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.
9/25/2006 Guardian’s effect doesn’t wear off.
9/25/2006 After the triggered ability resolves, Opal Guardian is no longer an enchantment.
9/25/2006 A face-down creature spell will cause Opal Guardian’s ability to trigger (if it’s an enchantment).
8/1/2008 A noncreature permanent that turns into a creature can attack, and its abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of his or her most recent turn. It doesn’t matter how long the permanent has been a creature.