Stalking Leonin — ruling 1

There are several ways to secretly choose an opponent, including writing that player's name on a piece of paper that's kept with Stalking Leonin.

Stalking Leonin — ruling 2

If you have multiple Stalking Leonins, you may choose a different opponent for each one. Be careful to keep track of which opponent is chosen for each Stalking Leonin.

Stalking Leonin — ruling 3

If Stalking Leonin leaves the battlefield and returns, it's considered a new object. You may choose a new opponent and you may activate its last ability again.

Stalking Leonin — ruling 4

If you didn't choose an opponent when Stalking Leonin entered the battlefield, you can't pay the cost to activate its last ability. This will usually happen because you gained control of another player's Stalking Leonin or because a creature already on the battlefield became a copy of Stalking Leonin.

Stalking Leonin — ruling 5

If a creature is attacking a planeswalker you control, it's not attacking you. Stalking Leonin's last ability can't target it.

Stalking Leonin : règles, fonctionnement et interactions (MTG)

Stalking Leonin 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

When this creature enters, secretly choose an opponent. Reveal the player you chose: Exile target creature that's attacking you if it's controlled by the chosen player. Activate only once.

How it works : Stalking Leonin

Original English text — Wizards of the Coast / Scryfall

#1August 25, 2017· Wizards of the Coast
There are several ways to secretly choose an opponent, including writing that player's name on a piece of paper that's kept with Stalking Leonin.
No other card is named in this ruling.
#2August 25, 2017· Wizards of the Coast
If you have multiple Stalking Leonins, you may choose a different opponent for each one. Be careful to keep track of which opponent is chosen for each Stalking Leonin.
No other card is named in this ruling.
#3August 25, 2017· Wizards of the Coast
If Stalking Leonin leaves the battlefield and returns, it's considered a new object. You may choose a new opponent and you may activate its last ability again.
No other card is named in this ruling.
#4August 25, 2017· Wizards of the Coast
If you didn't choose an opponent when Stalking Leonin entered the battlefield, you can't pay the cost to activate its last ability. This will usually happen because you gained control of another player's Stalking Leonin or because a creature already on the battlefield became a copy of Stalking Leonin.
No other card is named in this ruling.
#5August 25, 2017· Wizards of the Coast
If a creature is attacking a planeswalker you control, it's not attacking you. Stalking Leonin's last ability can't target it.
No other card is named in this ruling.