Illusionary Mask (CE)

(This card has square corners and a gold bordered back. Not tournament legal.)
X: You can summon a creature face down so opponent doesn’t know what it is. The X cost can be any amount of mana, even 0; it serves to hide the true casting cost of the creature, which you still have to spend. As soon as a face-down creature receives damage, deals damage, or is tapped, you must turn it face up.