Arena
Land
, 
: 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.)
Rob Alexander
Rulings
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.
(wotc • 2006-09-25)
Either or both creatures can be tapped prior to using the ability and it still works.
(wotc • 2004-10-04)
The power of the creatures when the effect resolves is used to determine how much damage is done.
(wotc • 2004-10-04)
In multiplayer games, you can choose a different opposing player each time it the ability is activated.
(wotc • 2004-10-04)
Rulings
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.
(wotc • 2006-09-25)
Either or both creatures can be tapped prior to using the ability and it still works.
(wotc • 2004-10-04)
The power of the creatures when the effect resolves is used to determine how much damage is done.
(wotc • 2004-10-04)
In multiplayer games, you can choose a different opposing player each time it the ability is activated.
(wotc • 2004-10-04)