Source: The Book of Many Things

Armor (light, medium, or heavy), very rare (requires attunement)

While wearing this armor, you can use your reaction to give yourself advantage on a saving throw you make against a spell. Once this property is used, it can’t be used again until the next dawn.

In addition, while you wear this armor, you can use it to cast Antimagic Field, requiring no spell components. Once this property is used, it can’t be used again until the next dawn.