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.
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?
Best Answer
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.