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.