Portal Mage — ruling 1

You may cast Portal Mage outside of a declare attackers step. If Portal Mage enters the battlefield outside of a declare attackers step, its ability simply doesn’t trigger.

Portal Mage — ruling 2

Reselecting which player or planeswalker a creature is attacking ignores all requirements, restrictions, and costs associated with attacking.

Portal Mage — ruling 3

Reselecting which player or planeswalker a creature is attacking doesn’t cause “whenever this creature attacks” abilities to trigger. Notably, the Curses in this set (such as Curse of Vitality) won’t trigger while Portal Mage’s ability is resolving.

Portal Mage — ruling 4

If you reselect which player or planeswalker an attacking creature is attacking, that creature is still considered to have attacked the player or planeswalker as declared, but it is now attacking the new player or planeswalker.

Portal Mage — ruling 5

If an ability targets something controlled by the “defending player” of an attacking creature and the defending player for that creature changes before that ability resolves, the ability won’t resolve because its target has become illegal.

Portal Mage : règles, fonctionnement et interactions (MTG)

Portal Mage 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

Flash When this creature enters during the declare attackers step, you may reselect which player or permanent target attacking creature is attacking. (It can't attack its controller or their permanents.)

How it works : Portal Mage

Original English text — Wizards of the Coast / Scryfall

#1August 25, 2017· Wizards of the Coast
You may cast Portal Mage outside of a declare attackers step. If Portal Mage enters the battlefield outside of a declare attackers step, its ability simply doesn’t trigger.
No other card is named in this ruling.
#2August 25, 2017· Wizards of the Coast
Reselecting which player or planeswalker a creature is attacking ignores all requirements, restrictions, and costs associated with attacking.
No other card is named in this ruling.
#3August 25, 2017· Wizards of the Coast
Reselecting which player or planeswalker a creature is attacking doesn’t cause “whenever this creature attacks” abilities to trigger. Notably, the Curses in this set (such as Curse of Vitality) won’t trigger while Portal Mage’s ability is resolving.
No other card is named in this ruling.
#4August 25, 2017· Wizards of the Coast
If you reselect which player or planeswalker an attacking creature is attacking, that creature is still considered to have attacked the player or planeswalker as declared, but it is now attacking the new player or planeswalker.
No other card is named in this ruling.
#5August 25, 2017· Wizards of the Coast
If an ability targets something controlled by the “defending player” of an attacking creature and the defending player for that creature changes before that ability resolves, the ability won’t resolve because its target has become illegal.
No other card is named in this ruling.