Decaying Blade

As part of the spell, make a melee weapon attack. On a hit, the attack leaves the wound festering with necrotic energy. The first time that creature is healed before the beginning of your next turn instead takes necrotic damage equal to your spellcasting modifier.

This damage is further amplified as you level up. At 5th level it becomes 1d8 + your spellcasting modifier. At 11th level the damage die increases to 2d8, and at 17th level it further increases to 3d8.