Illusions deal with spells to deceive the senses or minds of others. Spells that cause people to see things that are not there, hear noises not made, or remember things that never happened are all illusions.
And from 5e:
Illusion spells deceive the senses or minds of others. They cause people to see things that are not there, to miss things that are there, to hear phantom noises, or to remember things that never happened. Some illusions create phantom images that any creature can see, but the most insidious illusions plant an image directly in the mind of a creature.
Pretty similar. The 5e definition touches on the distinction between "phantom images that any creature can see" and images planted "directly in the mind of a creature". The 2e PHB actually goes into far greater detail regarding illusions (in Chapter 7, in a section that definitely should have instead been in the DMG):
Spells of this school fall into two basic groups. Illusions are creations that manipulate light, color, shadow, sound, and sometimes even scent. Higher level illusions tap energy from other planes, and are actually quasi-real, being woven of extradimensional energies by the caster. Common illusions create appearances; they cannot make a creature or object look like nothing (i.e., invisible), but they can conceal objects by making them look like something else.
Phantasms exist only in the minds of their victims; these spells are never even quasi-real. (The exceptions to this are the phantasmal force spells, which are actually illusions rather than phantasms.) Phantasms act upon the mind of the victim to create an intense reaction - fear being most common.
I say that this section should have instead been in the DMG because it goes into great detail about adjudicating illusions - believability, monster and NPC expectations and reactions, when a saving throw is warranted, how to handle "damage" from illusions, whether illusions can scare a creature to death, when to roll for system shock, and the like (I'll get to this later).
As is often the case with AD&D, it's verbose, but not at all unhelpful or unwelcome. There is even more text on adjudicating illusions in Appendix 2 of the PHB - really, a section titled "Adjudicating Illusions" is in the Player's Handbook!
3e goes into even greater detail when distinguishing the types of illusions. There are figments, glamers, patterns, phantasms, and shadows:
A figment spell creates a false sensation. Those who perceive the figment perceive the same thing, not their own slightly different versions of the figment. (It is not a personalized mental impression.) Figments cannot make something seem to be something else.
A glamer spell changes a subject’s sensory qualities, making it look, feel, taste, smell, or sound like something else, or even seem to disappear.
Because figments and glamers are unreal, they cannot produce real effects the way that other types of illusions can. They cannot cause damage to objects or creatures, support weight, provide nutrition, or provide protection from the elements. Consequently, these spells are useful for confounding or delaying foes, but useless for attacking them directly.
Like a figment, a pattern spell creates an image that others can see, but a pattern also affects the minds of those who see it or are caught in it. All patterns are mind-affecting spells.
A phantasm spell creates a mental image that usually only the caster and the subject (or subjects) of the spell can perceive. This impression is totally in the minds of the subjects. It is a personalized mental impression. (It’s all in their heads and not a fake picture or something that they actually see.) Third parties viewing or studying the scene don’t notice the phantasm. All phantasms are mind-affecting spells.
A shadow spell creates something that is partially real from extradimensional energy. Such illusions can have real effects. Damage dealt by a shadow illusion is real.
Someone who is more well-versed than I in the workings of 3e could probably explain why all of these distinctions are necessary or how they interact with the game's other rules.
5e makes no attempt to delineate different types of illusions. As is often the case in the most current edition, the spells' individual descriptions simply say what each does in game terms - that is, whether the spell creates a sensation which can be observed by anyone (and which sensation or combination of sensations it produces) or a sensation entirely within the minds of the affected creatures, whether it can be interacted with, whether it imposes some condition, whether it can cause damage, how to overcome it, and the like. One can assign most of the 5e illusions to any of the above categories: minor illusion is a figment, disguise self is a glamer, hypnotic pattern is (isn't it obvious?) a pattern, phantasmal force is a phantasm, and creation is a shadow.
Conjuring Enchanting Illusions
Illusion is one of the more consistent schools of magic in 5e. The spells found there are about what one would expect. There are some oddballs, and they are usually spells which would fit into the latter three categories presented in 3e - patterns, phantasms, and shadows.
