Thunderchord

As you make a discordant note with your instrument, a cone of thunderous power shoots forth.  Each creature in a 30-foot cone must make a Constitution saving throw. A creature takes 6d8 thunder damage on a failed save, or half as much damage on a successful one.

At Higher Levels. When you cast this spell using a spell slot of 4rd level or higher, the damage increases by 1d8 for each slot level above 3rd.