So what happens if one of my PC's casts True Polymorph to turn into an adult gold dragon, concentrates on the spell for the full hour to make it permanent, then uses the the dragons Change Shape feature to turn into some random humanoid. Would True Sight see only the original form of the PC, only the dragon form,
or would it see both the dragon form and the original PC?
True Polymorph says (emphasis mine)
… If you concentrate on this spell for the full Duration, the transformation becomes permanent. …
and then you add on another Magical Polymorph
Change Shape: The dragon magically polymorphs into a humanoid or beast…
Which means that the PC is now double Polymorphed, but since True Sight says
… and perceive the original form of a Shapechanger or a creature that is transformed by magic.
To me the wording of True Sight means that it sees only the original PC even if the transformation is permanent since it's still a magical effect, but I'm curious what either an official ruling has said and what you all think.
Best Answer
Creatures with True Sight see the PC
The wording on True Polymorph has changed in later printings and on D&D Beyond. It now says:
As a result it is clear that the PC is still under the effect of the spell (and thus transformed by magic).
The transformation of an Adult Gold Dragon states (emphasis mine):
You have quoted the correct portion of True Sight:
Importantly it states that creatures with True Sight can perceive the original form of the creature. This is distinct from the "true form" specified in the Adult Gold Dragon's Change Shape ability. It also pierces multiple layers of magical transformation.
Thus creature with True Sight see through both sets of magical transformations and see the PC.