Death Storm

You summon a storm of dark magic in a point within range. That storm covers a 30 foot radius sphere. Any hostile creature that starts its turn in the sphere or enters the sphere for the first time on its turn must make a Dexterity saving throw. On a failure, they take 3d12 lightning damage and 2d6 necrotic damage. The target's maximum hit points is reduced by the necrotic damage taken. 

At higher levels: For each spell slot cast above 5th level, the necrotic damage increases by 1d6