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While a spell with X in its cost is on the stack, its converted mana cost takes the chosen value of X into account. Mistmeadow Skulk can be chosen as a target for a Blaze (which has mana cost ) if X is 0 or 1, for example, but it can't be chosen as a target for a Blaze if X is 2 or more.

A face-down creature has a converted mana cost of 0, so Mistmeadow Skulk doesn't have protection from it .

The protection ability means the following: - Mistmeadow Skulk can't be blocked by creatures with converted mana cost 3 or greater. - Mistmeadow Skulk can't be enchanted by Auras with converted mana cost 3 or greater. It also can't be equipped by Equipment with converted mana cost 3 or greater. - Mistmeadow Skulk can't be targeted by spells with converted mana cost 3 or greater. It also can't be targeted by abilities from sources with converted mana cost 3 or greater. - All damage that would be dealt to Mistmeadow Skulk by sources with converted mana cost 3 or greater is prevented.