Lullmage's Domination

• WWizards of the Coast2020-09-25
  Official Ruling
  In a multiplayer game, once an opponent leaves the game, they won't be considered for such an effect, no matter how many cards they had in their graveyard before leaving the game.
• WWizards of the Coast2020-09-25
  Official Ruling
  To cast Lullmage's Domination, choose a value of X, choose a creature with the appropriate mana value, then determine the spell's cost, subtracting if appropriate. This can't reduce the in the spell's cost. For example, if you wish to target a creature with mana value 2 whose controller has eight cards in their graveyard, you'll pay .
• WWizards of the Coast2020-09-25
  Official Ruling
  An ability that offers a bonus if an opponent has eight or more cards in their graveyard won't provide additional benefits if more than one opponent has eight or more cards in their graveyard. There are also no additional benefits no matter how many cards are in an opponent's graveyard, as long as there are at least eight.
• WWizards of the Coast2020-09-25
  Official Ruling
  Once Lullmage's Domination resolves, the control effect lasts until the creature leaves the battlefield or you lose the game. It doesn't expire at the end of turn.
• WWizards of the Coast2020-09-25
  Official Ruling
  If a creature on the battlefield has in its mana cost, X is considered to be 0.

Rulings via Scryfall