Shield Savant

Regardless of your armor proficiencies, you gain proficiency with shields as well as the following benefits:

• If you are subject to a spell or effect that forces you to make a Dexterity saving throw, you can spend your reaction to interpose your shield between yourself and the source - adding your shields AC bonus to your saving throw.
• If an ally within 5ft of you is attacked, you can expend your reaction to add your shield's AC bonus to your ally's AC.

If you have or later take the Fighting Style: Protection, when you expend your reaction you can add your shield's AC bonus to your ally's AC AND the enemy's attack is made at disadvantage.