Lurrus of the Dream-Den

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.

You must follow the normal timing permissions and restrictions of the spell you cast from your graveyard.

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.