Arena

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

Mystery Booster 2

{3}, {T}: Tap target creature you control and target creature of an opponent's choice they control. Those creatures fight each other.

Magic Online Promos

Time Spiral Timeshifted

{3}, {T}: Tap target creature you control and target creature of an opponent's choice he or she controls. Each of those creatures deals damage equal to its power to the other.

HarperPrism Book Promos

o3,oc
T: Choose one of your creatures and have your opponent choose one of his or her creatures. Both target creatures become tapped. Your creature does its power in damage to your opponent's creature, and your opponent's creature does its power in damage to yours.

Either or both creatures can be tapped prior to using the ability and it still works.

In multiplayer games, you can choose a different opposing player each time it the ability is activated.

The power of the creatures when the effect resolves is used to determine how much damage is done.

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 doesn't resolve.