Is there any way for a character to create a magical disguise that wouldn't be automatically defeated by a creature with Truesight?

Question

I'm attempting to come up with a solution to the question in the title, but have been unsuccessful thus far.

Some monsters have the Truesight ability which states the following (emphasis mine):

A monster with truesight can, out to a specific range, see in normal and magical darkness, see invisible creatures and objects, automatically detect visual illusions and succeed on saving throws against them, and perceive the original form of a shapechanger or a creature that is transformed by magic. Furthermore, the monster can see into the Ethereal Plane within the same range.

This seems like it'd be nearly impossible to utilize any kind of magical disguise against this creature, however a means to thwart players' using Truesight would be to rely on additional coverage via the Nondetection spell, which says:

For the duration, you hide a target that you touch from divination magic. The target can be a willing creature or a place or an object no larger than 10 feet in any dimension. The target can't be targeted by any divination magic or perceived through magical scrying sensors.

and Alter Self's Change Shape option, which says:

You transform your appearance. You decide what you look like, including your height, weight, facial features, sound of your voice, hair length, coloration, and distinguishing characteristics, if any. You can make yourself appear as a member of another race, though none of your statistics change.

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As written, this spell combination would defeat players taking advantage of True Seeing spells. However, monsters do not follow the same rules as players, so I don't think this would work if the players were to attempt to deceive a monster with the True Sight sense as the ability is not inherently divination magic (to my knowledge).

My question is twofold. Are there any rules which specifically state that the monster True Sight ability is considered a magical divination effect that could be countered by Nondetection? If not, is there another combination of spells that could be used to counter a monsters' True Sight ability for the purposes of effectively passing off a magical disguise?

Answer 1

Fabricate

You can use Fabricate to create a disguise. This disguise tricks truesight since it is not an illusion, nor does it alter the form of a creature.

A classic disguise to fabricate would be a trench coat, fedora, and fake glasses/mustache combo. You could also fabricate a newspaper for you to pretend to read.

Minor Illusion

Truesight lets you "automatically detect visual illusions and succeed on saving throws against them" - however it doesn't automatically dispel them or allow you to see through them.

Careful use of Minor Illusion could create a disguise, the monster would know there is an illusion, and the illusion would become faint. Although, that may well be enough. You could try telling them "don't look, I'm wearing illusory clothing because I lost my real ones!" If they are polite/embarassed, they will avert their gaze.

Disguise Self

Alter Self's little brother. It has some advantages in this case. A creature with truesight detects visual illusions and succeeds on saving throws against them. This sucks because many illusion spells will become faint if detected. Disguise Self, on the other hand, doesn't have any mechanic like that. To detect that there is an illusion, a check must be passed, but once passed the illusion doesn't falter.

Disguise Self also does not transform the target. This makes Disguise Self decent for disguise against truesight. Yes, the monster will know you are disguised, but they won't see through it.

Auditory Disguise

Truesight doesn't do squat against auditory illusions. Combine this with other methods - use Disguise Self to make you look like a sheep, then use a fake voice to tell the target that you are their cousin, turned into a sheep. There's a lot of possibility with this one.

Answer 2

Yes there are spells that can "trick" Truesight

but it will be expensive and probably painful

The spell Reincarnate can grant a character an entirely new body although it will not be one of their choosing.

If they want to return to their old form they can first cast Clone to preserve their original form.

Clone specifies "At any time after the clone matures, if the original creature dies, its soul transfers to the clone, provided that the soul is free and willing to return" Which could be interpreted to allow the caster's soul to choose not to return to the clone but rather to be reincarnated first. There could still be a problem, however, if the reincarnated form does not qualify as "the original creature". I would suggest it does because Reincarnate states that the DM rolls on a table to "determine what form the creature takes when restored to life." This clearly identifies the reincarnation as merely a new form rather than a new creature.

My answer is addressing the second part of your question because Dale M's answer already addresses the first part.

Answer 3

Truesight is neither divination or magic

The Sage Advice Compendium includes a list of questions to see if something is magical:

• Is it a magic item?
• Is it a spell? Or does it let you create the effects of a spell that’s mentioned in its description?
• Is it a spell attack?
• Is it fueled by the use of spell slots?
• Does its description say it’s magical?

For a creature with truesight, the answer to all of these is “no”. Therefore it’s not magic and it follows that it’s not divination magic.

Your plan doesn’t work with True Seeing either

The target of True Seeing is the creature touched, not the creature being looked at so the first clause of Nondetection doesn’t work. True seeing also doesn’t create a “magical sensor”, it enhances the creature touched so the second clause of Nondetection doesn’t kick in either.