Improved Vengeance (3.5e Feat)
From D&D Wiki
You improve your ability to channel your hatred into raw destructive power.
Prerequisite: Vengeance Class Feature, Charisma 15+
Benefit: You gain a +1 bonus to damage per dice when calculating the damage of your vengeance ability. In addition if you sacrifice a 1st level spell or higher, apply 2x your charisma bonus to your damage roll. If you sacrifice a 4th level or higher spell you add 3x your charisma modifier to your damage roll. If you sacrifice a 7th level or higher spell you add 4x your charisma modifier to your damage roll. If you sacrifice a 9th level spell you add 5x your charisma modifier to damage.