Can Mind Blank be detected?

Question

After thinking about uses for Mind Blank and how it interacts with a bunch of spells, I then tried to think of counters and ways someone could get past it with one problem always coming up. How do you know when someone is under its effect?

Mind Blank spell description for reference:

Until the spell ends, one willing creature you touch is immune to psychic damage, any effect that would sense its emotions or read its thoughts, divination spells, and the charmed condition. The spell even foils wish spells and spells or effects of similar power used to affect the target's mind or to gain information about the target.

It seems like since it blocks divination spells and is powerful enough to foil a wish spell, a simple Detect Magic which is a divination spell would also not work for finding it. If that is the case, then is it even possible to detect someone under the affect of Mind Blank?

Answer 1

Ask them

Are you now, or have you ever been, a user of mind blank?

Interestingly, the spell zone of truth isn't divination - it's enchantment and doesn't seem to be affected by mind blank. This puts it an interesting position where it doesn't fall under the divination umbrella - and it also isn't giving information about the target. It is instead interpreting what they are saying: an external response analysis.

Once they're in the zone, ask them directly if they are under the effects of the spell mind blank. And yes, it's still possible for a sufficiently weaselly clever DM to come up with a non-lie response that doesn't say, but it's a pretty solid path to finding out.

This is a fairly straightforward and very unsubtle method, so its actual effectiveness may be pretty limited.

Answer 2

Mind blank and any other magical effects affecting the target would not be detected by detect magic.

Mind blank states:

one willing creature you touch is immune to psychic damage, any effect that would sense its emotions or read its thoughts, divination spells, and the charmed condition. The spell even foils wish spells and spells or effects of similar power used to affect the target's mind or to gain information about the target.

Notably, detect magic is of the Divination school of magic, and is a spell used "to gain information about the target", in this case, any magical effects affecting the target. Mind blank foils wish, which is "the mightiest spell a mortal creature can cast". Detect magic poses no challenge for mind blank. I've written a similar answer for nondetection, part of which I reproduce below.

Victims of AoE spells are referred to as targets.

This isn't easy to find, but there is a rule in the PHB which specifically uses "target" to refer to someone affected by an AoE spell, in the section "Targeting Yourself" (emphasis mine):

If you are in the area of effect of a spell you cast, you can target yourself.

Additionally, we see in the rules for Saving Throws (emphasis mine):

Many spells specify that a target can make a saving throw to avoid some or all of a spell's effects. The spell specifies the ability that the target uses for the save and what happens on a success or failure.

AoE spells are obviously in view here. Further, in the DMG's rules for Adjudicating Areas of Effect, we see (p. 249-250; emphasis mine):

If you would like more guidance, consider using the Targets in Areas of Effect table. To use the table, imagine which combatants are near one another, and let the table guide you in determining the number of those combatants that are caught in an area of effect. Add or subtract targets based on how bunched up the potential targets are. Consider rolling 1d3 to determine the amount to add or subtract.

[There is a table here]

For example, if a wizard directs burning hands (a 15-foot cone) at a nearby group of orcs, you could use the table and say that two orcs are targeted (15 ÷ 10 = 1.5, rounded up to 2). Similarly, a sorcerer could launch a lightning bolt (100-foot line) at some ogres and hobgoblins, and you could use the table to say four of the monsters are targeted (100 ÷ 30 = 3.33, rounded up to 4).

In the rules for using miniatures on a combat grid, we see the following about areas of effect (p. 251; emphasis mine):

The area of effect of a spell, monster ability, or other feature must be translated onto squares or hexes to determine which potential targets are in the area and which aren’t.

For a more specific example, the spell descirption of fireball even calls creatures it affects "targets":

A target takes 8d6 fire damage on a failed save, or half as much damage on a successful one.

Therefore, a creature under mind blank cannot be a target of detect magic, in the sense that the rules refer to victims of AoE spells as targets, because the creature under mind blank is "immune to...divination spells". .

Answer 3

Right now?

Going to be tough.

As you've pointed out, mind blank foils most things that would tell you that mind blank is in play.

With some effort

...you might be able to deduce it.

If your target is apparently not taking psychic damage, you can't sense its emotions or read its thoughts, you can't use divination on it, and/or it's not succumbing to your charms, then maybe, hmmm, it's somehow mind blanked.

Ask them

If you can actually interrogate them, then zone of truth or command and some careful wording might be able to get them to spill it, as other answers have alluded to. Of course, if you have them under your thumb, maybe you can just wait it out. But that's a lot of if's.

Ask someone else

Is there someone else who knows? Can you detect thoughts, charm, speak with dead on them? Maybe their buddy knows they're mind-blanking today, and you can use to figure that out.

What about commune? You might be able to flat out get an answer, if your deity or divine proxy actually knows.

Depending on who you're dealing with, maybe even legend lore will tell you something.

Some GM adjudication will be called for. The spell says:

The spell even foils wish spells and spells or effects of similar power used to affect the target's mind or to gain information about the target.

The GM will have to determine the extent of foiling "information about the target". To me, it's not reasonable that it will keep you from getting information from other sources, but the GM will need to rule.

