If you cast a card “without paying its mana cost,” you can’t pay any alternative costs. You can, however, pay additional costs. If the card has any mandatory additional costs, such as that of Tormenting Voice, those must be paid to cast the card.
Any of the cards you don’t play will remain in exile.
If the card has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. John Stanko
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You must follow the normal timing permissions and restrictions for cards you cast this way.
If no spells have been cast in the turn you activate Magus of the Mind’s ability, X is one.
You may play a land exiled this way only if you have an available land play this turn.
