Vengeance is Mine

From LegendWiki

Vengeance is Mine [Iconic]

You’re not obligated to scream the name of this feat as you activate it, but it is highly encouraged.

Benefit: Once per [Round], the first time you make an attack roll that is not an intentional miss, or spend a standard action to activate a spell or a spell-like or supernatural ability, you gain one focus point. As an immediate action, when you are damaged by an opponent’s ability or attack, you may spend any number of focus points you possess to deal damage to that opponent equal to your level per focus point spent, to a maximum of the amount of damage dealt to you by that ability or attack to which any [Immunity] you possess does not apply before applying [Resistance] or other abilities that would reduce that damage. An opponent may only be damaged once by a feat named Vengeance is Mine for each time it activates an ability or makes an attack to trigger this feat.