Amulet of Immutability

Alter: This adamantine amulet protects you from magic that would threaten to alter your form against your will. While wearing this 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.

Attunement: Yes
Rarity: Rare
Type: Amulet
Cost: 2205gp

Griffon's Saddlebag, Vol 2