Patterns and phantasms are illusions which affect the mind to "create an intense reaction - fear being the most common". Sounds kind of like an enchantment, no? Examples include fear, hypnotic pattern, phantasmal killer, and weird. It makes sense to me that these spells are in the realm of illusion, because it's the images being created which cause the mental effect. Whereas enchantments affect the mind directly, illusion can affect the mind only to the extent that the target is susceptible to the illusion itself.
Fear in particular is often brought up as being an oddball. In AD&D, the wizard "sends forth an invisible cone of terror", which, yeah, that doesn't really sound like an illusion at all. But in 5e, the caster projects "a phantasmal image of a creature's worst fears". The spell affects a 30-foot cone, so presumably each creature is seeing a different image, assuming that their fears are different. I've always thought of this as being like that scene in Fellowship of the Ring where Gandalf gets really tall and bellows at Bilbo "I am not trying to rob you!" He instills fear in Bilbo, but he does it by changing the way he looks and sounds.
Shadows, on the other hand, create "something that is partially real from extradimensional energy", which sounds a lot like conjuration. The "extradimensional energy" is usually drawn from the Plane of Shadow, and is magically woven into the illusion to make it quasi-real. Conjuration, on the other hand, involves "the transportation of objects and creatures from one location to another" or the creation of "objects or effects out of nothing." Conjuration transports or creates real things - shadow magic does not (it uses extradimensional energy to make an image temporarily real). Phantom steed is shadow magic, but find steed is conjuration. Creation is shadow magic, but create food and water is conjuration.
There are a few other oddballs, like color spray, which creates "flashing, colored light", yet other spells which create light (light, daylight) are evocation, as is prismatic spray (a more notorious spray of multicolored light). 3e classifies color spray as a mind-affecting pattern, so it's meant to be similar to hypnotic pattern.
Magic mouth is also odd, since nothing in the spell's description identifies the mouth as being illusory. It can make something like a painting or a statue appear to speak, but there's nothing saying that the mouth itself isn't real. 3e classifies the spell as a glamer, so the intention is clearly for it to be a trick of light and sound, not a real mouth that is conjured or an ordinary object like a statue that is magically altered.
I was surprised to see that silence is an illusion (3e classifies it as a glamer). I could see an argument for it being an abjuration (a magical barrier that traps sound), but I suppose it makes sense as a glamer (a spell which can make something sound like something else, or, in this case, make something sound like nothing at all).
Lastly, simulacrum - this is a doozy of a spell, and I'm not going to get into the weeds on it, but basically, the caster creates "an illusory duplicate" of a creature, which is "partially real and formed from ice and snow". However, the illusory duplicate is a creature (a construct, specifically) with its own hit points and all of the abilities and statistics of the creature it is based on. 3e classifies the spell as a shadow, so I guess shadow magic is used to give the duplicate a semblance of life.
This is a spell people love to whine about. A lot of DMs don't like that a high-level caster can make an almost exact duplicate of themselves or someone else in the party. A party of all wizards (or bards, with Magical Secrets), could hypothetical double the size of their party if they all cast this spell. I think it's iconic and awesome, and something that high-level casters should be able to do.
Besides, the spell is plenty limiting - even if a character can get their hands on all that ruby dust, how readily available is the ice and snow to make a snow man? Sounds to me like every campaign needs a calendar with distinct seasons and a system for randomly generating weather!
Real Eyes Realize Real Lies
The way in which illusions work in D&D has changed a lot throughout the editions. Most notably, there has been a great deal of consternation regarding belief in illusions. In OD&D, phantasmal forces can only do damage if "believed to be real". In AD&D, phantasmal force grants a saving throw only to creatures which "disbelieve" the illusion. Allegedly, this led to AD&D players shouting "I disbelieve!" at anything and everything they encountered, on the off chance that they were experiencing an illusion and could benefit from a saving throw.
2e cracks down on this nonsense to a reasonable extent. From Appendix 2 of the PHB:
Disbelief must be stated by the player, based on clues provided by the DM. Players stating disbelief must give a reason for disbelief based on sensory information available to the character. Failure to give such a reason results in failure to disbelieve. The DM can impose additional requirements or delays in recognizing illusions (such as Intelligence checks) as needed, such as when one player is obviously parroting a discovery made by another. Disbelief automatically forfeits a saving throw if the effect is real.
