The party has been caught mid-heist and is now running down the city streets, hoping to escape their pursuers. They turn down a narrow alley, for the moment passing out of sight, “Honey-bells comes to a stop, turns about and casts Minor Illusion, creating the illusion of a 5 foot hole in the ground stretching the width of the alley!”
Is this allowed? (Ignore that it’s an impractical strategy since the average grunt pursuing you can clear a 5 foot jump easily, and now you’ve wasted an action on a spell rather than dashing.)
The Illusion school cantrip Minor Illusion reads in relevant part:
You create a sound or an image of an object within range that lasts for the duration. The illusion also ends if you dismiss it as an action or cast this spell again.
If you create an image of an object–such as a chair, muddy footprints, or a small chest–it must be no larger than a 5-foot cube. The image can’t create sound, light, smell, or any other sensory effect. Physical interaction with the image reveals it to be an illusion, because things can pass through it.
If a creature uses its action to examine the sound or image, the creature can determine that it is an illusion with a successful Intelligence (Investigation) check against your spell save DC. If a creature discerns the illusion for what it is, the illusion becomes faint to the creature.
The relevant question here is whether or not a hole is an “object.” The PHB isn’t helpful since we don’t have a clear definition of an object, except that we know creatures are not objects. So, let’s turn to some other sources for guidance:
Merriam-Webster’s definition 1a reads, “something material that may be perceived by the senses.” A hole may be perceived by the senses, but it is not material. What allows you to perceive the hole is perceiving the material (the sides of the hole) around it.
The OED’s definition is, “a material thing that can be seen and touched.” Again, a material thing, but this definition creates some problems for illusionists as the thing created here can be seen, but it cannot be touched.
So, we have a strong case against being able to create an illusion of a hole, as a hole is not an object, but rather the absence of one. That would fit with the inability of Minor Illusion to make it seem as if an object has disappeared. Just as you cannot turn an object invisible, you cannot turn the road and the five feet of dirt below it invisible either.
But could you create an illusion of a small box, open on one side? I can’t imagine that you wouldn’t be able to do so. If you can do that, then how about this:
“Honey-bells comes to a stop, turns about and casts Minor Illusion, creating an illusion of a box, 5x5x5, open on one side. The open side is even with the street, with the rest of the box below street level. Basically, a hole in the ground — but with wood paneling on 5 of its sides.”
There is one bit of rule language that would prohibit this outright, and that would be limiting the spell’s range to a space you can see. The caster wouldn’t be able to see below ground to place the illusion there. Interestingly, Minor Illusion doesn’t have that limitation — makes sense for causing distracting sounds around a corner or in the next room, but presents some fun options for casting visual illusions through walls as well.
It doesn’t need that limitation though, because here’s what happens next:
“The guards round the corner to the alley and [roll perception check] rush to Honey-bells and set upon him with their clubs.”
“But what about the hole in the ground?”
“You didn’t create an illusion of a hole. You created an illusion of a box …which would appear filled with street. If they looked closely they might be curious to see the wooden frame of the box, but they’re not examining an otherwise normal-looking road. Now, does a 14 hit?”
If you created an illusion of a small box, open at the top, and conjured it on a table at the same location as a candle, and then you looked into the box… you’d see the candle. With the hole in the ground, it appears filled with ground.
Our DM will rewind the clock though since the player didn’t understand how the spell works, while Honey-bells, a skilled illusionist, would never have made that mistake.
“Honey-bells comes to a stop, turns about and casts Minor Illusion, creating an illusion of a 5 foot square piece of cloth laid out across the alley. On the cloth is a photo-realistic painted optical illusion of a hole.”
Artists are able to create rather compelling optical illusions on two-dimensional surfaces, such as this piece of art by Alex Makviov:
And of course we have this classic gag from the Road Runner cartoons:
So, can Minor Illusion create a magical illusion of an object that itself contains an intricate but mundane (non-magical) optical illusion?
Rule as written provides no limitation on the intricacy of the illusory object. However, reading the rule that way would allow for some really bizarre illusions. Honey-bells produces a cloth with an illusion of a hole, but not a 5 foot deep hole. Since the illusion is 2 dimensional, it could be an illustration of a hole 500 feet deep.
As much as fun as I think this idea is, it potentially opens the door to more abusive uses than evading some local law enforcement. What I’d rule is that due to the nature of the optical illusion (that these really require the exact right perspective from the viewer to be convincing and a fast-moving viewer is the least likely to be tricked) that the person seeing the illusion has advantage on their Investigation checks.
All this is somewhat moot though, because the pursuing guards could just leap across the illusion, bypassing it none the wiser. So, what is Honey-bells to do?
For now, I’m going to stick to terrain manipulation. An illusion of a wall won’t be very helpful since the wall can only be 5 feet high and the guards would fall through it the moment they try to climb.
Honey-bells should, before the heist, learn the layout of the neighboring streets quite well and try to find an alley that dead-ends with a climbable wall. Then during the pursuit, cast Minor Illusion to make the wall appear 5 feet taller, and ideally jettying out over the alley a few feet, so as to make scaling the wall appear an impossibility and thus preventing the pursuers from ever discovering the truth.
Outside of a city environment, there may be opportunities to evade pursuit by making a hole or chasm that already exists appear narrower than it really is. A 5 foot rocky outcropping on the near or far side of a ravine could induce an enemy to accidentally walk out over the edge or try to jump to a spot that is actually thin air — in either case, by the time the illusion is discovered, it’s too late to react.
A similar false safe step could be used on partially-crumbled stairs, a rope bridge with planks that are missing, or other perilous terrain. But, a foot pursuit down city streets doesn’t seem to be the right time for this particular gag.