Adjudicating Illusion Spells in D&D 5e

In D&D, Roleplay by Paul3 Comments

Waterdeep: Dragon Heist is the most recent adventure released by WotC and as an urban adventure it is likely to test the skills of players (and their characters) in entirely different ways. I’ll be running this adventure as an online game for some of my regular gaming group and one of the players asked, “So illusionists then… They’re either good or rubbish based upon DM leniency. How do you plan on running it?”

Well, Martin, this article is for you! (And any other DMs or Players looking for a bit of guidance).

Adjudicating Illusion Spells

By “illusion spells” I mean those spells that have a freeform effect usually involving the creation of some entity that the caster wants others to believe is real, rather than all spells of the Illusion school.

Spells that fall into this category include:

  • Minor Illusion
  • Silent Image
  • Major Image
  • Programmed Illusion

As they have a less clearly defined effect than something like Invisibility or Mirror Image and are much more open to differences in interpretation.

Hard Limitations

All of these spells are known to be illusions when they interact with real world objects as objects (they try to hit someone) pass through them (they get hit by an arrow). Where the caster gets to control the illusion they can accommodate for that change to the extent the spell allows.

For instance, a major image spell (edit: I had previously said a minor illusion could do this) can be used to make an image move from point A to point B naturally, so long as the caster is in range of the illusion. This includes slipping and sliding on ice, or a slight stumble as they change direction mid-sprint etc as a realistic flourish. The caster can make the illusion dodge around a stationary table.

However, the spell doesn’t allow the caster to have the character duck and dive in response to a fight looking like its evaded swipes. There’s no AC stated for the illusion and this isn’t part of a movement, as well as stretching fictional believability (in the example of an arrow, I’d give them AC10).

Where illusions are allowed to have more complex movements they can accommodate for the placement of real objects helping the whole appear more legitimate. For example, as part of a Programmed Illusion you could have the illusionary prince appear to bang their hands on a real, stationary table and produce an illusory sound. If someone placed a cushion beneath the hand the illusion would go through the cushion and produce the same sound though, thus revealing it for what it is.

Creatures that have additional senses the illusion cannot account for, e.g. Tremorsense, will immediately know it to be fake. If Tremorsense is their only sense then they won’t even know the illusion is there!

However, creatures that rely upon enhanced senses to be able to see hidden or invisible creatures, could still be fooled by an illusion depending upon the exact setup. Creatures looking from the Ethereal Plane into the Material Plane could also be fooled by an illusion.

When an illusion is revealed to be “fake”

Where the illusion breaks these hard limitations, any characters that can see the illusion immediately know it to be an illusion. If able they can communicate that to other characters that didn’t see it immediately.

However, the illusion doesn’t fade away, become translucent or be dispelled unless the spell states explicitly otherwise. It remains as solid looking and real as it did when cast, and therefore some characters may not believe it is an illusion, realise which entity is the illusion etc depending upon a host of factors.

So if Gruggak the Goblin shouts out “The ‘uman isn’t real!” his friends may not know which of the five humans in front of them Gruggak is referring to. Even if they did they’d have to fall into the soft limitations outlined below to know it was an illusion. They may still trust Gruggak and just take it as read without having to find out for themselves though, and that’s an area of DM ruling-in-the-context-of-that-encounter.

Soft Limitations

Where a character doesn’t have first-hand experience of the illusion being revealed for what it is, they may still suspect an illusion rather than reality.

For example:

  • Their companion may have told them
  • The illusory creature appeared out of thin air without the usual conjuration shenanigans
  • The lion doesn’t roar (Silent Image)
  • “How did something that large even get down here?” or “There’s something just odd about this situation…”

At this point the spells direct for an Intelligence(Investigation) check vs Spell Save DC of the caster to discern the illusion for what it is. This takes an action, so in combat it’s may still be a good debuff! If the check is failed the character believes it is real until they hit a hard limitation, or the situation changes so sufficiently they can make another roll (though I’d impose disadvantage as they struggle with self-doubt from the previous failure).

Common sense has to apply here in terms of whether an illusion is suspected or not, and as a DM I would take into account the character’s Passive Perception, their general Wisdom score, the environmental factors, and any attempt by the caster to seat the illusion more closely in reality.

For instance, a Silent Image spell cast mid-combat with the image coming from around a corner might not suffer initially from the lack of sound as the usual sounds of combat would have naturally muffled its steps anyway.

In situations like this I might simply rule there’s not sufficient evidence for that character to consider it’s an illusion through to imposing disadvantage on the Intelligence (Investigation) check.

Thinking out loud, I am reticent to suggest a Wisdom (Perception) check be called on for a character to notice the lack of sound when in combat as I want to reward the illusionist’s creative instinct and there’s probably been some disadvantage created in having to manage the Silent Image in that way. I feel like this creates too many rolls, a possibly arbitrary success threshold (what’s the DC… the Stealth of the illusion? The spell save DC of the spell? In that case what does the Investigation roll do?). If the character has a massively high Perception then I’d’ve considered that as part of whether I’d grant them a roll to check illusion in the first place.

If the caster or characters are going all in on the illusion, they may want to use a Charisma (Deception) or Charisma (Performance) check to impose disadvantage on the investigation roll as their actions are so congruent with the illusion. In addition, perhaps using the Minor Image cantrip to present a war cry around the corner before the silent image charges around sets the situation just well enough and would grant disadvantage on any Intelligence (Investigation) roll to ascertain the truth, or just mean I rule no roll is able to be made.

Making an illusion exactly the same as a person, object, place etc

Unless the spell specifically states otherwise the spell cannot create an exact duplicate of a person, object or place. The caster’s mental image of this isn’t so clear that this is possible. It is entirely possible to create an illusion that is as good as needed but those wouldn’t bear great scrutiny, and I’d grant advantage on the Intelligence (Investigation) check.


So there you have it, my take on adjudicating Illusion spells in 5e. If you’ve got criticism, suggestions or think I’ve nailed it please comment below!


  1. You lost me at minor illusion can move, you also imply that the minor illusion is a creature. That’s the spell silent image.

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