Void Cannon

8th Level Coactimancy
Casting Time: 1 action
Range: Self (150-foot line)
Components: V S
Duration: Instantaneous
Classes: Bard, Sorcerer, Wizard
You bring forth a massive beam of cosmic destruction. A 150-foot long, 5-foot wide beam of pure destruction surges forth in a direction you choose. Each creature in the line must make a Dexterity saving throw. A creature takes 12d8 Force damage on a failed save, and half as much on a success.
If a creature originating from a plane of existence besides the one you are currently in fails their saving throw permanently reduce their maximum HP by half the damage taken. Only the strongest of magical rituals can reverse this.