they are using the same method. Over and over and over and over again. Force them to resort to new methods. Even if just for a while. If they go back to it. Bring back a little bit of the challenging aspect. Or have a herald of say, Oghma or some other deity. Azuth. And tell them that they are using too much arcane energy of a single school. And they must either stop, or help him increase the limit. Where they are not allowed to use any illusion spells of 3rd level or lower.
Get creative. Challenge them. Make them change tactics. If they complain about it, tell them the problem. Tell them that they are using a spell so well, that it's breaking the game. But I will, in parting give you a round of applause. *Clapping* Because many DMs, Would simply BAN the cantrip from the game. And I'm proud and happy that you instead, come seeking help and think of how to deal with it. Not banning the spell, for creative use.
(PS: Don't tell them about rope trick. Many think it's a boring spell. I haven't told my littlebrother to look it up yet because his gloomstalker has it. And it's crazy good, but he hasn't realised it yet. And I don't intend to tell him, at least not just yet. So if it's getting out of hand, think of rope trick. ^^) Hope this helped! Sorry for the mass amount of text. This is more text than the vampire stat block. If you read it all, thanks man! And I hope it works out! ^^
If they tried to create something that otherwise would have odour, movement, sound, psychic presence, etc, that could be detected, I’d allow a save or possibly automatic detection (think how unnatural a frozen person in a movie looks when everything else is moving).
If they are perfectly trying to mimic something the target is familiar with or if they are trying to create something that must perfectly blend with an existing environment, I’d allow a contest check using an appropriate pair of skills. This might even force a little more creativity as they try to fit the illusion to their skill set.
From another perspective - are they having fun getting around encounters or situations doing this? A LOT of fun? If so, just let them use this creative method to do so.
That doesn't mean it's infaliable.
Create a spellcaster enemy or NPC who uses the same tricks, or who is well-studied in illusions and who is always capable of seeing through illusions. Perhaps your Big Bad is such an exact creature, or else someone like that is a close ally of the Big Bad. Then, when they encounter the PCs doing this, they begin to teach their minions how to see past illusions and similar tactics, giving them ADV on such attempts.
Create anti-magic fields. Utilize creatures with blindsight or truesight. Have an enemy with a homebrew net item that, if thrown on a caster, nullifies any active magic effects. Have a pseudonatural-magical disaster that sweeps across the nation and causes illusions to be transluscent/colored oddly/otherwise ineffective/imperfect. Have an NPC offer to teach the illusion caster "more powerful illusions" but have them teach them the wrong thing in such a way that does the opposite of what's desired, or makes the PCs light-up like Faerie Fire, or the like.
But my belief is that, at the end of the day, aside from setbacks, if the players are having fun utilizing such tactics, don't take it away from them entirely - but do make it harder and harder for them to get away with the same bullshit over and over, as the enemies grow wiser and wiser to their shenanigans.
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The minor illusion spell has been a major pain in the butt for me as a DM. My group is a bunch of math professors, a medieval history guru, and a biology PhD. Their ability to get creative with interpretations is... challenging. This is the problem with 5e and a bunch of math nerds - we love rules and without them, we assume there are no boundaries or we reach the edges of those grey boundary lines and jump up and down on them.
So... the uses that they are putting the spell minor illusion to are foiling even high-level encounters. As a DM, I do not want to discourage the use of creativity, but the ability to create any visual (stationary) illusion in a 5-foot-cube at will as a cantrip is incredibly over-powered. How do you deal with this?
I know that enemies can take an action to disbelieve, but to be honest, the way the players are using the spell (as walls, doors, images of themselves) wouldn't provoke the desire to disbelieve (otherwise, each combat would start with everyone taking an action to disbelieve.
I would love to hear your advice on this.
Are you stuck in a pre-made adventure with a pre-made grid and your players won't let you adapt? If so it's hard to offer advice! I don't know an in depth example. But if you have room to get creative I'd start pre-planning enemies with built-in tools to foil fake walls, higher stats to make disbelieving easier. Maybe reinforcements show up and snap bad guys out of their disbelief. The big prize gets relocated by a reactive and increasingly paranoid ecosystem.
You never want them overdoing the same puzzle solution repeatedly! It's worth improvising some odd reactions from the universe.
