Darkfire Smite

The next time you hit a creature with a melee weapon attack during the spell’s duration, your weapon flares with blackened flame, and the attack deals an extra 3d6 cold damage.

Creature hit must make a constitution saving throw against your spell save DC. On a failed save, the creature is weakened. The first hit made against the target on each turn will cause darkflame to flare up and inflict an additional 1d6 cold damage. Creature may expend an action to roll a Constitution saving throw against your spell DC. On a success, the effect of the spell ends.