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At the beginning of your upkeep, if twenty or more creature cards are in your graveyard, you win the game.
If you don’t have twenty or more creature cards in your graveyard by the time your upkeep starts, the ability won’t trigger that turn.
Mortal Combat’s ability has an “intervening ‘if’ clause,” so the condition is checked again when the triggered ability resolves. If you don’t still have twenty creature cards in your graveyard, the ability does nothing.