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Do you have an example of them using it and what it led to?
Minor Illusion can potentially be a major pain. Just remember, it shouldn't be able to accomplish something that would take a higher level spell (like silent Image). It also says physical interaction reveals it to be an illusion, it does not say what has to physically interact or who it is revealed to be an illusion too. So if your PCs are using it as cover and poking their heads out to attack, anything that sees that can infer it is an illusion no action required.
You can also employ a few variant rules and/or house rules that can prevent it from being overly abused. With the Identify a spell reaction from XGtE (https://www.dndbeyond.com/sources/xgte/dungeon-masters-tools#Spellcasting), monsters can make a DC15 arcana check to recognise an illusion is being created. You can also house rule that a creature with a passive investigate higher than the check needed to perceive the illusion and sees the spell cast, succeeds automatically.
If they are using it to modify rooms, forest glades, or any other area that NPCs and monsters are familiar with there is a good chance the illusion will be noticed as out of place. If a new chair suddenly appeared in your living room you'd notice, right? Walls and doors will only work on people who don't know the area the wall or door is in, which is more likely to be the PCs.
An illusionary person is going to be revealed as such at the first hit in combat. Or the first time the person speaks (minor illusions can't do that,), this includes most spell casting. Also people created by Minor Illusion don't move. How long does it take to spot a waxwork or mannequin?
A duplicate person must be five feet tall or less, including headgear, or must be posed so as to fit in that five-foot cube. Remember that a standard door is taller than five feet, and a five foot wall is lower than most ceilings.
If you are the DM you can impose any limits you like. The rulebook is not the sum total of limits on the world. Remember the rulebook nowhere states that torches can be used to light anything on fire. There are no actual rules for lighting things on fire other than with a tinderbox, a Gnomish firestarter, and a few spells. Torches, and candles, are not on the list of "things which light other things on fire". I give this as an example of not everything true about the game world being given a rule.
The existence of illusions is a known thing in D&D. People are well aware that they exist. So they are much more likely to try to disbelieve things they see and hear than you or I would be.
I was going to say the same thing. Who wouldn't notice a new half height wall, or miniature door that doesn't match the rest? Or a person who doesn't move, doesn't react to the wind, has leaves passing through them....?
You could decide that "it must be no larger than a 5-foot cube" means it must be no longer than 5’ in any dimension, rather than it must no be larger than the volume of a 5’ cube. That rules out most doors and walls, as well as any character over 5’ tall. Additionally a creature trying to decide between attacking a PC and the minor illusion of that PC doesn’t have to break the illusion to notice the differences between the two (motion, sound, etc.) Or you can have them fight a score of illusionists....
Minor Illusion doesn't let you create "any visual illusion in a 5-foot-cube", for starters. It's very picky: it creates an image of an object, and objects have a very specific definition in D&D: "a discrete, inanimate item like a window, door, sword, book, table, chair, or stone, not a building or a vehicle that is composed of many other objects." That means no atmospheric effects, no lighting or shadow effects, no creatures (including images of themselves), arguably not even loose liquids. It also means no walls - at least not a wall that's seamlessly connected to the rest of the structure - because that wouldn't be a discrete item of reasonable size, it'd be an extension of the building.
Once you realize how incredibly mundane the spell is, it shouldn't pose a significant problem.
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Good call on the object definition. I was also thinking about how your viewing angle would only hide someone if you view a 5 foot cube sized object from straight on and only if you're short enough to not see over the top of the cube. You're not going to hide from a giant behind a 5' tall anything if they're close enough, and if someone is walking by once they shift their viewing angle enough they'll see you crouching behind the cube.
As InquisitiveCoder pointed out, those are things that Minor Illusion can't actually do. Re-creating the surrounding area is more than a single small object. And it can't create illusions of creatures. I think the worst is you could make a Metal Gear Solid style box to hide under as long as no one saw you enter into the illusion (which would reveal it as fake)
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I was not referring to the identify spell, to be clear. I was referring to the "identifying a spell" optional rule that I linked.
My answers were to deal with possible cheaty uses during combat.
A Minor Illusion of the surrounding area is likely to be of less use to hide behind than the players expect in any case. in order to be convincing the parts of illusion that are thin air are going to be transparent any way. So I would disallow the hiding behind an illusion of "empty space" trick. Also known as I cast an illusion of empty space on myself. The spell you are looking for to do that is called "Invisibility".
