If you use the ability to cast a creature card face down, you must keep track of the amount and types of mana you spent on {X}. If that creature spell is moved from the stack to anywhere other than the battlefield, the resulting creature leaves the battlefield, or the game ends, the face-down card is revealed. If its mana cost couldn't be paid by some amount of, or all of, the mana you spent on {X}, all applicable penalties for casting a card illegally are assessed.