How does Major Image interact with Blind Fighting?

Question

The rules seem to give little indication if the blindsight gained by the Fighter's Blind Fighting is actually able to automatically foil high-level illusions (such as Major Image). But this text from the spell seems the most relevant:

The image appears at a spot that you can see within range and lasts for the duration. It seems completely real, including sounds, smells, and temperature appropriate to the thing depicted

To me, this sounds like that Major Image automatically foils blindsight, unless the character makes a successful Investigation check (but this burns the character action). If my interpretation is correct, this would imply that Major Image (assuming no caster is actively controlling it), while most of the time it is seen as something "static" and "non-reactive", actually does have a reactive part in it regarding perception (even special senses).

My example is the bat echolocation. Suppose there is a permanent Major Image of a 20-foot cube of stone, and we will also assume that no caster will be actively controlling it. The moment the echolocation range comes into a square affected by Major Image, the spell would reactively reflect sound waves, just as if it was a real stone cube.

My doubt comes from this next part of the spell description:

Physical interaction with the image reveals it to be an illusion, because things can pass through it

The problem, here, is that basically anything regarding perception is a physical interaction. After all, humans see things because light is reflected of objects (or some objects themselves, such as stars, emit light). But perception is indeed physical interaction - I don't see any way around this. Of course, this would make Major Image useless even without blindsight, since normal sight is physical interaction with the object you are seeing, and I don't think this is the intent of the rules.

Thus, my interpretation here would be that the text above refers to physical contact rather than interaction, implying that creatues or solid objects can pass through the illusion, releaving the "stone cube" to be a fake. But there are possibly exceptions to this, such as the illusion of a fog.

And now comes the "things can pass through it" problem. If we interpret "things" as meaning "solid objects" (like arrows) or even spells, that's a thing, but what about light and sound? Does the Major Image let the sound waves of the bat echolocation pass through it - but this would automatically reveal the illusion - or does it react to them, reflecting them?

My interpretation would be that Major Image does need to react to light, sound and temperature, otherwise normal sight/hearing would foil it automatically.

I have read (in the thread linked below) that this is DM call, but I am the DM in a campaign, thus I would like some feedback on how this is ruled by others.

Related: Is a creature with Blindsight affected by illusions?

Answer 1

Physical interaction here likely means touching or hitting it

Fundamentally, as these details are not covered in the spell or rules, it will come down to the DM making a ruling. But you state you know that, you are the DM, and are looking for considerations what a reasonable ruling could be.

Remember that D&D is not a physics simulation, and the terms used by it thereby do not reflect the terms used in modern physic (for example, here is a discussion on the term energy).

I think in the context of major image, "physical" interaction means "bodily" interaction, where you are actually touching, pushing, running into the illusion, or hitting it with a weapon or missile, a liquid like holy water, acid or alchemists fire splashed on it. Not just solid objects, but anything with substance, because the illusion will not bounce back those items, or have them stick as it should, by which you will discover it is an illusion. But it would exclude things you cannot touch, like sound or light.¹

I believe the best way to think about this is what the spell aims to achieve: it fools observation or perception that does not include feeling the object as a material thing -- only if you try that, you reveal it as an illusion because things pass through it.

The echolocation sound of a bat or a ray of light seems not to be a good match to a "thing" in this context. So they would be fooled like other perception forms. If you use this approach, it also would fool blind fighting, until you hit it.

It is tricky where to draw the line between "normal" light and something like the light from Sunbeam spell, which deals damage, but also is not a thing. Here for example is an answer suggesting energy attacks in form of magic missiles would count as physical objects, but these are actual "glowing darts of magical force", not just light. I think this is best left to the DM to adjudicate in the case of damaging light effects.

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¹ If you create the illusion of something that is not a physical object, like fog or darkness, then by the consensus answer, even physical interaction may not be sufficient to reveal it as an illusion, and you would need some other clue or reason to suspect it to be one.

Answer 2

Illusions require DM adjudication

Illusions are a very trick school of spells. Incredibly versatile, but the versatility is at the discretion of the DM.

The rules are unclear about how illusions interact with every edge case - especially edge cases that came after the spell was created. In these cases, it's always up to the DM how they want to narrate the effects of what the player is asking the illusion to perform.

What to do?

Well, you've pretty much got two choices here: the illusion works or the illusion doesn't work. And this is where specifics start to matter.

At my tables, I'd be asking about the illusion itself. What is it, how is it intended to perform? The characters know what they're fighting, so they know what they're trying to do and can explain how it applies to the monster(s) that they're trying to fool. This has the positive benefit of allowing players to get more creative, too. The more they can tie in their in ideas to what they want them mechanically to do, the easier the rulings are and the more interesting the combat gets due to the narration.

With that info in hand, I'd try and think about what makes narrative sense for the scene and make my ruling. And all of that would be on a case-by-case basis.

In the case of the limited info we have above for the theoretical use-case, I'd likely rule that having the Blind Fighting technique would be a passive interaction with the illusion identifying it as not there. But Groody's answer is just as reasonable for being ruled at a table.

Answer 3

Major Image foils everything but Truesight and physical interaction

There are no secret rules

It seems completely real, including sounds, smells, and temperature appropriate to the thing depicted.

“Completely real” means, well, completely real - it’s not an ambiguous concept.

It does not say “completely real except to blindsight”, or “completely real except on Thursdays”. It even provides a non exhaustive (because that’s what “including” means) list of phenomena that are “completely real”. Blind Fighting gives you Blindsight but there is nothing in either Major Image or Blindsight that suggests this does anything.

Physical interaction is called out as an exception. In this context, it means what everyone understands by “physical interaction” not what scientists when they’re actually working in a laboratory mean. Scientists when playing D&D, like me, understand that “physical interaction” means coming into contact with something material.

A creature with truesight can see the illusion for what it is because detecting visual illusions is part of that ability and, unlike Mirage Arcane, Major Image does not have specific rules about Truesight.