Irreversible Smite

Be sure the blade is swung with purpose and intent, for it cannot be in error without consequence.
The first time you hit with a melee weapon attack during this spell’s duration, cruel shadows creep down your weapon and into your target’s wounds. The attack deals an extra 3d6 necrotic damage to the target. Additionally, if the target is a creature, its flesh festers and refuses to repair. Until the spell ends, the target cannot regain hit points.

At Higher Levels: When you cast this spell using a spell slot of 4th level or higher, the necrotic damage increases by 1d6 per spell slot level above 3rd.