Chapter 7's section on illusions states "disbelieving is an action...and takes a round". So, shouting "I disbelieve!" at anything and everything is not exactly a foolproof strategy. It takes a full round to attempt to disbelieve an illusion (the character foregoes attacking, casting spells, and perhaps even moving or defending themselves, as they focus on the supposed illusion and attempt to see through its tricks), and if the "illusory" fireball happens to be real, the character forfeits their saving throw against it - they were too busy trying to think the fireball away, and got blasted in the face by it.
On top of all that, to even attempt to disbelieve an illusion, the player must provide the DM with some sort of logic for doing so. This is usually tied to believability, which is exhaustively detailed in Chapter 7:
The key to successful illusions or phantasms is believability, which depends on three main factors: what the caster attempts, what the victim expects, and what is happening at the moment the spell is cast.
The type of image chosen by the caster affects the reaction of the victim. ... Spellcasters are well-advised to create images of things they have seen.
The next important consideration is to ask if the spell creates something that the victim expects. ... The key to a good illusion is to create something the victim does not expect but can quickly accept.
This then leads to the third factor in the believability of an illusion, how appropriate the illusion is for the situation. ... The best illusions reinforce these expectations to your character's advantage.
The text is littered with helpful examples for each of these three factors, which I've left out for brevity. Suffice to say, in typical 2e fashion, there's a lot of verbose advice given, which seems largely meant to help the DM adjudicate things in a way informed by "common sense".
2e also provides some guidance for situations in which some creatures are affected by illusions while their allies are not:
In many encounters, some party members will believe an illusion while others see it for what it really is. In these cases, revealing the truth to those deluded by the spell is not a simple matter of telling them. The magic of the spell has seized their minds. Considered from their point of view, they see a horrible monster (or whatever) while a friend is telling them it isn't real. They know magic can affect people's minds, but whose mind has been affected in this case? At best, having an illusion pointed out grants another saving throw with a +4 bonus.
2e also touches on Intelligence as a factor in disbelieving illusions, though it isn't very specific:
Intelligence is the best defense against illusions. Low and nonintelligent creatures are more vulnerable to illusions, unless the illusion is completely outside their experience or the illusion touches on an area of the creatures' particular competence.
One has to consider how this would affect an animal, such as a wolf. It has low Intelligence, but heightened senses. A wolf may be startled by the illusion of a bear, but if it doesn't smell like a bear, the illusion won't be convincing. Thus, the limits of believability must be determined by the effected creature's experience and senses.
Furthermore, creatures with sufficiently high Intelligence are immune to low-level illusions. A creature with an Intelligence of 19 is immune to 1st-level illusions, and a creature with an Intelligence of 25 is immune to 7th-level illusions.
Others have taken the idea of Intelligence as a determining factor and run with it (interestingly, this post suggests instead that low-Intelligence creatures ought to be less susceptible to illusions - they simply don't fully process them). Using Intelligence this way might be useful for DMs who don't trust themselves to be unbiased when determining whether their monsters and NPCs might reasonably come to disbelieve an illusion, or those who simply want more mechanical heft when adjudicating this sort of thing.
That brings us back to 5e, where Intelligence plays much more of a role in disbelieving illusions. Many illusions in 5e do not involve a saving throw at all. Generally, an illusion in 5e only requires a saving throw if it can damage the target or inflict some condition (phantasmal force, fear, hypnotic pattern, phantasmal killer, dream, or weird). The saving throw has nothing to do with disbelieving the illusion, but simply resisting the illusion's effects.
Because of 5e's very literal rules language, there are spells which explicitly state conditions under which illusions are automatically revealed. Minor illusion, disguise self, silent image, major image, hallucinatory terrain, seeming, programmed illusion, and project image are all revealed to be illusions when physical contact is made.
These same spells, with the addition of phantasmal force, all allow creatures to use their action to make an Intelligence (Investigation) check to attempt to see past their illusory nature. The DC is equal to the spellcaster's spell save DC. This is, essentially, 5e's version of the "disbelieve" action. Like in 2e, a character forfeits the bulk of their turn in order to attempt to disbelieve an illusion.
