Noxious Dragon
Creature — Dragon
Flying
When Noxious Dragon dies, you may destroy target creature with converted mana cost 3 or less.
Dragons of Silumgar's brood begin to digest their prey even before consuming it, letting their caustic breath do much of the work.
4/4
Illustrated by
Commander Legends#139UncommonEnglishLegality
Rulings
Nov 24, 2014
A face-down creature has a converted mana cost of 0. However, its converted mana cost 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.
Nov 10, 2020
If a creature on the battlefield has 
in its mana cost, X is considered to be 0.