Minor Illusion: An Attempt at an Explanation

Some time ago I ran a session where the party was searching a dwarven mine for the monster that had been killing miners for the past few weeks. The nervous elf keeps glancing around anxiously the entire time, afraid the monster could jump him at any moment. He gets an idea. He raises his hand and casts the illusion spell Minor Illusion to create a mirror so he can see around the corner. What does he see in it? After the session, this scene sparked a lengthy discussion among the players. I want to summarize what we figured out here.

The Fiction

The description of the spell in the Player's Handbook is long, and you'd think that means thorough. Instead, it raises a whole lot of questions about what's possible with the spell and what isn't. The following passages are essential to this problem:

If you create an image of an object, [...] it can't be larger than a 5-foot cube. The image can't create sound, light, smell, or any other sensory effect. [...] If a creature uses its action to examine the sound or image, the creature can determine that it is an illusion.

Player's Handbook, p. 223 (German edition)

Players typically imagine the image as a static, three-dimensional hologram. But that raises a number of questions, such as: Can the image cast a shadow? Is the image affected by light, wind, or vibration? If it's standing right there in the room, how can it fade away for one person but not another? Or, in our case: What does the fearful elf see when he tries to look around the corner using the mirror? None of these questions can easily be answered by the "hologram" explanation.

One possible solution to the problem starts with the description of the schools of magic. Since this is an illusion spell, we need to take a closer look at that school.

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, [...] but the most insidious illusion spells plant an image directly in the mind of a creature.

Player's Handbook, p. 203 (German edition)

With this explanation, the spell's description starts to make sense. The image isn't a hologram at all. It's an illusion within the minds of those affected by it. Importantly, the minds of creatures who haven't realized it's an illusion actively try to keep the illusion intact. They see things that aren't really there, for as long as they haven't seen through the illusion. That means the image can cast a shadow. The image can be affected by light, wind, and vibration. The spell turns your own mind against you. How well that works depends on the power of the character casting the spell. Affected creatures see what they expect to see.

This approach also answers the question of what the fearful elf sees when he looks around the corner with the mirror. He sees nothing. The mirror would have faded away for him. His mind knows it's an illusion and rejects it. But suppose he didn't know that. He expects an attack at every moment. The answer would be that he sees, in the mirror, the very monster he's so afraid of. Whether it's actually lurking there is a different question entirely. It's also a separate question how the elf reacts once he realizes there's no monster around the corner after all. That's the point where the battle of intellect against one's own manipulated mind begins.

A creature can use its action to examine the image or sound. If it succeeds on an Intelligence (Investigation) check against your spell save DC, it can tell that it is an illusion.

Player's Handbook, p. 223 (German edition)

If the elf wins that battle, he regains control over his own mind and it rejects the illusion. If he loses the battle, he becomes convinced this is reality. The mirror must be some powerful artifact, showing the future, the past, or something else entirely. The Mirror of Erised from Harry Potter sends its regards.

The Reality

One last point that causes confusion is the absence of a maximum number of creatures that can be affected by the spell. That suggests you don't need specific targets - everyone is affected. A creature entering the room after the spell was cast would presumably also see the illusion. To get a handle on how this affects the reality of the game, it helps to look at how magic works in D&D 5e in general, since every spell is built on the following assumptions.

Whenever a magical effect is created, the strands of the Weave twist and tangle to make that effect possible. With divination spells like detect magic or identify, you can catch a glimpse of the Weave.

Player's Handbook, p. 205 (German edition)

As with every other spell, the Weave of magic is altered. It's confirmed that the Weave can be perceived, one way or another. From that, we can conclude that any creature that can perceive the location of the altered Weave is affected by the illusion.

At this point you could argue that, under this interpretation, the spell Minor Illusion is too powerful. But there are limitations. The spell can't create an image and a sound in the minds of creatures at the same time. The spell has a time limit of one minute. The image can't move and vanishes if someone touches it. All of these are limitations that quickly raise suspicion and prompt an Intelligence check. On top of that, these exact limitations are precisely what set the spell apart from more powerful illusions.

Ultimately, the spell doesn't produce a hologram at all. It produces a battle against your own mind.