I would also use 2e's "common sense" guidelines to determine whether a check is possible in the first place - the player would have to give me some sort of justification as to why their character would disbelieve the illusion, although a player is unlikely to forfeit their turn anyway unless they have good reason.
Similarly, if one creature has already seen through the illusion or is unaffected by it, and alerts an allied creature to its presence, I would give the affected creature advantage on their Intelligence (Investigation) check to see through it (based on 2e's suggestion of +4 to the saving throw).
I would also utilize 5e's concept of "passive" skill checks to give additional clues to players. A passive skill check value equals the characters skill check modifier plus 10. A 1st-level wizard with 16 (+3) Intelligence and proficiency in Investigation would have a an Investigation modifier of +5, and a passive Investigation score of 15.
Thus, if this wizard encountered an illusion created by a monster or NPC with a spell save DC of 15 or less, they would "passively" notice something "off" about the effect. They would not automatically be immune to the illusion, but it would be enough to justify an Intelligence (Investigation) check to see past it on their turn (if they're willing to use their action to attempt to do so, as normal).
Scared to Death
Can illusions do damage? Is damage done by illusions "real" damage, which persists after the illusion has ended? Can illusions kill? This is probably the biggest, most confusing change in illusions from edition to edition.
In OD&D, "Damage caused to viewers of a Phantasmal Force will be real if the illusion is believed to be real," but also, "the illusion will continue unless touched by some living creature." If the orcs believe that the phantasmal unit of archers is real, does the illusory volley of arrows "damage" the orcs, or does the illusion cease to be once the arrows touch the orcs?
In AD&D, phantasmal force explicitly does damage: "the magic-user creates a visual illusion which will affect all believing creature which view the phantasmal force, even to the extent of suffering damage". An illusionist who concentrates on the spell can cause it to "react appropriately" to contact with an opponent, allowing it to persist even after being touched.
However, phantasmal force is a visual illusion only. I don't see how a visual illusion alone can convince the orcs that they are being fired upon with real arrows, since they wouldn't feel the arrows (or even hear them whistling through the air). Perhaps the visual component is so convincing that the target's mind tricks the orcs into believing they have taken damage? Much and more must be read between the lines.
Tom Moldvay's Basic D&D states that "[Phantasmal Force] never inflicts any real damage. Those 'killed' will pass out, those 'turned to stone' will be paralyzed, and so forth. These effects wear off in 1-4 (1d4) turns."
2e provides a broader ruling:
Illusions are spells of trickery and deceit, not damage and destruction. Thus, illusions cannot be used to cause real damage. When a creature is caught in the blast of an illusionary fireball or struck by the claws of an illusionary troll, he thinks he takes damage. The DM should record the illusionary damage (but tell the player his character has taken real damage). If the character takes enough damage to “die,” he collapses in a faint. A system shock roll should be made for the character. (His mind, believing the damage to be real, may cause his body to cease functioning!) If the character survives, he regains consciousness after 1d3 turns with his illusionary damage healed. In most cases, the character quickly realizes that it was all an illusion.
When an illusion creates a situation of inescapable death, such as a giant block dropping from the ceiling, all those believing the illusion must roll for system shock. If they fail, they die -killed by the sheer terror of the situation. If they pass, they are allowed a new saving throw with a +4 bonus. Those who pass recognize the illusion for what it is. Those who fail faint for 1d3 turns.
Illusions are a powerful thing in AD&D! In 2e, phantasmal force is a 1st-level spell, affects a 500 square-foot area - at 1st-level, plus 100 square feet per additional level! - and can create an effect which can instantly kill every creature caught within it. The limits regarding believability and the affected senses - the spell creates only a visual effect - are still present, but those don't seem particularly difficult to overcome for a savvy player.
Phantasmal killer creates an illusory 4 HD monster in the target's mind. If the illusory monster makes a successful attack roll against the target, the target dies instantly. Only one attempt to disbelieve is allowed, and the spell lasts for 1 round per level of the caster - that's a lot of attempts to instantly kill any creature!
