The Monster Box

The Monster Box enters the battlefield
tapped.
{x}, {t}: Exile cards from the top of target
opponent's library until you exile a creature card
whose mana value is equal or less than X. You
may cast that card without paying its mana cost
if the resulting spell's mana value is less or equal
to X. Then put all cards exiled this way that
weren't cast on top of the targeted player's
library, then the targeted player shuffles.