Lurrus of the Dream-Den

• WOTC Official Reminder Textofficial rules
  Lifelink — Damage dealt by this creature also causes you to gain that much life.
• WOTC Official Reminder Textofficial rules
  Companion — If your starting deck meets this condition, you may begin the game with this card outside your starting hand.

• WWizards of the Coast2020-04-17
  Official Ruling
  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.
• WWizards of the Coast2020-04-17
  Official Ruling
  Lurrus doesn't care about instant and sorcery cards in your starting deck. They may have any mana value.
• WWizards of the Coast2020-04-17
  Official Ruling
  You must follow the normal timing permissions and restrictions of the spell you cast from your graveyard.
• WWizards of the Coast2020-04-17
  Official Ruling
  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.
• WWizards of the Coast2020-04-17
  Official Ruling
  For spells with in their mana costs, use the value chosen for X to determine the spell's mana value. For example, if a permanent spell costs , you could cast it with X as 1 but not as 2.
• WWizards of the Coast2020-04-17
  Official Ruling
  If you cast a spell from your graveyard using another permission, Lurrus's effect doesn't apply. You can cast another permanent spell from your graveyard.
• WWizards of the Coast2020-04-17
  Official Ruling
  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.
• WWizards of the Coast2020-04-17
  Official Ruling
  If you reveal a companion outside the game, for as long as it remains there, you may pay 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.
• WWizards of the Coast2020-04-17
  Official Ruling
  Once you begin to cast the spell, losing control of Lurrus won't affect the spell. You can finish casting it as normal.
• WWizards of the Coast2020-04-17
  Official Ruling
  If you cast one permanent spell from your graveyard and then have a new Lurrus come under your control in the same turn, you may cast another permanent spell from your graveyard that turn.
• WWizards of the Coast2020-04-17
  Official Ruling
  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.
• WWizards of the Coast2020-04-17
  Official Ruling
  Lurrus doesn't let you play lands from your graveyard.
• WWizards of the Coast2020-04-17
  Official Ruling
  You must pay the costs to cast that spell. If it has an alternative cost, such as a mutate cost, you may cast it for that cost instead.
• WWizards of the Coast2020-04-17
  Official Ruling
  Paying 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.
• WWizards of the Coast2020-04-17
  Official Ruling
  The requirements of the companion ability apply only to your starting deck. They do not apply to your sideboard.
• WWizards of the Coast2020-04-17
  Official Ruling
  If a card in a player's deck has in its mana cost, X is considered to be 0.
• WWizards of the Coast2020-04-17
  Official Ruling
  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.
• WWizards of the Coast2020-04-17
  Official Ruling
  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.
• WWizards of the Coast2020-04-17
  Official Ruling
  If a permanent card is put into your graveyard during your main phase and the stack is empty, you have a chance to cast it before any player may attempt to remove that card from your graveyard.
• WWizards of the Coast2020-06-01
  Official Ruling
  Paying 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.
• WWizards of the Coast2020-06-01
  Official Ruling
  If you reveal a companion outside the game, for as long as it remains there, you may pay 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.

Rulings via Scryfall

• WO
  Wizards of the Coast2022-03-07

  Lurrus of the Dream-Den was banned in modern on March 7, 2022. Lurrus of the Dream-Den has remained a ubiquitous presence in the format across multiple archetypes. Lurrus's play rate (31% in Magic Online League decks that started with four wins) points to a card that is contributing to the homogenization of the Modern play experience. Due to play data, community feedback, and a desire to keep as diverse a range of card options as possible available to players in Modern, Lurrus of the Dream-Den is banned in Modern. Source: https://magic.wizards.com/en/news/news/march-7-2022-banned-and-restricted-announcement
• WO
  Wizards of the Coast2022-03-07

  Lurrus of the Dream-Den was banned in pioneer on March 7, 2022. While considerably less dominant in Pioneer today (20% in Magic Online League decks that started with four wins), we expect that Lurrus's metagame share will only grow as Pioneer's card pool expands. In the interest of preserving the diversity that Pioneer is enjoying rather than waiting for the critical mass in which Lurrus of the Dream-Den is problematic, it is banned in Pioneer. Source: https://magic.wizards.com/en/news/news/march-7-2022-banned-and-restricted-announcement
• WO
  Wizards of the Coast2020-05-18

  Lurrus of the Dream-Den was banned in vintage on May 18, 2020. In the weeks following the release of Ikoria: Lair of Behemoths on Magic Online, we've observed a rise in the popularity and win rate of Vintage decks using Lurrus of the Dream-Den as a companion. The deck-building cost imposed by Lurrus is less restrictive relative to the payoff of having Lurrus as a companion, leading to win rates above 55% and too large a portion of the metagame. Therefore, Lurrus of the Dream-Den is banned in Vintage. Source: https://magic.wizards.com/en/news/news/may-18-2020-banned-and-restricted-announcement
• WO
  Wizards of the Coast2020-05-18

  Lurrus of the Dream-Den was banned in legacy on May 18, 2020. As in Vintage, the wide card pool of powerful, low-mana-cost permanents in Legacy makes the power level of using Lurrus of the Dream-Den as a companion not commensurate to the deck-building cost. Collectively, Lurrus decks represent an increasingly large portion of the metagame, with several variants maintaining win rates above 55% in Magic Online league play. Matchup data indicates that metagame forces alone aren't sufficient to keep these decks in check, so we are also choosing to ban Lurrus of the Dream-Den in Legacy. Source: https://magic.wizards.com/en/news/news/may-18-2020-banned-and-restricted-announcement