Morvair, the Timebound

Players can't activate abilities of nonland
permanents that aren't on the battlefield.
At the beginning of each player's upkeep, that
player exiles the top card of their library. Until
the end of their next turn, they may play that
card and spend mana as though it were mana of
any color to cast it.
Whenever a player casts a spell from exile,
counter that spell unless its controller pays X,
where X is equal to the number of spells that
player has cast this turn.