Colossus Bracers (5e Equipment)
From D&D Wiki
Armor (shield), rare
A pair of large half-shields that seem like they would fit snugly on the forearms of the larger Medium-sized races. The basic bracers add +1 AC. They can also interlock to form a larger shield if the wearer crosses their arms (using a bonus action), granting an additional +2 AC. The wearer cannot perform any action requiring the use of their arms during this time (attacking or using an item, for example), and requires a bonus action to unlock.