Volo's Guide to Monsters contains the stat bloc for an NPC called Transmuter, who is a humanoid, CR 5, with the following trait:
Transmuter's Stone. The transmuter carries a magic stone it crafted that grants its bearer one of the following effects {formatted for ease of reading}:
- Darkvision out of a range of 60 feet
- An extra 10 feet of speed while the bearer is unencumbered
- Proficiency with Constitution saving throws
- Resistance to acid, cold, fire, lightning or thunder damage (transmuter's choice whenever the transmuter chooses this benefit)
If the transmuter has the stone and casts a transmutation spell of 1st level or higher, it can change the effect of the stone.
It is, I believe, possible for a high level caster to "morph" into a Transmuter statblock, either by True Polymorping into it directly, True Polymorphing into a creature that can then Change Shape into it (such as an Ancient Metallic Dragon), or by Shapechanging into it.
My question is: if you "morph" into an NPC Transmuter, do you gain access to its Transmuter's Stone?