Dimensional Shrapnel

As you exit a mirrorred surface, you speak a word of shattering and sending. Any creatures within a 30ft. cone facing the direction you exit are hit with a wave of force and glass shards. Creatures caught within the blast that 3d8 force damage. The creatures must make a Dexterity saving throw. They take 3d6 piercing damage on a failed save, or half as much damage on a successful one. The mirror surface is destroyed when cast.