Will the Rakshasa's Limited Magic Immunity prevent Detect Magic from detecting the effects of a spell like Disguise Self on the Rakshasa?

Question

A Rakshasa has the following trait:

Limited Magic Immunity. The rakshasa can't be affected or detected by spells of 6th level or lower unless it wishes to be. It has advantage on saving throws against all other spells and magical effects.

Rakshasas can cast Disguise Self at will.

If a Rakshasa has cast Disguise Self on themselves (choosing to have it effect them), can a PC who has cast Detect Magic see the effect of the Disguise Self spell?

This question can be rephrased in three different ways (which may be useful as a way to consider this effect):

• Does the PC see that the Rakshasa has the effect of an illusion spell on it?
• Does the Rakshasa's Limited Magic Immunity extend to spells that have been cast on it and thus mask the Detect Magic?
• Does the fact that the Detect Magic spell is cast on the PC (changing their sight) bypass the Limited Magic Immunity of the Rakshasa?

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Note: The answer to this question will apply more generally to other ongoing effects, but for the purpose of getting a succinct answer I am restricting this question to cover the interaction between Detect Magic and Disguise Self as they pertain to a Rakshasa.

Answer 1

Detect Magic will enable the caster to see that there is a magical illusory effect on the Rakshasa (without actually being able to tell the creature is a Rakshasa)

The relevant portion of the Rakshasa's Limited Magic Immunity, for determining the answer to this question, is (emphasis mine):

The rakshasa can't be affected or detected by spells of 6th level or lower unless it wishes to be.

This is pretty clear, if Detect Magic affects or detects the Rakshasa then Limited Magic Immunity applies. So we have two follow up questions to answer:

1. Does Detect Magic affect the Rakshasa?
2. Does Detect Magic detect the Rakshasa?

Does Detect Magic affect the Rakshasa?

No, it does not.

Detect Magic alters the senses of the caster so they can sense and see magical effects. It does not alter the objects, spells or creatures from which magic might be sensed.

Does Detect Magic detect the Rakshasa?

The answer to this question is potentially more complicated.

Detect Magic has this text:

For the duration, you sense the presence of magic within 30 feet of you. If you sense magic in this way, you can use your action to see a faint aura around any visible creature or object in the area that bears magic, and you learn its school of magic, if any.

The spell can penetrate most barriers, but it is blocked by 1 foot of stone, 1 inch of common metal, a thin sheet of lead, or 3 feet of wood or dirt.

So the straightforward answer would seem to be No, Detect Magic, detects magic, not creatures.

But, you may ask, surely if you know there is an illusion on a creature then that illusion becomes see-through, no?

Why is this question important to ask? Let's pre-suppose the affirmative answer.

If disguise self does become see-through when it is discerned, then once the user uses their action to find out the class of the magic (illusion) the user would discern the disguise, and it would become see-through. This would be classified pretty clearly as a detection of the Rakshasa (as it's true form would be revealed).

In fact the answer to this follow up question is No the illusion does not become see-though once it is discerned.

An illusion becoming see-through upon detection is not a general rule for all illusion spells. Instead it is an effect that is applied to specific illusion spells by their text.

If we take the examples of Major Image and Minor Illusion, both have the relevant qualifier in their descriptions:

If a creature discerns the illusion for what it is, the illusion becomes faint to the creature.

The portion of Disguise Self that deals with discerning that a creature who is disguised is:

To discern that you are disguised, a creature can use its action to inspect your appearance and must succeed on an Intelligence (Investigation) check against your spell save DC.

Importantly discerning that a creature is disguised does not make the illusion disappear or become faint.

So...are we detecting the Rakshasa with Detect Magic?

No. We are detecting the effects of the Disguise Self spell, not the Rakshasa itself.

As a result the Rakshasa's Limited Magic immunity does not negate the Detect Magic spell from detecting the Disguise Self spell.

But surely detecting the illusion is "detecting" the creature?

No, it's not the same thing.

Lets pretend instead that we wanted to detect a magic weapon being held by the Rakshasa using Detect Magic. If we rule above that Detect Magic does not detect the Disguise Self spell, then we also have to rule that the magic weapon being held by the Rakshasa is not detected by Detect Magic.

But, if the Rakshasa drops the magic weapon while our Detect Magic is still running, then suddenly we would detect the magic weapon the moment it leaves the Rakshasa's hands. This oddity would then show that the Rakshasa is a creature with some sort of field surrounding it that limits detection, which then immediately gives a knowledgeable magic practitioner the ability to identify that the creature is a Rakshasa...thus detecting it!

Whereas if we rule that the Disguise Self can be detected, we learn nothing about the underlying creature. We also don't actually learn that the spell is Disguise Self, merely that there is an illusion spell overlaying the creature we can see. It could be any creature under the effect of an illusion spell (which may or may not be Disguise Self).

Are there examples of spells that would attempt to "detect" the Rakshasa?

There are two pretty common examples:

• Locate Creature. Locate Creature very specifically tries to locate a specific creature, and the Rakshasa can choose not to be detected by that spell.
• Alarm. Alarm needs to detect a creature entering the alarmed area in order for the alarm to be triggered. The Rakshasa can choose not to be detected by the Alarm spell.

Answer 2

No, they would not be detected (unless they wanted to be)

You've quoted the important bit about the Rakshasa (emphasis mine):

The rakshasa can't be affected or detected by spells of 6th level or lower unless it wishes to be.

Detect Magic would fall under this purview since if the aura could be seen with that spell, then it would be the similar to detecting the Rakshasa itself by detecting that they are under a magical disguise.

If Detect Magic were able to identify the Rakshasa was disguised, then it would be effectively detecting something about the Rakshasa, which their Limited Immunity specifically prevents.

But wait, there's more (or less)

As @miniman answers in this question it does not seem possible detect Illusions with Detect Magic if the illusion is covering the object you are 'detecting'. Since the Rakshasa technically isn't visible, it isn't a viable target for detect magic and the Rakshasa doesn't need it's Limited Immunity to avoid detection.

Answer 3

Detect Magic should only work on the Rakshasa when it wants to be affected by it

Looking at the text of detect magic gives us the following, emphasis mine:

For the duration, you sense the presence of magic within 30 feet of you. If you sense magic in this way, you can use your action to see a faint aura around any visible creature or object in the area that bears magic, and you learn its school of magic, if any. [...]

What this means is that detect magic only allows the school of magic that is being used to be determined if the target is both visible, and either a creature or an object.

This may mean that while the presence of magic is detected (due to detect magic not specifying that it senses magic that is affecting objects or creatures specifically), the type of magic is not (since that would target a creature or object). Anything else will go beyond a strict RAW ruling, which I cannot answer.

It may be useful to look at this post about detect magic and illusions.