Magus of the Mind
Creature — Human Wizard
, 
, Sacrifice this creature: Shuffle your library, then exile the top X cards, where X is one plus the number of spells cast this turn. Until end of turn, you may play lands and cast spells from among cards exiled this way without paying their mana costs.
"I'll show you what you truly desire."
John Stanko
Commander 2017 (C17)
• #12Rulings
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.
(wotc • 2017-08-25)
Any of the cards you don’t play will remain in exile.
(wotc • 2017-08-25)
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.
(wotc • 2017-08-25)
You must follow the normal timing permissions and restrictions for cards you cast this way.
(wotc • 2017-08-25)
If no spells have been cast in the turn you activate Magus of the Mind’s ability, X is one.
(wotc • 2017-08-25)
You may play a land exiled this way only if you have an available land play this turn.
(wotc • 2017-08-25)
Rulings
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.
(wotc • 2017-08-25)
Any of the cards you don’t play will remain in exile.
(wotc • 2017-08-25)
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.
(wotc • 2017-08-25)
You must follow the normal timing permissions and restrictions for cards you cast this way.
(wotc • 2017-08-25)
If no spells have been cast in the turn you activate Magus of the Mind’s ability, X is one.
(wotc • 2017-08-25)
You may play a land exiled this way only if you have an available land play this turn.
(wotc • 2017-08-25)