Spell Pierce (Combat)

When you disrupt an opponent’s spell with an attack or counterspell, you cause that opponent to lose an additional spell slot.

Prerequisite(s): Disruptive, Improved Counterspell, Spellcraft 5 ranks, 1st-level eldritch knight

Benefit: Whenever you cause an opponent to lose a spell by disrupting it with an attack or by countering it, the opponent loses an additional unused spell or spell slot (for spontaneous casters) of equal level to the disrupted spell.

If they have no unused spells of an equal level, they lose a spell slot 1 level lower than the disrupted spell, and so on if they have no remaining spells of that level. They cannot lose a spell slot of a higher level than the disrupted spell, even if they have no remaining spell slots of a lower level.