Debilitating Strike

As you hit the creature, this spell exposes your foe's weak points. The target hit by the attack roll takes an extra 4d6 Force damage from the attack, and if the creature is under the effect of your Hunter's Mark spell it must succeed on a Constitution saving throw or the next time you or an ally of yours hits the creature with an attack, the creature has vulnerability to all of that attack’s damage.

Using a Higher-Level Spell Slot. The damage increases by 1d6 for each spell slot level above 4.