Can static illusions be cast on moving objects so the illusion moves with them?

Question

Can static illusions be cast on and move with moving objects?

As a 20th-level Illusionist Wizard, I can make an illusion of:

• a functioning volcano (Mirage Arcane)
• or a ferocious monster (Major Image)
• that can even deal damage (Phantasmal Force)

But using an illusion spell, can I, for example:

• put an illusory sweater on my dog?
• put an illusory blade on the end of my normal wooden staff that follows the staff even while I use it? (it would look scary!)
• put an illusory vase on a table, which moves with the table if the table is pushed? (it wouldn't get knocked over, of course)

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Note: This question is about the general function and utility of illusion spells as a school of spells in 5e rather than about a specific spell. If specific illusion spells give different answers, that is a valuable part of the answer to this question.

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Supporting Thoughts:

Illusions can be cast at a place on the ground, and they stay there unless the spell gives you the ability to move them.

If the ground moves, does the illusion move? Does the D&D world spin like Earth does or fly through space? If so, the illusion usually moves with the ground on which it was cast.

Can an illusion spell such as Minor Illusion or Silent Image be cast on an object such as a sword or a table? If that object is then moved, does the illusion move with it? What about an illusory hat on my ally's head?

Put another way, what is the reference frame in which an illusion is cast, and is this something the caster can choose?

Clarification: I am not asking if I can pick up an illusion and move it, only if the illusion can be attached to something other than a fixed point on the ground and what happens when that something moves. A Minor Illusion would still be a simple, stationary object sort of illusion.

Answer 1

Yes, within some restrictions.

There are a wide range of illusion spells, each of which have their own specific conditions and limitations. This means that we have to look at each spell in turn. Generally, though, you should be able to create at least some of the effects you're looking for.

Illusions on creatures

Creating moving illusions on creatures is much easier, and there are several spells that allow for this. Specifically, spells like Disguise Self and Seeming allow you to change the appearance of your body, clothing, and any items you carry, including weapons: (text from Disguise Self)

You make yourself, including your clothing, armor, Weapons, and other belongings on your person, look different until the spell ends or until you use your action to dismiss it.

In fact, the text of disguise self gives an example of an illusory hat:

For example, if you use this spell to add a hat to your outfit, Objects pass through the hat, and anyone who touches it would feel nothing or would feel your head and hair.

Thus, altering your weapon to look scarier or adding sweaters to dogs is perfectly plausible with disguising illusion spells.

Illusions on objects

It's a little more difficult to make an illusion that moves on objects. You could accomplish everything you ask about with Major Image, but you have to spend an action on moving the image around:

As long as you are within range of the Illusion, you can use your action to cause the image to move to any other spot within range. As the image changes location, you can alter its appearance so that its movements appear natural for the image.

However, because you can have the illusion alter itself to appear natural during the movement, you can alter the sweater on the dog to move like a real sweater or have the vase wobble on the table. This is a rather costly spell to use, given that you have to be in range and constantly spend actions in order to maintain it. Note that Minor Illusion doesn't say that you can move it around with an action, so you can't use it to create these effects.

You could use a Programmed Illusion to replicate some of these effects, but given that the illusion is fully scripted, it doesn't seem like the kind of dynamic response you're talking about.

Illusions on Terrain

For spells like Mirage Arcane, the terrain itself is changed in appearance. This means that if the terrain moves (as the world turns, for instance), the illusion will also move with it. It's worth noting that the world isn't always round and rotating, but that's usually up to the DM. Generally, the rules don't specify a particular frame of reference for illusions.

Answer 2

Most likely not, by RAW.

Unless the spell states that the illusion can move on its own, or you are able to move it yourself (and do so,) then the spell is effectively "static." In which case, the only RAW basis by which the illusion should be moving at all is if its reference frame is also moving.

(And yes, we know that for most spells to even function properly, they must refer to a reference frame rather than some concept of "universal spacetime coordinates." Otherwise every teleportation spell would leave the caster floating in empty space, as the planet he was standing on moved away from his intended destination point in the few seconds it took to cast it.)

Armed with just that knowledge, we can then logic out how "static" illusion spells should work through the following:

• Consider if you're on the ground watching a sky ship, and you fix its location in your mind as you are about to teleport to it. A companion suddenly distracts you with a question, and you turn away to answer. After a minute you finally end the conversation, and without turning to see if the ship has moved you hurriedly cast your teleport spell. The most reasonable expectation would be that, if the ship has moved, you teleport to where you planned to relative to your position, NOT the ship's position--even if this would mean you fall. To expect otherwise would allow the spell to work even if the ship had moved beyond the spell's range. Conclusion #1:

The frame of reference for a spell should be relative to the caster, not the target or destination point.

• Consider if you're on a large moving vessel, and teleport, say, from a holding cell below deck to the deck of the sky ship you're on. The most reasonable expectation is that this works without incident; the spell loses its value completely if attempting such a teleport results in falling out of the sky (or worse, if the DM expects you to adjust for the speed of the vessel and to move your teleportation destination point to compensate.) Conclusion #2:

The planet the caster is on isn't necessarily the spell's frame of reference; rather, it should use the frame of reference closest to the caster.

• Finally, consider if you've cast the "Fly" spell on yourself, and are flying right above a sky ship, keeping pace with it. If you then use Minor Illusion to create an illusory treasure chest on the deck of the ship, the most reasonable expectation is that the illusion appears, and then slides back through the ship, eventually hovering in mid-air behind it as it moves forward. This is because we don't expect the caster's personal movement to affect an otherwise "static" spell. If it did, not only could you send your minor illusions flying simply by sprinting for a few seconds as you cast the spell, you could also "animate" them by standing still to cast the spell, then walking around as you watch the illusion match your movement. Conclusion #3:

The caster cannot personally be the frame of reference for his spells; so they must instead use the next-closest frame of reference.

Obviously there will be spells that specify for themselves how to handle all these situations, but for those that do not, these conclusions should give a solid basis for reasonable applications of static illusions.

If you're in your house and cast a minor illusion of a sweater on your pet, the sweater stays in mid-air when the pet walks off. Even if the pet is in motion when you cast the spell (or even if you are,) the illusion still remains motionless.

But if you have a moving house boat, and cast a minor illusion of a painting on an interior wall, whether the painting stays on the wall or eventually winds up hovering over empty water depends on whether you're standing on the boat (making it your frame of reference,) or whether you're flying alongside the boat (making the planet your frame of reference) when you cast the spell.

By RAW, a static illusion's motion (or lack thereof) should only ever be determined by the caster's closest non-personal frame of reference, not the target or destination.

Yeah, magic's weird.