Companion

Companion is a keyword ability featured in Ikoria: Lair of Behemoths. , and the name for creatures with this keyword ability. There are ten legendary companions in Ikoria.

Description
The companion ability lists a deckbuilding rule. If your starting deck follows that rule, then the creature can serve as your chosen companion.

You may have up to one chosen companion for each game. That chosen companion doesn't start in your main deck; rather, it starts outside the game. This means it doesn't count toward meeting the minimum deck size in the format you're playing, but in Constructed formats, it does count toward your sideboard size. (If you're playing casually without sideboards, it's just in your collection outside the game, but all other rules still apply.) Just before the game begins, players in turn order reveal their chosen companions to all players.

The rules for how to bring your chosen companion into the game were introduced with the release of Ikoria: Lair of Behemoths on April 16, 2020 and updated on June 1, 2020. Both sets of rules are listed here.

Original rules
Under the original Companion rules, once per game, a player can cast their chosen companion from their sideboard. Doing so follows all the normal rules for casting a creature spell, so a player can normally do so only during their main phase if they have priority and the stack is empty. Casting the companion this way brings it into the game for good, after which it can be destroyed, exiled, returned to the hand, shuffled into the library, etc. and won't return to the sideboard until the game ends.

Current rules
Under the new Companion rules, once per game, any time a player can cast a sorcery, the player can take a special action and pay to put their chosen companion from outside the game into their hand. This special action does not use the stack and cannot be responded to. Once a companion is brought into the game in this manner, it remains in the game until the game ends.

Rules
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Rulings

 * 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 requirements of the companion ability apply only to your starting deck. They do not apply to your sideboard.
 * 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.
 * 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.
 * The companion ability has no effect if the card is in your starting deck. You may put a card with a companion ability into your starting deck regardless of whether or not your deck meets the requirements. For example, Zirda, the Dawnwaker may be in your starting deck even if your other permanent cards don't all have activated abilities.
 * You may have one companion in the Commander variant. Your deck, including your commander, must meet its companion requirement. Although the Commander variant does not use a sideboard, a companion is not counted as one of the deck's 100 cards.

Controversy
Companions were made to push the design space of what was considered powerful, but they were quickly deemed too powerful. All ten of the companions were reasonably-sized creatures, some with further engine capacity. The power level of these cards was so great that they even upset several eternal formats. Less serious, but still prominent for the health of the game, was the fact that the companion mechanic provided players with access to the same cards every game lead to extremely repetitive games. One of the most prominent companions was Lutri, the Spellchaser, whose restriction of "each nonland card in the deck must have a different name" was inconsequential for Commander; this caused the Commander Rules Committee to ban it on release day.

The deckbuilding restrictions, which were intended to balance the companions, turned out to only be a minor inconvenience in multiple formats. The most egregious was Lurrus of the Dream Den, whose restriction was "no permanent cards above 2 CMC". For most decks in the eternal formats (Modern, Legacy and Vintage), this was only a minor sacrifice, as three-mana permanents are often already considered expensive haymakers in these high-powered formats, and its continuous ability to recast these permanents provided incredible value for effectively all decks that have any permanents at all. It took less than two weeks for Lurrus decks to take over and dominate the eternal formats. Lurrus was soon banned in Legacy, but although it was causing just as much trouble in Vintage, it couldn't simply be restricted there, as players only needed one copy of it anyway due to the Companion mechanic. Thus it was banned in Vintage, obtaining the infamy of being the first card banned in this format for power level reasons since Mind Twist was banned in 1996.

After banning Lurrus in Vintage and Legacy and Zirda, the Dawnwaker in Legacy, Wizards of the Coast announced that if Companions would cause long-term health issues in other formats, they would be willing to take steps up to and including changing the rules of how the companion mechanic works. They followed through with their announcement a few weeks after this announcement, and the keyword received a rules change to how it is now.

Due to such unprecedented events, players questioned the capability of R&D and especially Play Design, which lead to Mark Rosewater questioning if pushing into unexplored space should outweigh game balance. He later admitted that Companions were the biggest design mistake of the year in which they came out, though he believed they could have been very popular if executed correctly. As a result of the rules changes, Lurrus was eventually unbanned in Vintage on February 15, 2021.