Illusionary Mask

You actually cast the card face-down, much as when playing a spell with Morph. It can be responded to and countered.

The effect that turns it face-up is a replacement effect. It doesn't use the stack and can't be responded to.

Both the amount and types of mana you spend on {X} are taken into account while you're choosing a creature card from your hand. For example, if you spent {U}{U} on {X}, you can choose a creature card with mana cost {U}{U}, {1}{U}, {2}, or {W/U}{W/U}, among other possibilities, but not one that costs {2}{U} or one that costs {G}.

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.

You may look at a face-down spell or permanent you control at any time.

While the creature card is face down, it's a 2/2 creature with no name, mana cost, color, creature type, abilities, or expansion symbol. Since it has no mana cost, its mana value is 0.

Only the controller of the face down creature can look at it.

The creature enters face down, so none of its "enters" abilities will trigger or have any effect. Also none of its "As this enters" abilities apply.

The creature's "enters" abilities (and any other abilities relating to the creature entering) do not trigger when it turns face up.