Oracle Text

3, T: Tap target creature you control and target creature of an opponent's choice he or she controls. Those creatures fight each other. (Each deals damage equal to its power to the other.)

Card Rulings

10/4/2004 The power of the creatures when the effect resolves is used to determine how much damage is done.
10/4/2004 In multiplayer games, you can choose a different opposing player each time it the ability is activated.
10/4/2004 Either or both creatures can be tapped prior to using the ability and it still works.
9/25/2006 If either target is illegal at the time the ability resolves (whether because it has left the battlefield, because it has gained Shroud or Protection, or for any other reason), the ability taps the remaining target, but no damage is dealt. If both targets are illegal, the ability is countered.