Blessing of Belzenlok

You can cast this spell using your reaction when a creature you can see within range makes a melee weapon attack. If the attack hits, it deals an additional 2d4 necrotic damage, and the attacking creature takes 1d4 necrotic damage.

At Higher Levels. When you cast this spell using a spell slot of 5th level or higher, the additional damage increases to 4d4, and the attacking creature instead regains 1d4 hit points if the attack hits.