If a creature on the battlefield has {X} in its mana cost, X is considered to be 0.
24-11-2014
wotc
A face-down creature has a mana value of 0. However, its mana value changes when it's turned face up. Thus, if a creature is turned face up in response to being targeted by Noxious Dragon's ability, it may become an illegal target.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.