Complete a Fatality during an Xbox Live Versus game.

Once youve beaten your opponent, there will be a short period of time where they just stand there swaying before they fall over and die. You have these few seconds to complete the combination of moves necessary to perform a fatality maneuver. Each character has their own unique set of fatalities and youll know youve performed it properly when you see the animation of your foes grisly demise.

Credit: Andrew x360a