Crushing Blow

The next time you hit a creature with a weapon attack before this spell ends, the weapon gleams with dark necrotic energy as you strike. The attack deals an extra 3d8 necrotic damage to the target, which has to make a constitution save or be paralyzed for a turn 

At Higher Levels. When you cast this spell using a spell slot of 3rd level or higher, the extra damage increases by 1d8 for each slot level above 3rd.