you're about to find out who i am
a more militantly-minded version of the popular "you're about to get to know me"; instead of expressing its surface desire to become "properly introduced," this phrase is akin to the Shakespearean cry "into the breach once again dear friends" and is most likely used by an individual who wants to kick some serious tail
Guy 1: "I slept with your partner"
"Your Partner"'s partner: "You're about to find out who I am"
"Your Partner"'s partner: "You're about to find out who I am"