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Once you put your companion into your hand, it behaves like any other card you’ve brought into the game. For example, if it’s countered or destroyed, it’s put into your graveyard, remaining in the game.

Wizards of the Coast has issued functional errata for the Companion mechanic. Instead of casting companions from outside the game: Once per game, any time you could cast a sorcery (during your main phase when the stack is empty), you can pay {3} to put your companion from your sideboard into your hand. This is a special action, not an activated ability. It happens immediately and can’t be responded to. It can’t be countered or stopped by cards like Phyrexian Revoker. For more information please see https://magic.wizards.com/en/articles/archive/news/june-1-2020-banned-and-restricted-announcement

Paying {3} to put your companion into your hand is a special action. It doesn't use the stack and players can't respond to it. Once you take this action, you may cast that card if it's legal to do so before any other player can take actions.

If you reveal a companion outside the game, for as long as it remains there, you may pay {3} any time you could cast a sorcery (that is, you have priority during your main phase and the stack is empty). Once you do, you put it into your hand and behaves like any other card you've brought into the game. For example, if it's discarded, countered, or destroyed, it's put into your graveyard, remaining in the game. This is a change from previous rules.

If a card in a player's graveyard has {X} in its mana cost, X is considered to be 0.

A card with mana value 0 has an even mana value.

If a replacement effect causes a player to exile the top four cards of their library instead of putting them into their graveyard as Gyruda's triggered ability resolves, the creature card you choose may be one of those cards in exile.

Your companion begins the game outside the game. In tournament play, this means your sideboard. In casual play, it's simply a card you own that's not in your starting deck.

Before shuffling your deck to become your library, you may reveal one card from outside the game to be your companion if your starting deck meets the requirements of the companion ability. You can't reveal more than one. It remains revealed outside the game as the game begins.

The companion ability has no effect if the card is in your starting deck and creates no restriction on putting a card with a companion ability into your starting deck. For example, Zirda may be in your starting deck even if your other permanent cards don't all have activated abilities.

The requirements of the companion ability apply only to your starting deck. They do not apply to your sideboard.

You may have one companion in the Commander variant. Your deck, including your commander, must meet its companion requirement. Your companion is not one of your one hundred cards.

If more than one player wishes to reveal a companion, the starting player does so first, and players proceed in turn order. Once a player has chosen not to reveal a companion, that player can't change their mind.

If you reveal a companion outside the game, for as long as it remains there, you may pay {3} any time you could cast a sorcery (that is, you have priority during your main phase and the stack is empty). Once you do, you put it into your hand and behaves like any other card you've brought into the game. For example, if it's discard, countered, or destroyed, it's put into your graveyard, remaining in the game. This is a change from previous rules.

The companion's other abilities apply only if the creature is on the battlefield. They have no effect while the companion is outside the game.