Black Flash

You make a melee weapon attack against a target imbued with thunderous magical energy, so much so that on the point of impact sparks of black lightning flash from the weapon. The target must make a Constitution saving throw. It takes 8d6 thunder damage on a failed save, or half as much damage on a successful one. On a failed save, the target also trembles for the spell’s duration. The trembling target has disadvantage on dexterity and strength saving throws made for the next minute.