Weird causes targeted creatures in a 20-foot radius to see "phantasmal images of their most feared enemies". Even those who successfully save versus spell are paralyzed for a round and lose 1d4 Strength for a turn (10 rounds). Those who fail have to actually fight the phantasmal creatures with an entire separate combat playing out entirely in the minds of the affected:
Actual combat must then take place, for no magical means of escape is possible. The foe fought is real for all intents and purposes; affected creatures that lose will die. If a creature's phantasmal nemesis from the weird spell is slain, the creature emerges with no damage, no loss of items seemingly used in the combat, and no loss of spells likewise seemingly expended. The creature also gains any experience for defeating the weird, if applicable.
Although each round of combat seems normal, it takes only one-tenth of a round. During the course of the spell, the caster must concentrate fully upon maintaining it.
Appendix 2 of the 2e PHB suggests some limits on what kind of phantasmal monsters can be created and how to handle their combat statistics:
Monster Special Attacks: Before the caster can effectively duplicate a monster's special attack, the wizard must have undergone it (a wizard cannot conjure up the twinkle in a medusa's eye correctly without actually experiencing it - i.e., having been turned to stone by one).
Option: Illusionary monsters attack using the wizard's attack values. This would be a subtle clue that the monsters are fake.
Option: Extend the spell level control to monsters - the caster can create monsters only if the total monster Hit Dice are equal to or less than the caster's level (an 8th-level caster could convincingly do one hill giant, two ogres, or four 2nd-level fighters).
Thus, a 20th-level wizard could force the affected creatures to face a phantasmal 20 HD very old red dragon. It's unclear to me if the affected creatures experience the phantasm together or separately - that is, would a party of affected player characters fight the phantasmal dragon as a unit, or would each of them need to battle the dragon individually?
Modern editions of D&D have simplified things, for better or worse. It is readily apparent which illusions are capable of doing damage, because they say as much. Unfortunately, the damage-dealing capability of these spells has been greatly reduced.
In 5e, phantasmal force does 1d6 damage per round (if the target fails the initial Intelligence saving throw, if concentration is maintained, and if the target doesn't successfully disbelieve the illusion). An instant-death illusion of a falling stone block it is not, but the caster could make it look like, I don't know...individual bricks are falling from the ceiling and plunking the target on the head every round.
Another wrinkle is that the spell does 1d6 psychic damage, but the creature "perceives the damage as a type appropriate to the illusion". Since the spell allows the caster to create an effect which occupies a 10-foot cube, the caster could create an ogre and have it do what seems to be bludgeoning damage, but what ogre does 1d6 damage?
There's also the issue of damage resistances and immunities. If a raging barbarian is resistant to bludgeoning damage but not to psychic damage, how do they react to taking phantasmal bludgeoning damage which is actually psychic damage? If I tell the player, "You take 4 bludgeoning damage" and they say "I'm resistant to bludgeoning damage, so I instead take 2 damage", do I say "Well actually, it's psychic damage" or "No, you're not resistant to this damage...for reasons."
It's entirely unconvincing, and almost any player will see through the ruse immediately. Maybe this is a feature, rather than a bug. Maybe phantasmal force is meant to be an obvious illusion, since it is a low-level one. Maybe.
Phantasmal killer now frightens the target if they fail the initial Wisdom saving throw. Then, at the end of each of their turns, the target repeats the saving throw, taking 4d10 damage on a failed save - not even close to instantly killing most creatures at 7th-level, which is when a caster can first learn it. What's in a name?
Weird works similarly, but affects all creatures in a 30-foot-radius sphere. It still does 4d10 damage per round, despite being a 9th-level spell. Meteor swarm, by comparison, does 40d6 damage in four separate 40-foot-radius sphere. Power word kill does effectively 100 damage. Wish can do basically anything.
In the 2024 version of 5e, all three spells have received some love. Phantasmal force now does 2d8 damage per turn, phantasmal killer does the same damage but does so immediately upon a failed save (rather than requiring a second failed save), and weird does 10d10 damage initially and 5d10 damage on subsequent turns. It's a step in the right direction, but still rather underwhelming compared to what AD&D offered.
Illusionist wizards in the 2024 version will also have a few conjuration spells always prepared, which they can cast as illusions. This is similar to AD&D's idea of using illusions to create phantasmal creatures, rather than having phantasms do arbitrary damage determined by the spell. I was somewhat opposed to this at first, as it makes the Illusionist feel like a Great Value Conjurer, but the more I've read about how illusions worked in AD&D, the more I've come around to it.
