AC/DC

You inspire your allies through song creating an aura of rousing music that extends 30 ft from you and moves with you. The aura grants friendly creatures in the area that can hear you a +1 bonus to attack rolls. Once per round as a reaction you can grant advantage to an attack a friendly creature makes in the aura on their turn. You must use this reaction before the attack hits or misses.

Additionally the attack bonus increases every 3rd level, to +2 attack +1d4 thunder damage at 3rd, +3 attack +2d4 at 6th, and +4 attack +3d4at 9th level.