I am trying to give you an answer to the query within the title, however have been unsuccessful so far.

Some monsters have the Truesight capability which states the next (emphasis mine):

A monster with truesight can, out to a selected vary, see in regular
and magical darkness, see invisible creatures and objects,
robotically detect visible illusions and succeed on saving throws
towards them, and understand the unique type of a shapechanger or a
creature that’s reworked by magic. Moreover, the monster can
see into the Ethereal Aircraft inside the identical vary.

This looks as if it would be almost unimaginable to make the most of any sort of magical disguise towards this creature, nevertheless a way to thwart gamers’ utilizing Truesight could be to depend on extra protection by way of the Nondetection spell, which says:

For the period, you conceal a goal that you simply contact from divination
magic. The goal could be a keen creature or a spot or an object no
bigger than 10 ft in any dimension. The goal cannot be focused
by any divination magic or perceived via magical scrying
sensors.

and Alter Self’s Change Form possibility, which says:

You remodel your look. You resolve what you appear like,
together with your peak, weight, facial options, sound of your voice,
hair size, coloration, and distinguishing traits, if any.
You may make your self seem as a member of one other race, although none
of your statistics change.

As written, this spell mixture would defeat gamers profiting from True Seeing spells. Nevertheless, monsters don’t comply with the identical guidelines as gamers, so I do not suppose this could work if the gamers had been to aim to deceive a monster with the True Sight sense as the power will not be inherently divination magic (to my information).

My query is twofold. Are there any guidelines which particularly state that the monster True Sight capability is taken into account a magical divination impact that might be countered by Nondetection? If not, is there one other mixture of spells that might be used to counter a monsters’ True Sight capability for the needs of successfully passing off a magical disguise?