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When an opponent casts a creature spell, if Opal Guardian is an enchantment, Opal Guardian becomes a 3/4 Gargoyle creature with flying and protection from red.
A noncreature permanent that turns into a creature can attack, and its {T} abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn’t matter how long the permanent has been a creature.
A face-down creature spell will cause Opal Guardian’s ability to trigger (if it’s an enchantment).
After the triggered ability resolves, Opal Guardian is no longer an enchantment.
Guardian’s effect doesn’t wear off.
