Lurrus of the Dream-Den — ruling 1

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.

Lurrus of the Dream-Den — ruling 2

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

Lurrus of the Dream-Den — ruling 3

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.

Lurrus of the Dream-Den — ruling 4

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.

Lurrus of the Dream-Den — ruling 5

Lurrus doesn't care about instant and sorcery cards in your starting deck. They may have any mana value.

Lurrus of the Dream-Den — ruling 6

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

Lurrus of the Dream-Den — ruling 7

For spells with {X} in their mana costs, use the value chosen for X to determine the spell's mana value. For example, if a permanent spell costs {X}{W}, you could cast it with X as 1 but not as 2.

Lurrus of the Dream-Den — ruling 8

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

Lurrus of the Dream-Den : règles, fonctionnement et interactions (MTG)

Lurrus of the Dream-Den 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

Companion — Each permanent card in your starting deck has mana value 2 or less. (If this card is your chosen companion, you may put it into your hand from outside the game for as a sorcery.) Lifelink Once during each of your turns, you may cast a permanent spell with mana value 2 or less from your graveyard.

How it works : Lurrus of the Dream-Den

Original English text — Wizards of the Coast / Scryfall

#1June 1, 2020· Scryfall
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.
No other card is named in this ruling.
#2June 1, 2020· Scryfall
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 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
No other card is named in this ruling.
#3June 1, 2020· Wizards of the Coast
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.
No other card is named in this ruling.
#4June 1, 2020· Wizards of the Coast
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.
No other card is named in this ruling.
#5April 17, 2020· Wizards of the Coast
Lurrus doesn't care about instant and sorcery cards in your starting deck. They may have any mana value.
No other card is named in this ruling.
#6April 17, 2020· Wizards of the Coast
If a card in a player's deck has in its mana cost, X is considered to be 0.
No other card is named in this ruling.
#7April 17, 2020· Wizards of the Coast
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.
No other card is named in this ruling.
#8April 17, 2020· Wizards of the Coast
You must follow the normal timing permissions and restrictions of the spell you cast from your graveyard.
No other card is named in this ruling.
#9April 17, 2020· Wizards of the Coast
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.
No other card is named in this ruling.
#10April 17, 2020· Wizards of the Coast
Once you begin to cast the spell, losing control of Lurrus won't affect the spell. You can finish casting it as normal.
No other card is named in this ruling.
#11April 17, 2020· Wizards of the Coast
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.
No other card is named in this ruling.
#12April 17, 2020· Wizards of the Coast
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.
No other card is named in this ruling.
#13April 17, 2020· Wizards of the Coast
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.
No other card is named in this ruling.
#14April 17, 2020· Wizards of the Coast
Lurrus doesn't let you play lands from your graveyard.
No other card is named in this ruling.
#15April 17, 2020· Wizards of the Coast
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.
No other card is named in this ruling.
#16April 17, 2020· Wizards of the Coast
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.
No other card is named in this ruling.
#17April 17, 2020· Wizards of the Coast
The requirements of the companion ability apply only to your starting deck. They do not apply to your sideboard.
No other card is named in this ruling.
#18April 17, 2020· Wizards of the Coast
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.
No other card is named in this ruling.
#19April 17, 2020· Wizards of the Coast
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.
No other card is named in this ruling.
#20April 17, 2020· Wizards of the Coast
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.
No other card is named in this ruling.
#21April 17, 2020· Wizards of the Coast
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.
No other card is named in this ruling.
#22April 17, 2020· Wizards of the Coast
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.
No other card is named in this ruling.
#23April 17, 2020· Wizards of the Coast
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.
No other card is named in this ruling.