Magus of the Arena — ruling 1

If either target leaves the battlefield before the ability resolves, the ability taps the remaining target, but no damage is dealt. If both targets leave the battlefield, the ability doesn't resolve.

Magus of the Arena — ruling 2

Magus of the Arena's controller always chooses their creature first.

Magus of the Arena — ruling 3

If an effect changes the targets of this ability, the second target can be changed to any creature the chosen opponent controls. The choice of opponent can't be changed.

Magus of the Arena — ruling 4

Tapped creatures can be targeted. Tapping them again will do nothing, but they'll still deal damage.

Magus of the Arena — ruling 5

If an effect changes the targets of this ability, the second target can be changed to any creature the chosen opponent controls. The choice of opponent can't be changed.

Magus of the Arena : règles, fonctionnement et interactions (MTG)

Magus of the Arena is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

, : 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.)

How it works : Magus of the Arena

Original English text — Wizards of the Coast / Scryfall

#1February 1, 2007· Wizards of the Coast
If either target leaves the battlefield before the ability resolves, the ability taps the remaining target, but no damage is dealt. If both targets leave the battlefield, the ability doesn't resolve.
No other card is named in this ruling.
#2February 1, 2007· Wizards of the Coast
Magus of the Arena's controller always chooses their creature first.
No other card is named in this ruling.
#3February 1, 2007· Wizards of the Coast
If an effect changes the targets of this ability, the second target can be changed to any creature the chosen opponent controls. The choice of opponent can't be changed.
No other card is named in this ruling.
#4February 1, 2007· Wizards of the Coast
Tapped creatures can be targeted. Tapping them again will do nothing, but they'll still deal damage.
No other card is named in this ruling.
#5February 1, 2007· Wizards of the Coast
If an effect changes the targets of this ability, the second target can be changed to any creature the chosen opponent controls. The choice of opponent can't be changed.
No other card is named in this ruling.