Noxious Dragon
Fate Reforged — uncommon
Creature — Dragon
Noxious Dragon rulings:
2020-11-10: If a creature on the battlefield has in its mana cost, X is considered to be 0.
2014-11-24: 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.