Tower Shield, 2nd Variant (5e Equipment)
From D&D Wiki
This massive shield is used frequently in organized armies for its effectiveness at protecting soldiers and aiding in the formation of lines.
Trenched In. While holding this shield, if you did not move, attack, or cast a spell during your turn, at the end of your turn you may go into a defensive stance. You stay in this defensive stance until the start of your next turn, hiding yourself behind your shield. While in this stance, you are treated as having half cover.
Formation. When you are within 5 feet of a friendly creature that has a shield, as an action you may enter into a defensive formation with that creature until the start of your next turn or until they move more than 5 feet away from you or either of you drop your shield. While in this defensive formation, both of you are treated as having half cover.