Well put Dave. Best case scenario, they can make the illusion of a wall with a very realistic picture of what was behind it.
I once had a player who was also a brilliant marine biologist playing a halfling warlock/rogue(arcane trickster). His deviousness still haunts me to this day.
He thoroughly explored the use of Minor Illusion and constantly had me scratching my head and wondering things like, "How would someone react to suddenly finding themselves standing in the lower 2/3 of an iron maiden?"
Truthfully his use of the sound effect was more bothersome than the visual; he'd have enemies fighting and doubting each other more often than I can recall. If your crew of intellectual giants hasn't yet explored that, stall it as long as you can.
Anyway, one of the ways I made him adapt was first to have enemies learn his tactics. The long term villains adjusted and acclimated to his illusions after surviving encounters; it turned into a chess match of sorts. They would start using spells like True Seeing, Detect Magic, Detect Thoughts, and Locate Creature, so he switched to Nondetection, Dispel Magic, and Nystul's Aura and so forth.
Another way I kept things interesting was to include enemies that were automated to some extent or had blindsight. Oozes and grimlocks were particularly effective during a stint in the Underdark. Illusions don't hinder things that can't see them, so those creatures just keep on trudging.
I also included defensive mechanics in encounters that launched spells and such at living targets within range (e.g. Glyph of Warding on steroids). That forced the party to do more than just fool the monsters, they had to react to fortifications as well.
Regardless, that campaign had some very memorable moments that left me in awe of what a creative player can do. I completely understand the struggle!
A few more challenges that might help:
* Adding more environmental hazards like hidden pits, trap walls, and auto-teleportation tiles to split the casters from the Rogues/Rangers/Bards. Have some archers or monsters with reach attacks (ropers? displacer beasts? hydra?) attack them while they are separated.
* Give your supervillain a homunculus. Or plant a spy on them as an NPC who is there as a "guide" or fake prisoner. Intelligence works both ways.
* Anti-magic zones.
* Feature more monsters that not only see through illusions, but have some tricks of their own: naga, gynosphinx, umber hulks, doppelganger sorcerers, warlocks running their own alter self or witch sight powers. Give them a taste of their own medicine.
A couple ideas could include:
*Blindsight - "A monster with blindsight can perceive it's surroundings without relying on sight". Characters using a sight based illusion? Too bad, monsters " see" through it. (Bad pun unintended)
*Tremorsense - "A monster with tremorsense can detect and pinpoint the origin of vibrations within a specific radius". Using illusions to create hidey-holes? Monster sees through it. Creating duplicates? Monster feels no vibrations and doesn't care. (Warning - use of the spell " prestidigitation " to create harmless vibrations could bypass tremorsense unless you also get creative.
*Truesight - The monster sees everything. No saves or discernment needed. And when I say everything, it includes the ethereal plane and high level spells.
*Touchy feely NPCs. No longer does a covert nod constitute a hello, this NPC needs a nice long hug, slap on the back, some physical.
*Superstitious NPC - have a group walk down the hall / tunnel, talking about something (plot point or other) and one needs to knock on wood. Conveniently, the characters are hiding behind their minor illusion door; and oh dear, the NPC's hand just went right through. Let's have an initiative roll everybody.
*Environmental effects - This was touched upon earlier. Wind, dust, leaves, floor falling (fun way to turn the tables), whatever. The illusion is not affected? Free discern rolls with bonuses for all! And to all, a good fight!
I hope these help, and happy roleplaying!
I usually roll for something. When my gnome wizard (who is 3 ft tall) creates a 4 to 5 ft large tree stump, or rock, the creatures that walk past roll wisdom saves. If they succeed, they may roll a perception check. Yes. It is two rolls. But remember, a dire wolf can smell real good. And maybe they assume the creature is behind the stone or the illusion. So suddenly the wolf jumps at them! The minions may remember that there wasn't a door there. When I quickly make a wizard NPC for the town (Assuming the town would have one), I usually take into account that they would probably have a nondetection (or mind blank if high enough level), and a true sight. Wizards in my world(s), experiment with stuff. Creating potions, salves, and other objects or effects that would assist them in everyday life. Maybe a wizard/sorcerer/bard, and arcane (maybe even divine) spellcasters would be able to better see through such a low level illusion. You can rule that the DC of a spell, such as minor illusion is 10 + spell level. If the creature has a CR or class (or arcane spellcaster) levels equal to the DC. They can make a perception or wisdom save against their spell save DC. Seeing through the illusion on a success. Also, like invisiblity. Sounds are not masked.