I'm not sure how to feel about all of this. Clearly, the illusions of D&D's early days - with even the weakest spells having the potential to instantly kill anyone who failed a saving throw - were far to powerful, while also being nebulous enough to be entirely defanged by DM fiat and "disbelieving" players. The illusions of modern D&D are much more mechanically hard-coded - as is almost everything else in the game, at least when it comes to doing damage in combat - but feel totally lacking in potency compared to what came before.
Is there a way to split the difference? Damned if I know.
So You've Pissed Off an Illusionist
I've been running my buddy's 5e campaign as a guest DM for a month or two now. I was burned out from running my own game, and so was he, but I felt up to the task of designing a couple of quick adventures for his campaign, where the player characters are 9th-level - much more exciting than 4th-level AD&D, in my opinion.
I start planning all of my adventures by determining the primary antagonist. For this adventure, I rolled up an archmage. Because I love specialist wizards, all of my wizards have a school in which they focus their studies. For this one, I rolled a d8 and got a 6, which according to alphabetical order meant that my archmage was an illusionist.
I started to think about how to design an adventure around an adversarial illusionist, and it came to me very quickly. Really, just look at the spells available to an illusionist and think of all the trouble they can get up to.
I decided that this arch-illusionist had it out for the party - the circumstances are too specific to get into here, but there was a good reason (in the eyes of a megalomaniacal archmage, that is). Looking at the list of illusion spells I see disguise self, illusory script, Nystul's magic aura, creation, dream, seeming, project image, and others.
Here's what happened: While the party was returning to the city after their most recent adventure, they started having terrible dreams, which prevented some of them from resting, leading to accumulating levels of exhaustion (one of 5e's nastiest mechanics).
When they got back to civilization, they found that they suddenly had a bad reputation. Shop owners had closed their doors to them, stating that the party had sold them phony goods or paid in counterfeit coins and gems. Apparently, they had forged documents to get access to a royal gala, and had angered a number of important NPCs there.
The party knew that something was amiss, and started contacting the few friends they had left in the city. The party eventually learned that people who looked exactly like them had been going around town sullying their reputation. Some people claimed to have seen the party traveling to and from a minor noble house's estate.
The party asked around and found out that this minor noble house had recently dismissed many of their servants and guards. The dismissed workers reported that the family had begun behaving strangely - they played elaborate pranks on each other and the staff, enjoyed different food and wine than usual, stopped entertaining guests, and the like.
The party went to the estate to investigate, and found that while it appeared ordinary at first glance, it was all an illusion - the estate had become a rundown frat house. Mercenary thugs were magically disguised as noble knights, goblins as the noble children, and a cabal of illusionists as the noble family. In a secret cellar, they found the real noble family locked in a cage, trapped in a magical dream state, believing themselves to be at a high society gala with the city's greatest movers and shakers.
At the center of the deception, of course, was the arch-illusionist. He appeared in every room of the house, taunting the party until they struck him, at which time he disappeared, and a magic mouth appeared to taunt them into finding him elsewhere. In AD&D style, I gave the illusionist the ability to summon phantasmal monsters for the party to fight - they could either "kill" the monsters (exhausting their resources in the process) or "disbelieve" them to determine that they were illusions (this being a master illusionist, that was no easy feat).
The party finally found the illusionist and battled an illusory dragon he had created. They cornered him, slew him, and...he melted into a pile of ice and snow. A magic mouth appeared again, swearing that they would never find him. He was everyone, and no one. He could be anywhere he wished, and when next they found him, it wouldn't be him at all. Since the party had overcome his house of illusions, he made a gentleman's agreement to let bygones be bygones, but he warned them - he would be watching.
And that is the power of illusionist. They can be anyone. They can make someone see, hear, smell, taste, or feel anything. They can make themselves appear anywhere - even in someone's dreams. The illusionist can force someone to battle enemies that aren't there, trap them in a prison within their own mind, or scare them to death. When the illusionist finally shows up, it might not be them at all. And just like that, they're gone.
