Enervating

A bolt of black lightning leaps from your target to you. The target must make a Constitution saving throw. The target takes 8d8 necrotic damage on a failed save, or half as much damage on a successful one. In addition, its hit point maximum is reduced by the amount of damage it takes. After a long rest, the target’s hit point maximum returns to normal. At Higher Levels. When you cast this spell using a 5th level or higher spell slot, the damage increases by 1d8 for each slot level above 4th.