Or, make a (long) arc that evolves around magic. I have a faint idea of an arc or at least multiple sessioned plot for my wizard. An area that has wild magic all around it. Giving weird buffs at odd times. Such as max damage, no expended spell slot. But everytime he casts a spell of 1st level or higher (maybe even cantrips), he has to roll a 20. Depending on how close to the center he is, the number changes. But say he just entered it, if he rolls a 1. A wild magic effect occurs. The closer he gets, he may have to roll conc checks to even cast the spell, and not lose it. And at the very center. No magic. Blab. Nope. Nothin'.
Challenge them in ways that endanger them. Perhaps they are starting to get famous for their illusions. And suddenly, bugbears are paranoid of all rocks. So they see them touch EVERYTHING. You let them get creative. But I think it's you who needs to be creative here. And I don't mean that in a bad way. Not as in you suck a DMing. No no.
But, it's totally okay to challenge them against their usual tactic. If they are using the same method. Over and over and over and over again. Force them to resort to new methods. Even if just for a while. If they go back to it. Bring back a little bit of the challenging aspect. Or have a herald of say, Oghma or some other deity. Azuth. And tell them that they are using too much arcane energy of a single school. And they must either stop, or help him increase the limit. Where they are not allowed to use any illusion spells of 3rd level or lower.
Get creative. Challenge them. Make them change tactics. If they complain about it, tell them the problem. Tell them that they are using a spell so well, that it's breaking the game.
But I will, in parting give you a round of applause. *Clapping*
Because many DMs, Would simply BAN the cantrip from the game. And I'm proud and happy that you instead, come seeking help and think of how to deal with it. Not banning the spell, for creative use.
(PS: Don't tell them about rope trick. Many think it's a boring spell. I haven't told my littlebrother to look it up yet because his gloomstalker has it. And it's crazy good, but he hasn't realised it yet. And I don't intend to tell him, at least not just yet. So if it's getting out of hand, think of rope trick. ^^)
Hope this helped! Sorry for the mass amount of text. This is more text than the vampire stat block. If you read it all, thanks man! And I hope it works out! ^^
If they tried to create something that otherwise would have odour, movement, sound, psychic presence, etc, that could be detected, I’d allow a save or possibly automatic detection (think how unnatural a frozen person in a movie looks when everything else is moving).
If they are perfectly trying to mimic something the target is familiar with or if they are trying to create something that must perfectly blend with an existing environment, I’d allow a contest check using an appropriate pair of skills. This might even force a little more creativity as they try to fit the illusion to their skill set.
From another perspective - are they having fun getting around encounters or situations doing this? A LOT of fun? If so, just let them use this creative method to do so.
That doesn't mean it's infaliable.
Create a spellcaster enemy or NPC who uses the same tricks, or who is well-studied in illusions and who is always capable of seeing through illusions. Perhaps your Big Bad is such an exact creature, or else someone like that is a close ally of the Big Bad. Then, when they encounter the PCs doing this, they begin to teach their minions how to see past illusions and similar tactics, giving them ADV on such attempts.
Create anti-magic fields. Utilize creatures with blindsight or truesight. Have an enemy with a homebrew net item that, if thrown on a caster, nullifies any active magic effects. Have a pseudonatural-magical disaster that sweeps across the nation and causes illusions to be transluscent/colored oddly/otherwise ineffective/imperfect. Have an NPC offer to teach the illusion caster "more powerful illusions" but have them teach them the wrong thing in such a way that does the opposite of what's desired, or makes the PCs light-up like Faerie Fire, or the like.
But my belief is that, at the end of the day, aside from setbacks, if the players are having fun utilizing such tactics, don't take it away from them entirely - but do make it harder and harder for them to get away with the same bullshit over and over, as the enemies grow wiser and wiser to their shenanigans.