Arrow of Justice

You can only target a creature after they attack one of your allies. You reach out with your hands as if drawing an imaginary bow. A green bow of crackling energy forms in you hands, and you begin to draw the bow back for a shot. An arrow of pure white light forms and notches itself onto the bow string. And you fire a shot that screams as it races through the air and explodes as it hits the target creature. The target creature takes 1d8 radiant damage, knocking him prone if they fail a constitution save of 17 or higher. They now have disadvantage on saving throws for 1 minute.

You gain an extra 1d8 damage at levels 3, 5, 8, 12, 16, and 20 (+2d8 at level 20).