Armor of Invulnerability

From Dungeon Master's Guide, page 152. Found on Magic Item Table I.

Description
Armor (plate), legendary (requires attunement).

You have resistance to nonmagical damage while you wear this armor. Additionally, you can use an action to make yourself immune to nonmagical damage for 10 minutes or until you are no longer wearing this armor. Once this special action is used, it can't be used again until the next dawn.