Thunder Shield

An invisible barrier of magical force appears and protects you or another person within 30 feet. Until the start of your next turn, you have a +5 bonus to AC, including against the triggering attack, and you take no damage from magic missile.  If the shield causes the attack to miss, and audible boom is unleashed and the attacker must make a strength saving throw.  On a failed save the attacker is knocked prone, takes 2d10 damage, and loses movement until the end of their turn.