Incursion Armor (5e Equipment)
From D&D Wiki
Armor (medium or heavy), very rare
Ordinarily appearing to be a small, thin backpack, this armor can be deployed as an action into full protective armor. While "packed" the armor confers no bonuses.
You gain a +1 bonus to AC while you wear this armor.
When you take acid, cold, fire, lightning, or radiant damage, you gain resistance to that damage type after the damage is taken for 1 minute.