12-14 feels about right to me, 3d8 thunder damage each to as many people you can fit in a 10ft radius, average 13.5 damage save for half, if you successfully hit 2 creatures in that that's 27 average damage total, 40.5 for 3 creatures.
to compare eldritch agonising blast at 11th level (assuming a +5 mod for easy math) deals 3(d10+5) (average damage of 16.5(+15)) between all three blasts together, targets within 120ft, which your attack can miss on each of those, (or 22(+20) damage at 17th level when you get 4 blasts)