In the end though

...if you're looking for a fool-proof way of just flat-out knowing, you're probably going to come up . . . blank.

Answer 4

Try to cast a helpful Divination spell on them, like Guidance

Guidance is a divination cantrip, so Mind Blank would make them immune to its helpful effects. If you try to cast Guidance on someone and it fails, you'd realize that because you wouldn't be concentrating on a spell. (Normally it's concentration for 1 minute, or ends early if the target chooses to add +1d4 to an ability check)

Or a harmful one like True Strike or Hunter's Mark.
We can add this to this short list of use-cases for True Strike :P
Finding a target immune to one of those spells would leave mind blank near the top of a short list of possibilities, depending on what else you know about the creature.

I don't have a rule citation for exactly what happens if you try to cast a spell on a creature that's immune to spells of that school in general. I'm assuming it fizzles, so its duration doesn't start and you aren't concentrating on it.

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Cast something that should be visible with Detect Magic, but isn't

Mind Blank stops Detect Magic from detecting any magic affecting the target, as the creature is immune to that divination spell. If you do something that should leave a detectable aura of magic on the target, but it doesn't, they might have Mind Blank up. But not Intellect Fortress; that doesn't block divination.

(Or Nystul's Magic Aura with the False Aura effect, if that works on creatures as well as objects. With a strict reading it only works on objects. Even if it does work on creatures, you should still be suspicious if a recent ally of convenience is using Nystul's to hide something, if you don't know why they'd be doing that. Still, you couldn't then assume Mind Blank specifically.)

Some other anti-divination effects like Nondetection stop the creature from being targeted by "divination magic", rather than making them immune to all "divination spells". (The target can't be targeted by any divination magic or perceived through magical scrying sensors.) Thomas Markov's answer argues that Detect Magic (even though range=self), creates an AoE effect that "targets" creatures or objects you're looking at. If your table uses that ruling, then Nondetection is another explanation for not being able to see magic on them.

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So for example, if the party needs to use Fly or Spider Climb on one member so they can carry a rope (or the other members 1 at a time) over some terrain, select the party member you don't fully trust and buff them up (hrm...). Have someone else use Detect Magic while you're concentrating on that helpful spell. (Detect Magic is also concentration.)

Or while they're taking a nap, cast Faerie Fire on them. Or just "as a joke". It's non-damaging, but doesn't require a willing target. (And it's a Dex save to avoid, so they auto-fail if actually napping.) Faerie Fire is maybe not ideal because you (or the DM) could argue that it's a visible magical effect stuck to the target, not intrinsic to the target's body, and thus Detect Magic could see that it's magic regardless of Mind Blank.

Many other non-damaging effects are charms, like Charm Person, and thus blocked by Mind Blank. (The Friends cantrip requires you to pick a creature, but has Range = self. It's of the enchantment school but doesn't mention creating the charmed condition. It's debatable whether it leaves an aura of magic on the target while active.)

The Message cantrip is Transmutation, and has a duration of 1 Round. (Not concentration but not instantaneous). Arguably it should leave an aura of transmutation magic around the recipient while it's being cast, so you can have Detect Magic up while you send a Message to someone.

The Resistance cantrip is Abjuration, so unlike Guidance it will take effect.

Prestidigitation would take some rule-bending to use on a creature: You make a color, a small mark, or a symbol appear on an object or a surface for 1 hour. - A creature's skin may not count as "a surface". And even then, it might just be an external effect, like I mentioned above for Faerie Fire.

Or convince them to cast anything on themselves.

In combat, with detect magic up:

Many effects, such as Slow, are non-charm and should show up with Detect Magic. (But wouldn't if the target has Mind Blank.)

Bane is an enchantment spell but doesn't cause the Charmed condition.

Hunter's Mark is divination, and will fail on a creature with Mind Blank. (Hex is enchantment.)

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Find some divination magic that isn't a spell?

Mind Blank specifically says you're immune to "divination spells", different from Nondetection protecting you from divination magic in general. (The target can't be targeted by any divination magic or perceived through magical scrying sensors.)

Nondetection would block include creature or item abilities to sense magic, but Mind Blank wouldn't. Also non-spell class abilities if there are any that detect magic.

Answer 5

Specifically answering the "think of counters and ways someone could get past it" part of the question, you can cast Dispel Magic on the target:

Choose one creature, [...] within range. Any spell of 3rd level or lower on the target ends.

[...]

When you cast this spell using a spell slot of 4th level or higher, you automatically end the effects of a spell on the target if the spell's level is equal to or less than the level of the spell slot you used.

To ensure Mind Blank ends, you have to cast this using 8th level spell slot.

After that you know creature is not affected by Mind Blank.

Note to DMs: When Dispel Magic is cast on an NPC/monster, the DM should have the habit of always calling for the spellcasting ability check, so the players don't get meta-knowledge about there having been spells to be dispelled, and about their levels.

Answer 6

Detect Magic works. Detect magic does not target a creature, it instead simply allows you to "sense the presence of magic within 30 feet of you."

The creature being immune to divination spells would only prevent spells such as Scrying that actually target the creature.