A hero's ex-lover is now his worst enemy.
So you have a hero (usually a protagonist) facing off against a Dark Action Girl, when suddenly, you learn about a shocking truth. In their youth, rather than being bitter enemies, they were former lovers! This trope occurs whenever a good aligned character is revealed to have a history of a former Childhood Friend Romance or a marriage with the villainess. Usually the marriage or romance fell through because the lover in question went mad with power and the hero didn't want any more to do with it. See also: Psycho Ex-Girlfriend, Not Good With Rejection, Evil Former Friend, We Used to Be Friends Needs More Examples