Guardião do Tesouro

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

If a spell has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

If you cast a spell "without paying its mana cost," you can't choose to cast it for any alternative costs. You can, however, pay additional costs. If the card has any mandatory additional costs, those must be paid to cast the spell.

If you don't cast the nonland card with mana value 3 or less, it'll be put on the bottom of your library in a random order with the other cards.

If your library has no nonland cards in it with mana value 3 or less, you'll reveal all the cards in your library, then put them back in a random order.