Vengeance Slot (3.5e Feat)
From D&D Wiki
You learn to naturally channel your hatred into a powerful vengeful attack against your foes without expending your magical energy.
Prerequisite: Vengeance Class Feature
Benefit: Once per day you may use your vengeance ability as if you had sacrificed one spell of the highest level you can currently cast without losing a prepared spell.
Special: You can gain this feat multiple times, it's effects stack.