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I've been thinking of various ways for a crafty Illusionist/Rogue to 'hide in plain sight' by essentially creating the cover that they want to hide behind.

Ideas include things like:

  • Create a wall that is 2.5ft closer than it really is and hide in said space
  • Create some innocuous barrier or container and hide within it
  • Create a 'sheet' that essentially has what an empty room looks like 'painted' on it, so someone walking past an open door would see an empty room.

From different discussions I've had, these all should work, but the one lingering one I have is, what if my illusion is simply 'empty space'?

If I were to create an illusion of empty space and then place myself in it, would I be essentially invisible?

What if I created an illusion that was partially-empty space, like a 5x5 cube of swiss cheese? Would it seem like there was a swiss cheese cube formed around my person or could my 'fake-empty-space-image' fill the holes?

Would my 'empty space' illusion simply need to work like the aforementioned 'sheet of what I expect you to see' in order to function (and thus most likely only work from limited viewing angles)?

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  • \$\begingroup\$ How is your "empty space" concept different from invisibility? Apart from the fact that it is much easier to create if possible. \$\endgroup\$ – Szega Jul 31 '17 at 13:51
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    \$\begingroup\$ @Szega To name a few, it's immobile and breakable with an investigation check or simply passing through the space with your face. \$\endgroup\$ – Ethan The Brave Jul 31 '17 at 13:52
  • \$\begingroup\$ Somewhat related: rpg.stackexchange.com/questions/97501/… \$\endgroup\$ – Ethan The Brave Jul 31 '17 at 13:55
  • \$\begingroup\$ What spell do you mean? \$\endgroup\$ – enkryptor Jul 31 '17 at 13:58
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    \$\begingroup\$ I think there's probably some distant relationship between the above question and this question. Second cousins, maybe? \$\endgroup\$ – Hey I Can Chan Jul 31 '17 at 15:47
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It is not possible with minor illusion, as with that

You create a sound or an image of an object (PHB 260)

and neither vacuum, nor air is usually considered an object. (There are discussions about this, looking them up is left to the reader.)

Nor could you use mirage arcane, as that explicitly states, that

The spell doesn't disguise, conceal, or add creatures. (PHB 260)

With silent image and major image there is a hint of possibility:

You create the image of an object, a creature, or some other visible phenomenon (PHB 276, 258)

It comes down to whether that the space inside the illusion is filled with air is a phenomenon. Even if we accept that it is, this illusion has to change relative to the viewer, which would require precise calculations from the caster and would be essentially impossible against more than one observer.

My personal ruling would be that this is an abuse of the spell and not allowed. It is much more streamlined and easier to handle this way. If a PC wants to be invisible, there is already a spell for it and not even that high level.

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    \$\begingroup\$ Nice answer. And yeah, air is not a visible phenomenon. \$\endgroup\$ – Miniman Jul 31 '17 at 14:38
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    \$\begingroup\$ "which would require precise calculations from the caster" which of course the magic takes care of for you \$\endgroup\$ – Dale M Jul 31 '17 at 21:05
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    \$\begingroup\$ @DaleM We have no reason to suppose that the spell has a way of sensing an observer and changing itself relative to them. It is not a creature. \$\endgroup\$ – Szega Jul 31 '17 at 23:08
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    \$\begingroup\$ @Szega it's an illusion - it's not much good if it can't "Illude" people. The "image" of an object must look like the object under different lighting conditions and different points of view otherwise it is not the image of the object. \$\endgroup\$ – Dale M Jul 31 '17 at 23:45
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    \$\begingroup\$ @DaleM I get your point. I think of a light illusion as a set of points that emit light as an object would reflect it. I feel this is the intended interpretation, but yours of one magical construct projecting an image in all directions could be valid. But for it to work properly, it would also need to account for distances, so it has to project to all points where eyes could possibly be. Eyes with likely different optical properties and either of a left/right pair. Thats a really convoluted set of photon trajectories. One that might not even be possible for certain phenomena (like this). \$\endgroup\$ – Szega Aug 1 '17 at 6:30
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Of course you can, just not a very good one. Since you're limited to illusory "objects," it's going to be, like you suggested, pretty much the same as your "sheet with a [photorealistic] picture of an empty room" only you've got one sheet on each side of an illusory cube.

Such an illusion would be roughly comparable to things like the chalk artists who draw pictures of holes and other impressive things on the sidewalk. Extremely believable when viewed from the intended angle, but stepping too far to one side or another results in the parallax giving it away almost immediately. If using a spell that allows the illusion to be altered on an ongoing basis you might be able to track a person or small group and adjust it to continuously be what they expect to see, but it's not going to be an easy thing to do and will become close to impossible if they split up.

How successful this would be as a tactic will be highly dependent on placement. Near a wall would work OK as the parallax will be minimized. And the further away from the observer the harder it will be to spot. In a room with say polished obsidian walls and floors where there aren't any distinguishing characteristics for the eye to use to see the parallax it might even hold up to anything short of someone walking into it.

Do note two things though:

First, if I were the DM, I'd probably ask for a perception (or maybe stealth, I'd have to think about it) check to determine how clearly you can visualize exactly what texture needs to be on the cube to make it appear empty from a particular location. (Though I'd give you a big bonus if you took the time to scout it out and actually look at it from that location.) This isn't a normal "hey, there's a barrel here" use of an illusion spell, this illusion has to merge flawlessly with its environment, even moreso than putting an illusory wall over a doorway. If you don't line the bricks up with the background, or if you cut off half a wall sconce, it just got a lot more likely that somebody will notice. They won't be able to see through it until they realise it's illusory, but "Hey Thak! There's a weird piece of wall standing in the middle of the room!" is generally not something you'll be pleased to hear.

Second, consider whether you actually want your DM to think of doing this. This is the kind of trick that gives a much larger advantage to well-entrenched defenders than to wandering adventurers since the defenders will have had time to study and perfect the appearance of their illusion to match their intended purpose.

Other than that though, "Illusion of what this place would look like except without me in it as seen from over there" has been a staple part of more D&D campaigns than I can count. It gets past some of the weaknesses of Invisibility at the cost of being usable in a much more limited set of circumstances and being easier to pierce once the other guys realise what's going on.

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  • \$\begingroup\$ Not all Illusion spells are limited to objects. \$\endgroup\$ – mattdm Oct 5 at 22:44
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Probably not

RAW, there are no strict rules on what exactly illusions can or cannot be, so the standard answer is "ask your DM".

If you are the DM, consider two things:

  • Spells only do what they say
  • A spell doesn't do X if there is another spell of the same or higher level that does X

So the result depends on what spell is used. Let's take the Minor Illusion for example.

Rules as intended the Minor Illusion spell creates an illusion of a simple object (see the 2019 Sage Advice Compendium):

An illusory object made by minor illusion is meant to be like a stool or a rock

It cannot create an atmospheric effect like fog, or lighting effect like light or darkness, or visual effect like reflections or invisibility. There are other spells for such effects - Fog Cloud, Light, Darkness and Invisibility.

Keep in mind that, aside from Invisibility spells, no illusion spells can make a creature completely invisible, or make them look like a part of terrain. Even the high-level Mirage Arcane spell can't conceal creatures inside the illusion:

The spell doesn't disguise, conceal, or add creatures.

Instead, Illusion spells can change creature's appearance (Disguise Self, Seeming spells) or substitute their visible location (Mirror Image, Mislead).

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