Amulet of Immutability (Patreon)

Amulet of Immutability
Wondrous item, rare (requires attunement)

This adamantine amulet protects you from magic that would threaten to alter your form against your will. While wearing the amulet, you have advantage on saving throws against transmutation spells of 5th level or lower that would transform or control you, such as enlarge/reduce, levitate, or slow.

In addition, when you are targeted by the polymorph spell, you can choose to automatically succeed on the saving throw.

I do not bend for you, mage. I will not become clay in your hands with which you can mold me. My body is mine own, and you cannot change that.