Birch's Bolt Blast

A fan of lighting shoots forth from your fingers. Each creature in a 15-foot Cone makes a Dexterity saving throw, taking 2d8 Lightning damage on a failed save or half as much damage on a successful one. Creature's who fail their save can’t make Opportunity Attacks until the start of their next turn.

Using a Higher-Level Spell Slot. The damage increases by 1d8 for each spell slot level above 1.